PRACTICAL MANUAL
on AGROMETEOROLOGY
Dr. L.R. Yadav Dr. S.S. Yadav Dr. O.P. Sharma Dr. A. C. Shivran
2012
Department of Agronomy S.K.N. College of Agriculture (SK Rajasthan Agricultural University) Campus : Jobner – 303329 (Rajasthan)
SKN COLLEGE OF AGRICULTURE
(S.K.RAJASTHAN AGRICULTURAL UNIVERSITY: BIKANER) JOBNER-303329 Distt. - Jaipur (Raj.) Phone: 01425-254022 (O), 01425-254022 (Fax) 01425-254023(R)
Dr. G.L. Keshwa Dean
FOREWORD
Weather and crop production are integral components of agriculture. During the era of global warming and climate change, the role of agrometeorology in agriculture has become more important in mitigating the challenges arised by climate change. Successful crop production depends on the prevailing weather conditions at different stages of crop growth. This manual has been prepared to enhance the understanding of undergraduate students regarding measurement of weather elements, their interpretation and role in crop production.
Dr. L.R. Yadav, Dr. S.S. Yadav, Dr. O.P. Sharma and Dr. A.C Shivran deserve congratulation for bringing out this manual for benefit of students, readers and all those involved in the measurement of weather data.
PREFACE
Agriculture and weather are the essential components of the crop production. Crop production depends upon the prevailing weather conditions at different stages of crop growth. Precise measurements of weather elements ar e required to understand the proper interpretation in relation to crop growth and development. Recently, the course curricula of undergraduate classes has been reoriented according to IV Dean‘s committee of ICAR. The practical exercises of this manual are according to new syllabi of agrometerology course running in the UG programme.
Sixteen exercises have been included in the manual. The material has been drawn from various publications with and without seeking permission from authors/publishers. The authors would like to express their gratitude of all of them. The authors would be grateful to receive suggestions from readers for further improvement of this manual. The authors wish to acknowledge the financial assistance received from ICAR for publication of this practical manual.
Special gratitude is expressed to Dr. G.L. Keshwa, Dean, S.K.N. College of Agriculture, Jobner for his able guidance and encouragement for preparation of this manual.
February, 2012
Jobner Authors
CONTENTS
S. Particulars Page Date Signature of teacher No no 1 Agro meteorological observatory site selection , installation and exposure of instruments 2 Calculation of local mean time 3 Handling of agro meteorological instruments and weather data recording 4 Measurement of total solar radiation 5 Measurement of long and short wave radiation 6 Measurement of bright sunshine hours 7 Measurement of maximum and minimum temperature 8 Measurement of soil temperature 9 Measurement of dew point temperature 10 Determination of vapor pressure and calculation of relative humidity 11 Measurement of atmospheric pressure 12 Measurement of wind direction and speed 13 Measurement of rainfall 14 Measurement and determination of evaporation 15 Processing and tabulation of weather data 16 Presentation of weather data
Exercise No. 1
Agro-meteorological observatory site selection, installation and exposure of instruments
An observatory is a specially designed station or place where the regular and simultaneous records of the weather data are made by physical measurements using various techniques, sensors, skills, recorders, instruments etc. by standard methods at hours recommended by IMD and WMO. IMD was established during 1875 with its central office at Pune which takes up this responsibility. Country is divided into 35 meteorological sub divisions.
To facilitate the collection of data at one point, five regional centres were established, which are located in five different places, for which the head quarters are mentioned against each as detailed below.
North zone — Delhi
East zone — Calcutta
South zone — Chennai
West zone — Bombay
Central zone — Nagpur
Weather affects agriculture at every stage, therefore, knowledge of crop weather relationship helps in opti-mizing the agricultural operations. Since meteorology and climatology are primarily observational science, adequate care has to be taken for getting most representative and accurate observations of weather parameters for their worthwhile application in weather and climate prediction.
In arid and semi arid agriculture, the weather aberrations are more as compared to the humid agriculture that adversely affect the agricultural production. The weather variables and their measurement, if taken in time for the weather forecast for short and medium range one can avoid from the great losses in the arid and semi arid areas. Therefore, urgent and rapid data collection and their dissemination is possibl e by well settled agromet observatory in the data measurement and management for weather forecast agencies like India Meteorological Department (IMD), National Centre for Medium Range Weather Forecast (NCMRWF) and State Agricultural Universities for their better use for the farming community.
Types of general observatories
On the basis of instrumental facilities, observer type, data observation frequency and mode of data transmission, IMD has divided general observatories into 6 classes:
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 1
10. Class I(Special station): Autographic recording and eye recording, all data elements are recorded at least thrice(0530,1130,1430,2030,0230 IST).
11. Class II (Synoptic station) : Eye reading, almost all data recorded at least twice(0830, 1730 IST).
12. Class III(Synoptic station) : Eye reading,data are recorded part time once a day(0830 IST).
13. Class IV(Climatological station) : Eye reading without barometer all data recorded except pressure once a day.
14. Class V(Rainfall station) :Eye reading only, rainfall is recorded daily.
15. Class VI( Less or Non instrument station) : Less or no instrument, wind direction, cloud, speed visibility etc. are recorded daily. Agro meteorological observatories
Agromet observatories are those stations at which physical elements of climate and biological, agricultural elements, generally of phenological nature or both related to agriculture are observed to explore crop- environment relationship. World Meteorological Organization (WMD) has divided agromet observatories into four categories-Principal, Ordinary, Auxiliary and Specific purpose agromet observatories.
Types of the observatories
The observatories are classified on the basis of the number and type of instruments available. Essential and optional availability of the instruments in each type observatory are as below:
(a) Auxiliary ( or class C) observatory:
Essential instruments
1. Single Stevenson screen with
Dry bulb thermometer
Wet bulb thermometer With one spare set of thermometers
Minimum thermometer
Maximum thermometer
14. Non-recording rainguage and measuring glass (with one spare measuring glass) Optional instruments
1. Wind vane 2. Anemometer 3. Dewgauge
(b) Ordinary (or class B) observatory:
Essential instruments
1. Single Stevenson screen with
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 2
Dry bulb thermometer
Wet bulb thermometer With one spare set of thermometers
Minimum thermometer
Maximum thermometer
17. Non-recording rainguage and measuring glass (with one spare measuring glass)
18. Soil thermometers at the depth of 5, 10, 15, 20 and 30 cms along with the stand
19. Class A pan evaporimeter with fixed point gauge covered with a wire mesh and an ordinary thermometer for measuring water temperature in the evaporimeter
20. Wind vane
21. Anemometer and
Optional instruments
1. Dewgauge 2. Sunshine recorder 3. Self recording rainguage and
4. Double Stevenson screen with thermograph and hygrograph.
(c) Principal (or class A) observatory:
1. Instruments of class B observatory
2. Self recording rainguage
3. Thermograph and 4. Hygrograph
Criteria for site selection
I. The site should contain a flat rectangular plot with 55 meters (180 feet) in north-south direction and 36 meters (120 feet) in east-west direction( Fig. 1),
II. The site must be representative of climate, soils and agricultural (cropping) conditions of the area and should be located at the centre of the farm,
III. The site must be free from water logging and easily accessible throughout the year,
IV. Site selected should be away from hills, buildings, streams and trees to avoid shade, shield or channel affects,
V. It should be away from steep slopes, water bodies and frequent irrigation.
VI. The recommendable distance from the obstacles from the raingauge and other instruments are at least twice
and 10 times the height of the obstructions, respectively. Fencing
After the site selection, it should be wire fenced, normally to a height of about 1.5 meter with a gate locking arrangement to protect from theft, animals and rodents which may cause damage to and cables.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 3
Surface conditions
The surface of the observatory should be cleaned regularly and be covered with thin grass as barren ground causes increased ground radiation. The grass should be periodically trimmed.
Coordinates of the observatory
Since data has far wider application, therefore, exact location of the observatory should be known. For t his purpose, coordinates of the site of the observatory, i.e. latitude and longitude to the nearest meter and elevation (height above mean sea level) to the nearest meter should be obtained. The elevation refers to the ground on which raingauge stands, in the absence of which it refers to the ground under thermometer screen.
Precautions for installation
1. The instrument should be robust, durable, accurate and simple for operation and should not require calibration graph without any electrical connection.
2. While installing the Stevenson screen which contains the thermometers, care should be taken that it should open in North direction to prevent direct sun shine during observation.
3. Tall instruments should be on one side of the observatory so that they may not shade small instrument.
Hours of observation:
Since Meteorological elements vary with time, it is necessary that they should be recorded at a particular time on every occasion. In India, the main observations are recorded as per the guide lines of I.M.D. at 0830 and 1730 IST.
At Agromet observatories, the observations are recorded at 07:00 and 14:00 hours LMT except evaporation and rainfall which are recorded at 08:30 hours IST. Radiation observations including sunshine are recorded as per LAT (Local Apparent Time). At meteorological observatories the hours are numbered consecutively from mid- night 00 hours to midnight 24 hours, the hours after noon being 13 hours, 14 hours and so on. Time as 2:30 p.m and 2:30 a.m are expressed as 1430 and 0230 hours I.S.T. respectively.
The instruments should be read in the following order: 1. Wind instruments 2. Raingauge
3. Thermometers 4. Barometer Type of observations
The observations are of two types:
1. Instrumental (for which instruments are used). Most of the observations recorded in observatory
2. Sensory (for which observer uses his senses and these are primarily visual). These are current weather including clouds, visibility, thunder storm, lightening, fog etc.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 4
Fig.1 Layout of agrometeorological observatory
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 5
Problem: 1. Draw the layout plan of a standard agrometeorological observatory.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 6
Dated______
Exercise No.2
Calculation of local mean time
Indian Standard Time (IST)
The surface of the Earth is divided into 24 ‗time zones‘ the way in which there are 24 hours in a day. The time established in each of the zone is called as ‗Standard time‘. The Indian Standard
Time (IST) is the Local Mean Time (LMT) for the meridian of lo ngitude 82° 30 ‗ E. This is the longitude of
Allahabad which is taken as standard longitude for our country. Since each degree is equal to four minutes of time, the IST is 5 ½ hours ahead of Greenwich Mean Time (GMT). The GMT is also known as universal time.
Local Mean Time (LMT
This is the local time based on the transit of the mean Sun. To calculate LMT from IST, it is essential to know the longitude of the station. The time of observation at India Standard time is 0700 and 1400 hours for all observations except rainfall and pan evaporation whose observations are taken at 0800 hours. As corresponding to each degree longitude local mean time (LMT) is higher by 4 minutes on the east and lower to the west, therefore, for determining LMT from Indian standard time (IST), following formula may be used:
LMT= IST- 4 (L1 - L2)
LMT = IST - 4 (82° 30' - longitude of the station) minutes)
Local apparent time
The interval between two successive transits of the Sun across the meridian is called the true solar day and the time based on the length of this day is called Apparent Solar Time. Local Apparent Time is the apparent solar time for any particular place such that the Sun passes across the geographical meridian at noon.
o o o Example1: Station Hyderabad, Longitude 78 30' East. i.e. 78.5 East, Standard meridian 82 30' i.e. 82.5
LMT = IST — 4 (82.5 — 78.5)
IST — 4 (4)
IST — 16 minutes.
If IST is 10 h 30 m, then
LMT = 10 h 30 m — 16 m = 10 h 14m.
Example 2 : Longitude of Jobner is 75° 28', then corresponding to 0700 IST and 1400 IST, the LMT will be ?
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 7
LMT = 0700 - 4 x ( 82° 30' - 75° 28') minutes = 0700 hrs - ( 4 x 7° 2') minutes
= 0700 - (28 + 4 x 2/60) = 0700 -28.13 minutes = 0632 hours
LMT = 1400 - 4 x ( 82° 30' - 75° 28') minutes = 1400 hrs - ( 4 x 7° 2') minutes
= 1400 - (36 + 4 x 2/60) = 1400 -28.13 minutes = 1332 hours
Jobner lies in the west, therefore, the time of observation will be 0732 and 1432 hrs.
Problem: Workout LMT of New Delhi( 77° 12'), Kolkatta( 80° 21'), Mumbai (72° 50' ),
and Jaipur (75° 48'). at 0700 hrs.
Solution:
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 8
Dated______Exercise No. 3 Handling of agro meteorological instruments and weather data recording
Meteorological operations are required for a variety of purposes which can be broadly classified into two categories: I. Forecasting II. Climatological study.
In the country, there is an international understanding to have uniformity in the observatory set up including instruments installation procedure as well as timing. Different weather elements with units and different instruments are shown in Table 3.1.
Different thermometers used for measurement of temperature in an agrometeorological observatory are:
Maximum thermometer : Maximum thermometer is a mercury-in-glass thermometer with a constriction in the bore below the lowest graduation. When the temperature falls after reaching the maximum value, mercury does not return to the region below the constriction, provided that the stem of the thermometer is approximately horizontal. The range of maximum thermometer graduation is from -20°C to 55°C.
Minimum thermometer : Minimum thermometer is a spirit thermometer, commonly used liquid being absolute ethyl alcohol.
Within the liquid there is a very light dumb bell shaped glass index which moves freely within the spirit but not readily eme rge from the liquid due to surface tension. The thermometer is tilted slightly so that bulb end is upward, the glass index slides along the tube until it reaches the meniscus. But when temperature rises, it remains stationary while the liquid moves ahead in the column. The range of minimum thermometer graduation is from -40°C to 50°C. Self recording of temperature:
Continuous air temperature is recorded by bimetallic thermograph that work on the principle of expanding of two different metals in length at different rates with the temperature variation. The outer strip of iron - nickle alloy having negative coefficient of expansion much less than the inner strip of brass and the different expansion is recoded at clock drum by pointer. The bimetallic thermograph is kept in big Stevenson screen.
Exposure and installation of air thermometers
To give a representative reading, the thermometers are protected from direct sunlight, sky, earth and surrounding objectives with a adequate ventilation. For this purpose thermometers are kept in Stevenson screen.
Stevenson screen: Stevenson screen is a wooden box whose side walls are louvered providing free movement of air with gusts suppressed. Its upper roof is double with space in between preventing solar radiation and consequent heat affecting the inside. These are of two types. Small Stevenson screen and large Stevenson screen. Sm all Stevenson screen has a size of 2' x
2.5' x 3' whereas large has 4' x 2.5' x 3' with 4' large lags mounted on the earth for fixing in concrete material ( Fig. 2).
Thermometers ( maximum, minimum, dry and wet) are kept in small Stevenson screen whereas se lf recording instruments like thermograph and hair hygrograph are kept in large Stevenson screen for
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 9 ventilation and protection from outside objects.
Measurement of soil temperature
For measuring of soil temperature, the mercury -in-glass thermometers are used for the depth of 5, 15 and 30 cm with their stem bent at right angles or other suitable angle with their scale facing upward are the most conve-nient. For greater depth, the soil thermometers are installed in greater depths like 50 cm in iron pipes.
Grass minimum thermometer
The idea about ground frosts which are very crucial from the point of view of agriculture, is obtained from Grass minimum thermometer. Grass minimum thermometer consists of sheathed alcohol minimum thermometer, simi-lar to the ordinary minimum thermometer and is used to measure temperature at soil surface covered with vegetation. This is also called terrestrial radiation minimum thermometer.
Table 3.1: Measured elements, units in general use and measuring instruments
S. Element Unit Instrument(s) No.
1. Temperature Degree Celsius (°C) Thermometer, hygrograph 2. Wind speed kmph, mps, knots Anemometer, Anemograph 3. Wind direction Degre es clockwise from north on Wind Vane, Anemograph the scale 00-36, where 36 is the wind from the north, 09 from the east and 00 refers calm. 4. Relative humidity Per cent (%) Dry and wet bulb thermometers, hygrograph 5. Precipitation millimetres (mm) Rain guage, dewgauge, snowguage 6. Evaporation millimetres (mm) Evaporimeters 7. Duration of hours (h) Sunshine recorder sunshine hours 8. Cloud cover Oktas (1/8 of the celestial dome) Visual, observed in the observatory
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 10
Fig. 2 Single Stevenson screen
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 11
Wind instruments:
The site for wind instruments must be as open as possible and should be away from tall structures. The instruments namely anemometer and wind vane should be placed on wooden posts or masonry pillars so that the height of the centre of the cup in case of anemometer and the arrow head should be 10 feet above the ground level. The pillars should be vertical to the ground surface. In order to maintain natural rotations of wind vane and anemometer, the instruments should be regularly lubricated by oiling.
Sunshine recorder
It is placed at elevated place and usually placed at a platform of 5-10 feet from the ground surface. It is kept on a horizontal plane. Open pan evaporimeter
This instrument is installed at a place free from water logging. The pan rests on a wooden plank which is painted white and placed about 5-6 cm above the ground surface. This allows free circulation of air. Problem:
1. Name the different thermometers which are kept in Stevenson screen. Solution:
2. Name the different instruments installed in an agromet observatory. Solution:
3. What precautions are to be followed in handling of instruments while recording weather data?
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 12
Dated______Exercise No. 4 Measurement of total solar radiation
Global solar radiation can be meas ured by converting radiant energy into electro magnetive force as in thermopiles. Total short wave radiation or global radiation on a horizontal surface can be measured by
Pyranometers (Fig.3).
In the absence of the instrumentation for observing solar radiation, the solar radiation receiving on the surface of the earth could be estimated by using the relationship between the duration of sunshine and radiation(Table 1). That 2 is, radiation (calories/cm /day) is directly proportional to number of bright sunshine hours. Higher the sun shine hours, larger the radiations received and vice verse. This can be given in Angstroms formula as: Q = QA(a+b n/N)
2 Where Q = the radiation actually received on the surface of the earth(cal/cm /day) QA = Angots value(extra terrestrial radiation value incident outside the
earths atmosphere
n = actual number of sunshine hours in a day as observed
N = Maximum possible duration of sunshine( depends on the latitude of the place)
a & b = constants depending upon the locations( latitude sun angle)
Case I- when n=N that is actual sunshine equal to maximum possible sunshine under clear sky
Q = QA(a+b n/N) = QA(a+b)
Maximum radiation received on the earth surface.
Case II - n =0, due to continuous overcast conditions through out the day
Q = QA(a+b 0/N) = QA(a)
Lowest radiation received due to scattering effect of clouds.
Q is not equal to 0, even if n=0. It is due to the fact that diffused radiation is received even on cloudy days.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 13
Table1. The value of a and b for different stations in India( Gangopadhyaya, 1970) o Station Latitude N ‘a’ ‘b’ Bangaluru 13.0 0.18 0.62
Chennai 13.1 0.30 0.44
Hyderabad 17.4 0.14 0.55
Pune 18.5 0.35 0.40
Nagpur 21.1 0.16 0.68
Ahemdabad 23.1 0.42 0.30
Jodhpur 26.4 0.31 0.49
New Delhi 28.4 0.31 0.46
Shillong 24.6 0.18 0.66 Depending upon the sky condition, the radiation varies between (aQA) to (a+b)QA . Problem:
1. Calculate the radient energy recorded at Jobner during different months.
Months Normal n N n/N QA
January 10.6 715
February 11.3 793
March 12.0 870
April 12.8 896
May 13.4 900
June 13.8 892
July 13.6 893
August 13.0 891
September 12.3 859
October 11.6 813
November 10.8 735
December 10.4 695
Solution:
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 14
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 15
Dated______
Exercise No. 5 Measurement of long and short wave radiation
Solar radiation affects to a large extent the micro-climate thereby, the crop growth and yield. Spectral quality of sunlight intercepted by the crop canopy and light that penetrates through the canopy are other important factors determining the crop growth in the system. The measurements should enable the evaluation of the photosynthetic efficiency of the system and matching of this with alternate designs of canopy structure. Instruments used for the measurement of radiation (Fig.3) for the study of micro-climatic regimes are:
(1) Line quantum sensor ( 2) Net radiometer (3) Spectroradiometer ( 4) Pyranometer
(5) Pyrano - albedometer etc. Radiometers are named according to the nature and direction of radiation which they absorb and indicate. The commonly used instruments are : Name of the instrument Radiation measured 1. Pyranometer or Solarimeter : Direct and diffuse solar radiation 2. Pyrheliometer Direct solar radiation 3. Pyrradiometer Solar and long-wave radiation 4. Pyrgeometer Long-wave radiation 5. Netradiometer Net radiation flux Measurement of different’ components of solar radiation.
Material required: Pyrano-albedometer, millivoltmeter etc.
The visible part of the spectrum is short -wave radiation. The solar radiation is a combination both direct and diffused radiation. So, to measure direct solar radiation, the diffuse radiation has to be subtracted from the total incident radiation. The total incident radiation, diffused radiation and radiation reflected from various surfaces can be measured with the help of a pyrano -albedometer. When radiation is incident on the sensitive element of the instrument, it produces electric current which is measured in millivolts in the millitivoltmeter. There is a c onversion factor for eac h instrument. When the reading in milli voltmeter is multiplied by conversion factor, the reading comes out in langley per metre per metre per second or watts per metro per met re per second. Procedure:
1. Connect the pyrano-albedometer properly to the millivoltmeter.
2. Keep the instrument ready for taking observations by setting the knob at
‗ON‘, in the millivoltmeter.
3. Level the instrument, if needed, using a small spirit level.
4. Total incident radiation, diffuse radiation and reflected part of the radiation shall be recorded simultaneously, as per the instructions given for the instrument, on the selected surfaces.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 16
5. Normally, a bare soil, a dry turf, a wet turf and a crop canopy may give values of real worth comparison.
6. Repeat the process at 15 minute intervals for at least 6 times on the same surfaces and record the observations in the recording sheet provided for this exercise.
7. Calculate albedo and direct radiation as per the formulae in point no.9 given below.
8. Observe the trends in albedo and absorbed radiation with variation in time.
9. Formulae: a) Direct radiation = Incoming radiation - Diffuse radiation
b) Albedo = Reflected radiation/Total Incoming radiation x 100 Graph: Plot a neat graph for all the measurements recorded above.
Units
23 1 Einstein = Avogadro‘s number of quanta, (6.023 x 10 ) -2 -1 -2 1 photon = 1 quantum of light, 1 J m s = 1 W m -2 -1 -2 1 cal = 4.184 J, 1 cal cm min = 1 Langley = 697.8 Wm The following conversion factors are mainly approximation.
At noon on a clear summer day in the wave band 400-700 nm, -2 10,000 foot-candles = 350 W m , 1 foot candle = 10.76 lux
The solar irradianc e at the Earth‘s surface in a temperate region at mid-day during the summer in the wave band -2 -2 400 - 700 nm = 350 W m and in the wave band 700 - 2400 nm = 350 W m The region 400 - 700 nm is called visible band or photosynthetically active wave band. The short -wave infrared wave band is 700 - 2400 nm.
Units of length
-1 -3 10 m = decimetre ( dm), 10 - 2 m = centimetre (cm) , 10 m = millimetre ( mm)
-6 -10 o 10 m = micrometre (micron), 10 - 9 m = nano metr e ( nm ) , 10 m = A ngs tr om uni t ( A )
Problems1: What are different radiation measuring instruments?
Solution:
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 17
Pyranomet re Pyrheilmet oer
Pyrano-albedometer
Fig.3 : Radiation measuring instruments
Problem 2: How will you calculate direct radiation and albedo?
Solution:
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 18
Dated______Exercise No. 6
Measurement of bright sunshine hours
Crops obtain energy necessary for life directly from sunlight. Sun gives out its energy within 0.15 -3.0 u. Solar radiation in general are c alled short wave radiations. Earth emits radiation in the wavelength of 3 -100 u and are called long wave radiations. Intensity, quality, quantity, duration and periodicity are five important factors in agricultural production processes. Photosynthesis takes place only in the visible light with wave length 0.4 -0.7 micron and these radiations are also called as Photosynthetically active radiations (PAR) They convert solar energy into chemical energy during the process of photosynthesis. Large amount of energy is also required during the process of transpira-tion of crops. Infestation of pests and diseases is also gene rally associated with the occurrence of prolonged periods of continuous cloudy days during the crop growing season.
2 At the top of the atmosphere 2.0 cal/cm /min of energy are received( taking this as 100%). Out of this only 47 % ( or 0.94 cal) reaches to the earth surface as 53% is lost by reflection, scattering and absorption. This energy on reaching the earth is again reflected differently, roughly 35%. On crop particularly, 25% is reflected and thus 75% of 0.94 cal equals to 0.705 is incident on the crop. Out of this only 41% energy is in PAR which is 0.29 cal and rest being used for heating. Photosynthesis has highest efficiency of 8% and, therefore, 0.29 x 8/100 = 0.023 cals or 1.15% of total solar radiation are utilized on a clear day.
The sunshine is measured by means of Campbell -Stokes Sunshine Recorder ( Fig. 4). One of the measurements needed for studying the total radiation reaching the earth‘s surface is the duration of sunshine. A sunshine recorder is used to measure the hourly or daily totals of the duration of sunshine. It makes accurate measurements to the nearest tenth of an hour. In Campbell Stokes sunshine recorder, the duration of sunshine is determined by concentrating the sun‘s rays, so that they fall on to a piece of pap er, and burn a trac e of it.
Description of sunshine recorder
The sunshine recorder consists essentially of a glass sphere, about 10 cm in diameter, mounted concentrically in a section of spherical bowl. The diameter of the bowl is such that the sun‘s ra ys are focused sharply on a card held in grooves cut into the bowl. Three overlapping pairs of grooves are provided in the spherical segment in order to take cards suitable for different seasons of the year. The sunshine recorder is installed on a masonry pillar of 5 or 10 feet above the ground.
Record cards: Record cards are made of good quality paste board which does not expand appreciably in length on wetting. They are printed in a colour such a medium shade of blue that absorbs solar radiation. Different types of cards are given in Table 6.1.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 19
Table 6.1:Types of cards and time of use
S.No. Name of card Type of card Time of use Remarks th st 1. Summer card Long curved cards 13 April - 31 August Inserted in bottom slot 2. Winter card Short curved cards 13 October to end of Inserted at top February slot th 3. Equinoctial cards Straight cards st March to 12 April Inserted in 1 st th and 1 S eptem ber to 12 middle slot October
Exposure
The purpose of the sunshine recorder is to provide a continuous record of bright sunshine. The sunshine recorder should ideally be set up on a firm and a rigid support in a place where there is no obstacle to the sun‘s rays at any hours of the day at any time of the year.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 20
Measurement of the records
A second card (C), of similar curvature, is used to evaluate the daily total of the duration of bright sunshine. Place this card along with edge of the one caring the record (R). Then use a sharp pencil to mark on the card C, lengths equal those of the successive trace on card R. The position of the card can be adjusted, so that these lengths from a continuous line. The length of the line should be measured to the nearest tenth of an hour. The sunshine card is read with the help of sunshine scale. One division of the scale is six minutes (0.1 hr). The reporting is done by adding together all the points of the burn. The burn may be very faint such as is usually the case near sunrise and or sunset.
Changing the card
A fresh card should be inserted in the recorder each day, whether there has been sunshine duri ng the day or not. A blank card provides evidence that the sky has been overcast. After rain, it is some times difficult to withdraw a card without tearing it. In this case, cut the card out carefully by drawing a sharp knife along the edge of one of the fla nges. Insert a new card and adjust to the 1200 hours time. Before inserting a new card, it is desir-able to clean dust that may have accumulated in the grooves into which the cards are to be placed.
Problems
1. Draw a neat sketch of Campbell-Stokes sunshine recorder and different cards.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 21
Dated______
Exercise No.7
Measurement of maximum and minimum temperature
Temperature
Air temperature is the temperature of the air recorded by the thermometer exposed in a standard type of screen called Stevenson screen.
Heat and temperature
Temperature is the measure of mean kinetic energy of per molecule of the molecules of an object, while the heat is the measure of total kinetic energy of all the molecules of that object.
Measurement of air temperature
Temperature of the air is one of the important factor in crop - weather relations. Its measurement helps in understanding the rate and amount of water loss in the process of evaporation from the soil and transpiration from the plant system for a given environment. Temperature is measured using three type of scales namely Fahrenheit , Ceisius and Kelvin. The melting point of ice on the t hree scales is 32°, 0° and 273° and boiling point of water is 212°, 100° and 373°, respectively. The relation between three scales are
K = C + 273
C = 5/9(F – 32)
F = 9/5 (C +32)
Temperature instruments
1. Maximum thermometer: This is a mercury thermometer and records the highest or maximum temperature reached during past 24 hours or since last setting. Maximum temperature generally occurs in the world between 14.00 to 1600 hrs (Fig. 5).
2. Minimum thermometer: This thermometer records the lowest temperature of air reached during last 24 hours or since last setting. Lowest temperature of the day generally occurs just before sunrise or clear day and after sun rise on cloudy day (Fig 5).
3. Dry bulb thermometer: This is a mercury thermometer which gives the prevailing temperature of air at 4' 3'‘ to 4' 6'‘ height. It is required to calculate relative humidity and vapour pressure.
4. Wet bulb thermometer: This is similar to dry bulb thermometer but the bulb of thermometer acts as a evaporat-ing surface. It is used for calculating dew point, relative humidity and vapour pressure.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 22
5. Thermograph: This instrument continuously records air temperature with passage of time (Fig.6). This is not a accurate instrument but its importance lies in its automatic recording of air temperature.
6. Infra-red thermometer: This is a sophisticated instrument used for measuring instant temperature. It contains thermocouple and thermistors upto accuracy 0.1 °C and response time less than one second. It is very accurate, rapid, portable and without any contact, temperature is obtained.
Diurnal temperature variation and its measurement
From sunrise until 2 to 4 pm when the energy supplied by incoming solar radiation is greater than that is being lost by earth in the form of long wave radiations, the air temperature rises. From 2 to 4 pm, the loss of that energy from the earth is greater than the energy received from the sun, the air temperature starts falling and reaches its lowest value just before sunrise ( about 4 - 6 am).
Maximum air temperature :
The maximum temperature attained by air during the day is measured by a thermometer called maximum thermometer. It is a mercury-in-glass thermometer with a constriction in the bore below the lowest graduation. It allows the mercury to be forced through with rising temperature but prevent it being drawn back with falling tempera-tures, provided the thermometer is kept at an angle of 10° from the horizontal with the bulb downwards. It allows the mercury in one way as the constriction acts as a valve. The observer resets the thermometer after reading by holding it firmly in hand by the remote end from the bulb and swinging it briskly downwards. The range of maximum thermom-eter graduation is from -20°C to 55°C.
Minimum thermometer :
The minimum temperature attained by air during the day is measured by using a thermometer called minimum thermometer. Minimum thermometer is a alcohol thermometer. Alcohol is sensitive for lower temperature than mercury. Within the liquid, there is a very light dumb bell shaped glass index which moves freely within the spirit but not readily emerge from the liquid due to surface tension. The thermometer is tilted slightly so that bulb end is upward, the glass index slides along the tube until it reaches the meniscus. But when temperature rises, it remains stationary while the liquid moves ahead in the column. The range of minimum thermometer graduation is from -40°C to 50°C.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 23
Fig. 5: Maximum and minimum thermometers
Recording and setting of air temperature thermometers
Procedure :
1. Open the door of Stevenson screen
2. Note the reading of maximum and minimum thermometers in sequence to the accuracy of 0.1°C and verify for the correctness of the observation
3. While taking reading of maximum thermometer, watch carefully first the mercury column and shining line and record the point.
4 If scale is not clearly visible then any black material can be rubbed on the scale and then wipe so that black material fills the marking on the scale. This will enable easy reading of the scale.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 24
5 To avoid error due to parallax for which your eye and the mercury level should be exactly in one line.
While recording the minimum thermometer, the set reading of the earlier day is first seen in the pocket register and then the end of the glass index farthest from the bulb is read.
1. The reading of the maximum thermometer should be at least as high or higher than any of the dry bulb tempera-ture readings taken since the previous setting.
2. The reading of the minimum thermometer should be as low or lower than any dry bulb reading taken at or since the previous setting.
Fig. 6: Thermograph
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 25
Setting of maximum thermometer :
1. After morning reading, it should be set.
2. For setting, grip firmly the thermometer and swing vigorously up and down in a semicircle.
3. Take care that it does not slip out of hand.
4. No jerks should be given.
5. After setting, the reading should be equal to that of dry bulb thermometer.
Setting of minimum thermometer :
1. After afternoon reading, it should be set.
2. For setting thermometer should be gently tilted keeping bulb with the meniscus.
3. The end of the index farthest from the bulb then indicates the minimum temperature of the air a t that moment.
4. Care should be taken to avoid the thermometers to protect from the direct sun light.
5. When replacing the thermometer in the screen, incline the instrument slightly by about 2° with the bulb end uppermost.
6. Place the other end in the support firm.
7. Then gently lower the bulb making sure that the index does not slide backwards the bulb.
Self recording of temperature
Continuous air temperature is recorded by bimetallic thermograph that work on the principle of expanding of two different metals in length at different rates with the temperature variation. The outer strip of iron - nickle alloy having negative coefficient of expansion much less than the inner strip of brass and the different expansion is recoded at clock drum by pointer. The bimetallic thermograph is kept in big Stevenson screen.
Stevenson screen: To give a representative reading, the thermometers are protected from direct sunlight, sky, earth and surrounding objectives with a adequate ventilation. For this purpose thermometers are kept in Stevenson screen. The objective of the screen is to shield the thermometers from radiation from the sun , ground and neighboring objects and from losing heat by radiation at night. The screen also protects the thermometer from precipitation while at ther same time allowing free circulation of air.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 26
Observations: Record temperature from different thermometers.
______
Date Time Temperature T es t r eadi ng at the Time of setting Dry Maxi- Mini- Dry Maxi- Mini- bulb mum mum bulb mum mu m ( °C) ( °C) ( °C) ( °C) ( °C) ( °C) ______
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 27
Dated______Exercise No. 8
Measurement of soil temperature
The surface of the earth gets heated up during the day and gets cooled during the night causing diurnal changes in the top layers of the soil. The crops have their root systems in these layers and extract plant nutrients and water from these layers of the soil, since the heat regimes of these layers are governed by the soil temperatures. Germination of crops are also affected by soil temperature. Therefore, soil temperature becomes extremely important. The movement of moisture in vapour form is mostly gover ned by temperature gradient in soil. The condensation of water vapour in the air in the form of dew or frost also occurs due to the excessive cooling of the earth surface due to emission of long wave radiation by it.
The soil temperature is measured by soil thermometers (Fig.7). These are mercury –in – glass ther-mometers of the enclos ed scale type. There is a bend of 120° angle just above the bulb, the rest of stem being straight, so that when the soil thermometer is installed at a particular depth of the soil, the bulb rests horizontally. The inclination of the stem at 120° also facilitates the reading of the scale. Thes e thermometers have graduation for every degree Celsius and the graduation starts from the the distanc e of 6.5 cm, 17.5 cm and 35 cm from the bulb for the 5,15 and 30 cm depth soil thermometers, respectively. Iron stands with sloping sides at 60° to the ground surface are provided to support the thermometers at the right inclination. In the observatory, so il thermometers are installed at a sight which is sufficiently away from obstructions and is free from water logging during the rainy season. Precaution should be tak en to remove the soil layer by layer and later replace the same in order, during the installation of the soil thermometers. The soil temperature should be read daily at 0700 hrs and 1400 hrs LMT correct to 0.1°C in the order of 5,15 and 30 cm depths.
Procedure
1. Install the thermometers with the help of iron stands as discussed above.
2. Read correct to 0.1°C soil temperatures at 0700 hrs and 1400 hrs LMT in the order of 5,15 and 30 cm depths.
Observation:
Fig. 7: Soil thermometer PRAC TICAL MANU AL ON AGR OMETEOROLOGY 28
Problem 1: What is importance of measuring soil temperature? Draw a sketch of soil
thermometer.
Solution:
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 29
Dated______Exercise No. 9
Measurement of dew point temperature
Dew point temperature is the temperature at which air would become saturated if cooled at constant pressure without addition or removal of water vapour. Thus, the actual vapour pressure is equal to the saturated vapour pressure at the dew point temperature. The closer the dew point temperature to air temperature, the nearer is the air to be saturated condition.The temperature beyond which air can no longer hold moisture . It means the air becomes fully saturated o r dew point of a given mass of air is the temperature at which saturation occurs when the air is cooled at constant pressure without addition or removal of water vapour.It is determined by the amount of water vapour in t he air and is entirely indepe ndent of free air temperature. It is thus a state of saturation when the air is holding maximum amount of water vapour pressure possible at the existing temperature and pressure.
If the air is cooled below the vapour becomes liquid, it is called condens ation. Higher the dew point, higher the moisture content of the air at given temperature. Thus, dew point of humid air will be higher than that of dry air . From the dry bulb and wet bulb temperature readings, the dew point temperature and RH can be obtained by re ference to
Hygrometric tables. If the height of the place of observation is less than 1500 feet , 1000 mb Hygrometric tables are to be used. Dew point temperature and RH corresponding to specified value of dry and wet bulb temperatures are given in the abo ve mentioned Hygrometric tables at interval of 0.2 °C. While using the tables , interpolation to the nearest 0.1 C has to be don e. The following example would illustrate the procedure:
Dry bulb temperature = 34.5 °C, Wet bulb temperature = 29.7 °C
The values from tables are as below:
Wet bulb
29.6 °C 29.8 °C
Dry bulb 34.4 °C 27.9 28.2
34.6 °C 27.8 28.1
Interpolating for dry bulb temperature, we get
Wet bulb
29.6 °C 29.8 °C
Dry bulb 34.5 °C 27.9 28.1
So final interpolated value of dew point temperature is 28 °C for the wet bulb temperature of 29.7 °C . Dew point can also be calculated by the formula:
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 30
T =T – G (T – T ) 1 1 2 Where, T = dew point temperature, T1 = Dry bulb temperature, T2 = wet bulb temperature and G = Glashier factor(Its value depends upon room temperature, given in Table 9.1)
Example: If the dry and wet bulb thermometers show the reading of 20°C and 15°C respectively, then find out the dew point temperature and relative humidity.
Solution:
T =T1 – G (T1 – T2)
T1 = 20, T2 = 15 , G in table at 20°C = 1.79 T = 20 – 1.79(20 -15)
= 20 – 8.95 = 11.05°C
Saturated vapour pressure from Raino table (Table 9.2) at dew point temperature(11.05°C) is 9.87 mm and at room temperature( 20°C) is 17.51 mm
RH = f/F x100 or 9.87/17.51 x 100 = 56.3%
Table 9.1 : Glashier factor for dry bulb temperature
Dry bulb temp °C Glashier factor Dry bulb temp °C Glashier factor 4 7.82 21 1.77 5 7.28 22 1.75 6 6.62 23 1.73 7 5.77 24 1.72 8 4.92 25 1.70 9 4.04 26 1.69 10 2.06 27 1.68 11 2.02 28 1.67 12 1.99 29 1.66 13 1.94 30 1.65 14 1.92 31 1.64 15 1.90 32 1.63 16 1.87 33 1.62 17 1.85 34 1.61 18 1.83 35 1.60 19 1.81 36 1.59 20 1.79
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 31
Problem 1: If the dry and wet bulb thermometers show the reading of 23°C and 18 °C,
respectively, find out the dew point temperature and relative humidity.
Solution:
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 32
Dated______Exercise No.10
Determination of vapour pressure and calculation of relative humidity
The important measures of humidity are vapour pressure, relative humidity and dew point temperature. The pressure of air is the total weight of all the gases including water vapour in small proportions. Since water vapour also contributes to this air pressure, the partial pressure due to water vapour alone is called vapour pressure. It is expressed in millibars or millimeters of Hg. Water evaporates into water vapour. As more and more water is evapo -rated, amount of water vapour increases in air. However, at any particular temperature, there is a maximum capacity for water vapour that air can hold.
The pressure exerted by water vapour under such a saturated condition is called saturated water vapour pressure. SVP increases with increasing temperature. The pressure exerted by water vapour actually present in air is called the actual vapour pressure(AVP) of air. The ratio of actual water vapour pressure and saturation vapour pressure under fixed condition of temperature is called relative humidity.
Measurement of Relative humidity
Water in its different forms is very important for all activities of the life. Growth and sustenance of life heavily depends on it. The relative humidity, is a measure of water vapour in the air. The instrument used for measuring the water vapour or relative humidity content of the atmosphere are called hygrometers. The two main type of the instruments used for measuring the relative humidity of the air near the earth‘s surface are
(a) Combination of dry and wet bulb thermometers (also called psychrometer) and (b) Hair hygrometers
Dry bulb thermometer: This is an ordinary mercury-in-glass thermometer graduated from -35 °C to 55 °C. This has a capillary stem of which one end is a bulb containing mercury and other end is sealed after removing air from the same. The stem is graduated for reading the value of temperature. Mercury level in the stem changes with the changes in air temperature denoting the air temperature.
Wet bulb thermometer: This is an ordinary mercury -in-glass thermometer graduated from -35 °C to 55 °C as like dry bulb thermometer whose bulb is wrapped by a piece of muslin cloth just sufficient to cover the bulb and is looped by cotton thread
(Cruex thread) that remains the bulb in wet conditions so it is called wet bulb thermometer. When water evaporates from the wet surface, the latent heat requirement is drawn from the bulb of the thermometer and so the mercury column comes down indicating a reduction of temperature. Cooling causes the temperature difference in dry and wet bulb thermometers that is used to calculate relative humidity by using relative humidity tables. Once the temperature of the dry and wet bulb thermometers a re obtained, hygrometric tables are used to determine the dew point temperature and relative humidity. The table appropriate for ventilation (wind speed around the thermometer bulb) should be used. When the wet bulb temperature is below the freezing point, alternate tables should be used depending on whether the wet bulb is covered with ice or coated with super-cooled water. Calculations:
Td and Tw are the dry bulb and wet bulb temperatures respectively, in degree celsius , then the actual vapour pressure
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 33
(AVP) in the air is given by:
AVP =e = EW – ½(Td-Tw)
Where, Ew = saturation vapour pressure in mm of Hg (at wet bulb
temperature Tw)
EW can be obtained from hygrometric tables against Tw.
Td = dry bulb temperature
Tw = wet bulb temperature
(Td – Tw) = wet bulb depression
Then RH (%) is given by
RH(%) = e/E x 100
Where, E is the saturation vapour pressure at the dry bulb temperature Td (from hygrometric tables), e is actual vapour pressure‘ e, E and Tw are in mm Hg and can be converted into millibars or pascals VPD = SVP – AVP
Example: Let, Td = 27.4°C and Tw = 23. 0°C
SVP at Tw = 21.1 mm of Hg( from table)
E = SVP at Td =27.4 mm Hg ( from table)
Then, AVP (mm of Hg) = e =Ew – ½(Td –Tw)
21.1-1/2 (27.4 – 23.0)
= 21.1 – ½ x 4.4
= 21.1 – 2.2 =18.9 mm of Hg VPD = E –e
= 27.4 – 18.9
= 8.5 mm of Hg
RH= e/E x 100
= 18.9/27.4 x 100 = 68.9 %.
With Stevenson’s Screen wet and dry bulb thermometers
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 34
For deriving the vapour pressure in the India Meteorological Department the formula is as follows: For temperatures of Wet Bulb above 0° C
X = f‘ - 0.480 (T-T‘) X p 610 - T‘ where X = pressure of vapour present in the air (AVP) f‘ saturation vapour pressure at temperature T‘ °C of the wet bulb. T Temperature of the Dry bulb in °C.
T‘ Temperature of the Wet bulb in
°C. p Pressure of the air. % Relative Humidity U = x/f X 1 00 where x = Pressure of vapour present in the air (AVP). f = Saturation vapour pressure at temperatue T ― C of the Dry bulb. u = Humidity in percentage.
By using Saturation Vapour Pressure in mn of Hg table, we can substitute the relevant values and find the vapour pressure present in the air (Actual Vapour pressure) and percentage humidity.
Example: Given T = 20 °C T = 30 °C p = 713 mm of Hg. (for Hyderabad, Altitude of 545 m AMSL) Note from svp table
Saturation vapour pressure at wet bulb temperature (f) = 17. 54 Saturation vapour pressure at Dry bulb temperature (f) = 31.83
Actual Vapour Pressure ‗x‘ =17.54 ((-0.480 (30-20)) x (713))/610-20
= 17.54 - 5.8
= 11.74 mm
Dew point temperature (T d) = 13.7 °C (By definition, look for temperature corresponding to AVP value in SVP table) Relative Humidity % = 11.74/131.83 = 36.9 %
Vapour Pressure Deficit = SVP (at T) - AVP
= 31.83 - 11.74
= 20.09 mm Hair hygrograph
It record the continuous changes in relative humidity on graph paper during the hours of the day. When a hygrometer is transformed into a self recording device it is called as a hygrograph (Fig. 8). This is used to record the relative humidity of the air continuously.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 35
Table9.2: Saturation vapour pressure over water in mm of Hg.for temperatures 0 to 50 ° C (Raino table) ______Temp .0 .1 .2 .3 .4 .5 .6 .7 .8 .9 _°C______0 4.58 4.61 4.65 4.68 4.72 4.75 4.79 4. 82 4.86 4.89 1 4.93 4.96 5.00 5.03 5.07 5.11 5.14 5. 18 5.22 5.25 2 5.29 5.33 5.37 5.41 5.45 5.49 5.52 5. 56 5.60 5.64 3 5.68 5.73 5.77 5.81 5.85 5. 89 5.93 5. 97 6.02 6.06 4 6.10 6.14 6.19 6.23 6.27 6.32 6.36 6. 41 6.45 6.50 5 6.54 6.59 6.64 6.88 6.73 6. 78 6.82 6. 87 6.92 6.97 6 7.01 7.06 7.11 7.16 7.21 7.26 7.31 7. 36 7.41 7.46 7 7.51 7.57 7.62 7.67 7.72 7.78 7.83 7. 88 7.94 7.99 8 8.05 8.10 8 .16 8.21 8.27 8.32 8.38 8. 44 8.49 8.55 9 8.61 8.67 8.73 8.79 8.85 8. 91 8.97 9. 03 9.09 9.15 10 9.21 9.27 9.33 9.40 9.46 9. 52 9.59 9. 65 9.71 9.78 11 9.84 9.91 9.98 10.04 10.11 10.18 10.24 10.31 10.38 10.45 12 10.52 10.59 10.66 10.73 10.80 10.87 10.94 11.01 11.09 11.16 13 11.23 11.31 11.38 11.45 11.53 11.61 11.68 11.76 11.83 11.91 14 11.99 12.07 12.14 12.22 12.30 12.38 12.46 12.55 12.63 12.71 15 12.79 12.87 12.96 13.04 13.12 13.21 13.29 13.38 13.46 13.55 16 13.64 13.72 13.81 13.90 13.99 14.08 14.17 14.26 14.35 14.44 17 14.53 14.62 14.72 14.81 14.91 15.00 15.10 15.19 15.29 15.38 18 15.48 15.58 15.68 15.78 15.87 15.97 16.08 16.18 16.28 16.38 19 16.48 16.58 16.69 16.79 16.90 17.00 17.11 17.22 17.32 17.43 20 17.54 17.65 17.76 17.87 17.98 18.09 18.20 18.31 18.42 18.54 21 18.66 18.77 18.88 19.00 19.12 19.24 19.35 19.47 19.59 19.71 22 19.83 19.95 20.07 20.20 20. 32 20.44 20.57 20.70 20.82 20.95 23 21.07 21.20 21.33 21.46 21. 59 21.72 21.85 21.98 22.12 22.25 24 22.38 22.52 22.65 22.79 22. 92 23.06 23.20 23.34 23.48 23.62 25 23.76 23.90 24.05 24.19 24. 33 24.48 24.63 24.77 24.92 25.07 26 25.22 25.37 25.52 25.67 25. 82 25.97 26.13 26.28 26.44 26.59 27 26.76 26.90 27.06 27.22 27. 38 27.54 27.70 27.87 28.03 28.19 28 28.36 28.52 28.69 28.86 29. 03 29.19 29.30 29.54 29.71 29.88 29 30.05 30.23 30.40 30.58 30. 75 30.93 31.11 31.29 31.47 31.65 30 31.83 32.02 32.20 32.38 32. 57 32.76 32.95 33.13 33.32 33.51 31 33.71 33.90 34.09 34.29 34. 48 34.68 34.88 35.07 35.27 35.47 32 35.67 35.88 36.08 36.28 36. 49 36.69 36.90 37.11 37.32 37.53 33 37.74 37.95 38.17 38.38 38. 60 38.81 39.03 39.25 39.47 39.69 34 39.91 40.13 30.36 40.58 40. 81 41.04 41.26 41.49 41.72 41.95 35 42.19 42.42 42.66 42.90 43. 14 43.38 43.62 43.86 44.10 44.34 36 44.58 44.83 45.07 45.32 45. 57 45.81 46.06 46.32 46.57 46.82 37 47.08 47.34 47.59 47.85 48. 11 48. 38 48.64 48.91 49.17 49.44 38 49.71 49.97 50.24 50.52 50. 79 51.07 51.35 51.62 51.90 52.18 39 52.46 52.74 53.02 53.31 53. 59 53.88 54.17 54.46 54.75 55.04 40 55.34 55.63 55.93 56.23 56. 53 56.83 57.14 57.44 57.74 58.05 41 58.36 58.67 58.98 59.29 59. 60 59.92 60.24 60.56 60.88 61.20 42 61.52 61.85 62.17 62.50 62. 82 63.15 63.49 63.82 64.15 64.49 43 64.82 65.16 65.50 65.84 66. 19 66.53 66.88 67.23 67.58 67.93 44 68.28 68.63 68.99 69.35 69. 71 70-08 70.44 70.80 71.17 71.53 45 71.90 72.27 72.65 73.02 73. 39 73.76 74.14 74.52 74.90 75.29 46 75.67 76.06 76.45 76.84 77. 23 77.63 78.02 78.42 78.82 79.22 47 79.62 80.03 80.43 80.84 81. 25 81.66 82.07 82.48 82.90 83.32 48 83.74 84.16 84.59 85.01 85. 44 85.87 86.30 86.74 87.17 87.61 49 88.05 88.49 88.93 89.38 89. 83 90.27 90.72 91.17 91.62 92.08 50 92.50 93.0 93.4 93.9 94. 4 94. 8 95.3 95. 8 96.3 96.7
______
Procedure
1. A band of human hair is fixed on the levers and any slight increase in the volume is transmitted to the pen arm.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 36
2. The pen arm is self-inked and works on levers.
Fig. 8: Hair hygrograph
3. A change in the length of hair is proportional to the log change of relative humidity
4. A calibrated chart is wrapped around a rotating drum. This completes one rotation In 24 hours, and works on clock mechanism.
5. The X-axis represents time and Y-axis, the relative humidity.
6. The chart has to be replaced everyday.
7. The dust on the hair should be cleaned and washed regularly. The hair should not be touched with hand.
8. This instrument should be expos ed in double Stevenson screen. The screen should be located in a place where
the air is not polluted with smoke, dust, oil and amrnonia releasing industries in the immediate surroundings.
Problem: If, Td= 20 °C and Tw = 16 °C, then calculate actual vapour pressure,VPD and relative humidity.
Solution:
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 37
Dated______Exercise No.11
Measurement of atmospheric pressure
The weight exerted by a column of air on unit surface of the earth is known as atmospheric pressure. This can be measured by an instrument called barometer (Fig. 9). There are two types of barometers, viz., 1. Mercury barometers 2. Aneroid barometers
Of these two, the most accurate instrument is the mercurial barometer. This is used as standard for calibrating the others. The following instruments are used to measure the atmospheric pressure.
1. Mercurial barometers:
There are two types of mercurial barometers. A) Fortin‘s barometer B) Kew pattern barometer Fortin’s barometer:
Principle: Balancing of column of air against a column of mercury in a sealed glass tube. The height of the mercury column is proportional to the pressure.
The Fortin‘s barometer is a familiar sight at most of the micro-meteorological laboratories and is an accurate one. It consists of a glass tube of uniform cross section and length, which is closed at one end. It is about one metre in length, filled with mercury and then inverted with it‘s lower end open into a movable cistern of mercury. The cistern vessel contains mercury with a flexible leather bag and screw at it‘s bottom. There are two scales on two sides of the tube, one in centimetres and the other in inches. For accurate readings vernier calipers is also attached.
The mercury column in the tube is supported by the pressure of the air on the surface of the mercury in the cistern. Procedure
1. To take the pressure reading, the height of mercury column is measured on main scale and then vernier scale is read.
2. To read the Fortin‘s barometer:
(a) Read the attached thermometer to the nearest degree before the time specified (or barometer observation.
b) Gently tap the cistern and tube of the instrument 2 to 3 times with the fingers.
c) Raise the surface of the mercury in the cistern by screwing up the plunger at the base until the tip of the ivory point just touches it‘s image in the clear mercury surface.
d) Set the lower edge of the vernier tangent to the top of miniscus.
e) Read the scale and the vernier. f) Check the reading by making a fresh setting.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 38
Barograph:
The sensitive element in this device is an aneroid capsule which consists of a closed circular vacuum box or boxes placed one above the other (Fig. 10). The box is made of an alloy of silver plated beryllium copper. As the atmospheric pressure rises or falls the walls of the box collapse or distant proportional to the impressed pressure changes. The motion is communicated to a lever system connected to a rotating drum on which recording is made. This is an instrument used to record the atmospheric pressure continuously. Units of measurements:
I. Height of mercury column is measured in inches, centimetres or millimetres. 2. The S.I. unit for pressure is Pascal and this is equal to a force of one newton per sq. m. One atmospheric pressure = 29-92 inches or 76 cm or 760 mm of Hg = 1013.250 millibar
= 101.325 kilopascal (kPa) 14.7 lbs / inch2
= 1.014 X 106 dynes / cm2 Problem 1: Draw the neat sketch of Fortin’s barometer
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 39
Fig. 9: Mercurial barometers
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 40
Fig. 10: Barometer and barograph
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 41
Dated______Exercise No. 12
Measurement of wind direction and speed
Wind is the air in horizontal motion c aused due to differences in atmospheric pressure. Wind has to be speci-fied by its direction and speed. The movement of wind is almost horizontal and the vertical component is very small. The air in horizontal motion near the surface of the earth is called surface wind. Since it relates motion, it is associated with direction and speed. The influence of the underlying terrain condition sharply diminishes with height and observa-tions indicate that wind at a height of 10 m above the ground is fairly representative of general surface wind for meteorological purposes. However, for agrometeorological purpose, the obs ervations are made at a height of 10 feet which is a representative height of crop canopy.
Wind Instruments :
Anemoscope: This records the direction of the wind continuously.
Aerovane : This measures the velocity and direction of the wind simultaneously.
Wind vane : This is used in observatories to find the wind direction.
Anemometer: It measures speed of the wind.
Wind vane
The common instrument to determine wind direction is wind vane (Fig.12). This instrument indicates the direc -tion from which the wind blows. It is a balanced lever which turns freely about a vertical axis. One end of the lever exposes a broad surface to the wind, while the other end is narrow and points to the direction from which the wind blows.Wind direction is the direction from which the wind is approaching the observer. It is expressed in degrees, measured clockwise from geographical north. The codes and 16 point wind direction is expressed in Table 12.1.
Wind direction known by wind vane is of the shape of a penchant with an arrow head installed on a metal frame free to rotate in the horizontal plane with the direction of the arrow pointing towards the wind direction of the wind. Below the indicator, a frame indicating 4 (north, east, south and west) or 8 or 16 points of the compass is fixed to the frame to facilitate the estimation of the direction.
Reading of wind vane at the time of observation
The direction is read by noting the direction to which the arrow head points. Wind vane is read by standing exactly in the line of the arrow of the instrument. Since we record 16 points of the directions and distance between two direction is less, so care is required. The nearest possible is recorded for alphabetical recording and the nearest 10 degree is recorded for wind direction in degrees. It may be noted that we report the windward side i.e. from where the wind is coming. Leeward side i.e. where the win d is going is never reported. In case wind is static, wind may be calm and in that case direction reported is 00.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 42
For example if the arrow head is pointing towards the middle of the region between North(N) and North-west(NW), the direction should be North- North-west
Measurement of wind speed
Wind speed is measured by Robinson Cup counter anemometer which has a mech anical arrange-ment for converting the rotational motion into linear motion in kmph which is converted into average wind speed in kmph by dividing 24 during last 24 hour (Fig.11). This instrument consists of four hemispherical cups fixed at the end of metal arms from a central point. The cup wheel is pivoted at the centre to a vertical spindle passing through a brass tube attached to the anemometer box. The cup is set in motion due to the pressure differences occurring between the two faces of the cup. The vertical cup is connected to a mechanical counter through a gear system from which the number of rotations made by the cups in a chosen interval of time can be counted. The counter is directly calibrated in kilometers.
To determine wind speed at the time of observation, the two successive readings of the anemometer should be taken at an interval of 3 minutes. The difference between the two readings when multiplied by 20 will give the wind speed in kilometer per hour, if the anemometer is calibrated in kil ometers.
For example: If the first anemometer is 4005.6 and the second reading is 4006.8, the wind speed will be 20 x ( 4006.8-4005.6) = 20 x 1.2 = 24 kmph.
Calculation of mean daily speed
The mean daily wind speed is calculated at the one hour of observation viz.,7 hrs local mean time at the agromet observatory. The anemometer reading of 7 hrs LMT reading of the previous day is subtracted from 7 hrs. LMT reading of the current day wind speed in kmph can be obtained. Thus, mean daily wind speed on a particular date corresponds to the 24 hour period ending at 0700 hrs LMT of that date.
Example: Anemometer reading at I hour of 20.7.2011 ——— 6754.9
Anemometer reading at I hour of 19.7.2011 ——— 6475.4
Difference ______
279.5 Mean daily wind speed on 20.7.2011 is given by 279.5/24 = 11.64 kmph
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 43
Fig. 11: Robinson’s cup anemometer
Fig. 12: Wind vane
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 44
Problem1:
Calcutate the wind speed at the time of the observation on 4.6.2011 and the mean daily wind speed on 5.6.2011 with the following data from cup anemometer:
Date Time Readi ng
4.6.2011 0710 hrs IST* 3345.9
0713 hrs IST. 3346.4
1.6.2011 0710 hrs. IST. 3575.5
*At the station concerned, 0710 hrs IST is equivalent to 0700 hrs LMT
1. Draw levelled diagram of wind vane and cup anemometer.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 45
Dated______Exercise No.13
Measurement of rainfall
Precipitation is defined as the particles of liquid water or ice dropping from the clouds and reaching on the ground in the form of rain, drizzle, snow hail, etc. and is measured as the depth or thickness of layer on the surface of the earth if there were no loss by evaporation. It is measured in mm. The simplest and most common method of measuring rainfall is to use a raingauge. These are of two types:
(a) Non-recording raingauge : Simons rain gauge or ordinary raingauge and FRP ( Fibre Glass Reinforced Polyster) rain gauge
(b) Self recording raingauge : Recording weighing rain gauge, Tipping bucket rain gauge and natural siphoning float type self recording rain gauge
Non-recording raingauge : Symon’s simple rain gauge or ordinary rain gauge
FRP rain gauge
Symon‘s rain gauge was the standard rain gauge of India Meteorology Department. Since 1969, on the recommen-dation of WMO , FRP rain gauges are used (Fig. 13). Its essential parts are:
2 1. Collector ( funnel) of circular shape with brass rim of 200 cm area. The collector has a deepest funnel and the complete rain gauge is tapered slightly with narrower portion at the top.
2. Cylindrical body on which funnel is supported
3. Receiver bottle with narrow neck made of polythene and rain falling into funnel gets collected in the bottle.
4. Base which is partially sunk and fixed in the ground and supports the cylindrical body
5. Measuring cylinder whose graduation is consistent with the diameter of funnel. The collected rain water in the bottle is measured with the appropriate specific measuring glass. The measure glass is graduated in tenths of millimeter. The capacity of measure glass is 20 mm of rain.
Installation
The rain gauge should be fixed on a concrete foundation , not upon a slope and never on a wall or roof. It should not be installed on a ground that has slope on the side of the prevailing wind since in that case quite a few rain drops will be carried away by the winds. A foundation of 60 x
60cm x 60 cm should be provided to rain gauge. The base of the rain gauge is cemented into this foundation so that the rim of the gauge is exactly 30 cm above the ground level.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 46
Fig. 13: Symon’s rain gauge and its body parts
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 47
Fig. 14: Self recording rain gauge
Time of observation
Daily at 0830 hrs IST giving rains of last 24 hrs. The rainfall is measured in mm and tenth of mm. For no rains, record 0.0 and for rainfall less than 0.1 mm, record ‗t‘ meaning trace.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 48
Procedure
1. Remove the funnel by rotating it.
2. Take out receiver containing rain water and measure it by measuring glass upto the first decimal.
3. If water is more, measure again by and count the full cylinders.
4. Check the value of non-recording raingauge by self recording one.
Self recording raingauge
This provide continuos record of the rainfall. This also determine the time of onset, cessation and intensity of the rain (Fig.14). These are three type
1. Float type
2. Tipping bucket type and
3. Weighing type
In India, float type self recording raingauge is used. In this type instrument rain is led in float chamber containing a light hollow float. The principle is that when water rises the float rises with time and this plotted on a graph wraped on clock drum of the self recording rain gauge.
Measurement of snowfall and hail
Snowfall is measured by snow gauges. These gauges are of 127 mm or 200 mm rim diameter and are
mounted on iron stands at height above the average snow level of the location. A known quantity of warm water is poured into the receiver in which snowfall has been collected. After melting of the snow, the total amount of water is measured. The
actual amount of snowfall is obtained by subtracting from this, the amount of warm water added. During hail also, water and hail stones collected in the receiver are measured in a similar fashion.
Calculation of rainfall amount from crop field
As we know, rainfall is measured in depth units. So if the area of the crop field (A) in sq cm and the rainfall amount ( R) in
cm are known, the volume of water (V) can be calculated in cu m or cu cm from following formula:
V = A x R
Since 1000 cubic cm is 1.0 litre and 1000 litres is 1.0 kilo litre or 1 cubic m.
For example, if the area of crop is 1 ha and rainfall recorded is 50 mm, the volume of rain water received in that crop field can be calculated as follows:
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 49
V = 1 x10000 x 10000 x 5 cu. cm
= 500000000 cu.cm
= 500000 litres or 500 kilo litres
Or
3 V = 1 x 10000 x 5 m
————————— = 500 cu. m or 500000 litres or 500 kilo litres
100
Problems
1. Draw diagram of different parts of a rain gauge.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 50
2. What is the quantity of rain water received in kilo litres by a cropping land of 1.5 ha from a rain fall spell of 85 mm?
3. How is snowfall is measured in hilly areas?
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 51
Dated______Exercise No.14
Measurement and determination of evaporation
Evaporation is a physical process in which the amount of water is converted into water vapor under environmental conditions. Water is lost from the earth‘s surface from water bodies, moist surface and through plants. Evaporation is one of the important aspect of the hydrological cycle by which water from the earth‘s surface is transferred to the atmosphere in the form of water vapour. When the soil is saturated, evaporation may be essentially equal to that from free water surface. Dry soils may show lesser evaporation. Evaporation is measured in depth units (mm/day) just like rainfall.
Following factors affect the rate of evaporation:
(a) Total radiation, solar and terrestrial
(b) Temperature, both the air and evaporating surface
(c) Wind speed at surface
(d) Relative humidity of the air at surface
(e) Atmospheric pressure and vapour deficit
(f) Nature of surface and surrounding including impurities and vegetation
(g) Amount of moisture in the surface available for evaporation
Measurement of Evaporation
Evaporimeters can be categorized into 3 main classes. They are: Float pan, Below ground sunken pan and above ground pan
1. Floating pans : These pans are used to measure water loss from water bodies
2. Pitche evaporimeter or Atmometer: Atmometer is hung at stevensson screen. It consist of a glass tube. The open end of the tube is covered by drier paper which is placed in position by metallic clip. The reading of atmometer is higher than USWB open pan evaporimeter.
3. Sunken pan evaporimeter : It is a below ground evaporimeter which gives good values of evaporation very close to potential evatranspiration.
4. USWB Class A open pan evaporimeter: This evaporimeter is most commonly used in observatories for measuring evaporation by IMD.
For evaporation measurement, generally a pan evaporimeter adopted by United States Weather Bureau pan evaporimeter (Class A pan evaporimeter) is used (Fig. 15). Observations are made by measuring the amount of water evaporated from an open pan. Measurements are carried out by adjusting the water level in the pan to fixed point. This is done by adding or removing a fixed amount of water.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 52
Fig. 15: U.S.W.B. class-A open pan evaporimeter
Description of pan evaporimeter
1. The pan consists of a cylinder body made of 20 gauge copper sheet tinned inside and painted white outside.
2. It is 25 cm deep and 120 cm in diameter (inside dimensions)
3. The pan is normally unpainted and filled with water to 5 cm below the rim.
4. The pan should be 3 to 5 cm above the ground for air circulation. On wooden stand of 4 feet x 4 feet x 4inch. This ensures the pan not to be in direct contact with soil. Water level is measured either by a hook gauge or by fixed point. The hook gauge consists of a movable scale with vernier fixed to the hook.
5. A stilling well, about 10 cm across and about 30 cm deep is situated within the pan.
6. Measuring cylinder: A measured volume of water is added with the help of measuring cylinder so that the original level of water to the tip of the reference point is restored. The measuring cylinder is of brass and graduated from 0 to 20 cm from top to bottom. It is fixed on the inner side of the cylinder. The diameter of the measuring cylinder is th th exactly 1/10 of the evaporimeter, therefore, the cross sectional area is 1/100 of the evaporimeter.
Thus one container of 20 cm added to the evaporimeter will raise water level by 200x1/10 = 2 mm in pan.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 53
Procedure
The observation of evaporation should be made daily at 0830 hrs IST. Initially the water is filled upto the fixed point tip. Due to evaporation water level will normally be below the tip of the rod at the time of observation, then add water to the evaporimeter using the measuring cylinder till the water level once again coincides with the tip of the reference point. Hence, the amount of water added, which is equal to the evaporation can be directly measured. For example, if on a particular day three full cylinders of water and 4 cm of water i.e., 64 cm has been added to bring the water level to the reference point, the evaporation can be determined by dividing the amount of water added by 100, since the area of pan is 100 times than that of the base of the measuring cylinder. Thus, evapora-tion during the day is 640mm/100 =6.4 mm
If rainfall has occurred after the previous obs ervation and that rainfall has exceeded evaporation during that period, water has to be removed from the pan till the water level reac hes the reference level indicated by the tip of the rod.
In case the rainfall has occurred during the period but is less than the evaporation, some water has to be added to the pan to bring it to the reference level. But, while calculating the evaporation, not only the water added, but the rainfall that has occurred should be taken into account.
The following example would illustrate these procedures :
i. Water added at 0830 hrs IST on 4.11.2011 to bring the water level to the reference point is 61 cm. Rainfall from 0830 hrs of 3.11.2011 to 0830 hrs on 4.11.2011 is Nil.
Evaporation on 4.11.2011 = 61 cm/100 = 6.1 mm
ii. Water removed at 0830 hrs IST on 10.7.2011 =74 cm. this amounts to a change in water level of 7.4 mm. The rainfall recorded at 0830 hrs of 10.7.2011 hrs IST is 8.4 mm. Had there been no rain, the level would have risen by 8.4 mm . So the evaporation for the day is 8.4 —7.4 = 1.0 mm.
iii. The water added at 0830 hrs IST on 5.6.2011 is 33 cm. This amounts to a change in water level of 3.3 mm.
If the rainfall recorded at 0830 hrs on 5.6.2011 is 1.6 mm, the actual evaporation is equival ent to the sum of the rainfall and the decrease in the rainfall. So the evaporation for the day is 3.3 + 1.6 = 4.9 mm.
Problems:
1. Calculate the pan evaporation From the following data
(a) Water added 65 cm Rainfall for last 24 hrs Nil (b) Water added 21 cm Rainfall for the last 24 hrs 17 mm (c) Water removed 156 cm
Rainfall for the last 24 hrs 20 mm
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 54
Solution
2. Calculate evaporation from pan for the data recorded as below 1
June 2011 – Rainfall = 0 mm, water added in pan =30 cm
2 June 2011 – Rainfall = 4.5 mm, water added in pan = 25 cm 3
June 2011 – Rainfall =17.8 mm, water removed from pan =
135 cm Solution:
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 55
Dated______Exercise No.15
Agro meteorological data management system
All the aspects relating to agrometeorogical data viz., observation, transmission, storage and analysis are important for their fruitful utilization in operational.
Data recording
Initial recording of observations : An observer takes data as read from the instruments or as observed visually in a register called ―Pocket‖ register in appropriate columns. The correction factor to be applied are recorded once for that month.
Daily register: In this register, only the final observations arrived at from raw (or observed) data after applying required corrections are recorded. This should be done soon after the observations have been completed. It is a permanent record. In this register month and the year are mentioned on the top of each page. Daily register should be in duplicate one for users and other for record.
st th Weekly register: There are 52 meteorological weeks in a year commencing from I January, Duration of 9 week in nd the leap year and 52 week are of 8 days each. At the end of each meteorological week, the weekly means of the data calc ulated by summing of the values for the week and dividing by the number of the observations in respect of the each parameter and are recorded in the respecti ve column of the weekly register which has columns for meteorologi-c al week number, period covered and year for each of the parameter for morning and evening.
Monthly register: This is to be maintained to refer the monthly averages of the data. As soon as the month is over, the daily data of a particular parameter are added together and divided by number of the days in the month. The work should be completed within 3 to 5 days after completion of the month.
For reporting the data following forms are used:
Crop Weather Station (CWS) 1 form: This form is a copy of the daily register and is filled every day. This needs to be sent every month to the IMD. Pune, if the observatory is registered with them. This contains daily information of 4 to 5 meteorological weeks. The morning and evening observations are recorded separately under the first hour(I) and second hour (II); respectively, wherever the observation is taken only in the morning, then it is recorded only under first hour (I). In CWS 1 form, the observations are rec orded as they are required to be reported e.g. the maximum temperature is read twice a day but reported for only first hour (I) and hence there is a place for only first hour in the form. Meteorological weekwise totals and means are worked out and written at appropriate places.
Crop Weather Station CWS 2 form: This is used for reporting the microclimatic observations. These observations are recorded at the micro-climatic post. Wherever it is available in the observatory. The dry bulb and wet bulb tem -peratures. For first and second hour are recorded separately. The observations are recorded for different heights starting form surface to 12 feet height. The observations on vapour pressure and relative humidity derived from the dry
PRAC TICAL MANU AL ON AGRO METEOROLOGY 56 bulb and wet bulb temperatures are also recorded for first and second hour. However, is not mandatory for all the observatories.
Quality control in the data:
Accuracy of data is very important. Following checks may be found useful for the purpose:
(a) Examine the previous reading of anemometer before recording the current reading.
(b) When wind instruments indicate calm, observe the movement of shrubs or crops growing nearby to confirm the same.
(c) The reading of wet bulb thermometer has to be lower or at the most equal to the dry bulb thermometer. When humidity is higher the difference is narrower and vice versa. Use distilled water in water can of wet bulb thermom-eter.
(d) The dry bulb and maximum thermometer readings are quite close, even if both the thermometer are not same.
(e) Avoid parallax error while reading.
(f) Minimum temperature recorded in the morning has to be lower than that of evening except in exceptional cases.
(g) Make sure about the compatibility of the rain measuring glass with the installed raingauge. While purchasing the new one, inform the diameter of raingauge.
(h) Do not cast shadow on the soil thermometers while recording readings.
(i) Observe minutely the movement of a bubble at the time when the water level is approaching towards the set level. Check the level of pan frequently especially during rainy season.
(j) Recheck the observations while recording.
Communication, archival and availability of data
Communication of data: Since both speed and accuracy are important in meteorological data communication, there-fore innovations in the field of telecommunication have found ready application in this area. Communication of current weather data is important for forecast preparation on every Tuesday and Friday therefore, it‘s essential that each AAS unit may regularly send real time meteorological data to NCMRWF through phone/Fax/and VSAT.
Monthly data is required for various type of R&D work for this data may send at the end of month in floppy and hard copy through post. This may done on a regular basis.
Problem:Prepare a flow chart of agro-meteorological data management system.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 57
Date______Exercise No.16
Synoptic charts and weather reports
After collection of data with various devices, the data need to be transferred to an appropriate place for analysis. In this regard, it is proper to know a little about synoptic climatology. The term synoptic climatology is applied to investigations of regional weather and circulation types. It is also used to refer any climatological analysis which makes some reference to synoptic weather phenomena. This field is concerned with obtaining an insight into local or regional climates by examining the relations hip of weather elements individually or collectively to atmospheric circulation processes. Synoptic climatology is defined as the description and analysis of the totality of weather at a single place or over a small area, in terms of the properties and motion of the atmosphere over and around the place or area.
There are essentially two stages to a synoptic climatological study: 1. The determination of categories of atmospheric circulation type
2. The assessment of weather elements in relation to these categories.
Besides agricultural meteorological observatories, synoptic weather stations also record weather data such as rainfall, temperature, radiation, low level wind and evaporation etc. The surface observatories collect information on various weather elements and based an these, daily forecasts, warnings and weather reports are prepared by weather forecating centres at Channai, Nagpur, Mumbai, Delhi and Kolkatta. The weather bulletins are being broadcast in regional languages through All India Radio and Television.
Synoptic report: Observed weather conditions are marked in brief coded form as a synopsis of the conditions. Such a brief report on weather conditions is known as synoptic report.
Synoptic chart/weather map: The regular observatories record weather elements at scheduled time and send these readings through a telegram to the main observatory at Pune. They reach Pune within an hour of observation and they are charted on outline map of India, using the international code of signals and abbreviations. These are called synoptic charts or weather maps.
In synoptic charts different weather phenomena and atmospheric characters are marked with different symbols as mentioned below. S.No. Symbols Weather element / character / phenomenon 1. Narrow block items Isobars 2. Numbers at ends of isobars Pressure values in mb 3. Shading Precipitation 4. Arrows Wind direction 5. Feathers in the arrows Wind velocity 6. Small circles with shading Amount of clouds
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 58
Besides, different symbols are also used for recording weather phenomena, in relevant columns of the pocket register and the Monthly Meteorological Register by the observer.
The duties of the observer: The routine duties of the observer include:
1. To make regular and careful observations and to note the general character of the weather and record in the pocket register.
2. To prepare and dispatch the weather telegram as per the instructions to the
different forecasting centers, immediately after the observations are taken.
3. To send, heavy rainfall telegrams to the various offices on warning list.
4. To prepare and post monthly meteorological and pocket registers, for each month to the controlling meteorological office.
5. To keep the instruments clean and maintain them property.
After sending the data by the observer, it should be decoded and the we ather observations for each station must be plotted at the appropriate location in a systematic manner following the international station model. Only weather maps in first-class forecasting c enters approach the completeness of the model. Printed maps and maps used for plotting usually have an appropriately numbered circle corresponding to each reporting land station and observations are plotted about this location in the appropriate position regardless of the number of observations shown. It must be, emphasized again that the weather pattern affecting a locality is an integral part of the much larger hemispheric weather pattern. It is necessary to plot a map over a large area. Even if observations are not to be plotted, it is necessary to know the plotting scheme in order to read and interpret weather charts already plotted.
Problem: What are synoptic charts? Also discuss duties of the observer.
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 59
PRAC TICAL MANU AL ON AGR OMETEOROLOGY 60