By Graeme Tepper Micrometeorology Research and Educational Services Title: Weather Essentials for Pesticide Application Revised Edition January 2014

WEATHER ESSENTIALS FOR PESTICIDE APPLICATION

Revised 2014 edition By Graeme Tepper MicroMeteorology Research and Educational Services Title: Weather Essentials for Pesticide Application Revised edition January 2014

GRDC Project Code: TEP00001 – General meteorology for pesticide application booklet

Author: Graeme Tepper MicroMeteorological Research and Educational Services (MRES) email: [email protected]

About the author Graeme Tepper has worked closely with the pesticide spray industry since 2004 and understands the application factors pertinent to drift from both aerial and ground sprayers. He retired from the Bureau of Meteorology (BoM) in 2005 after 34 years. The main focus of his career was on meso- scale and micro-scale meteorology. He researched and published papers on the development of fog and fog forecasting and upstream air mass blocking in stably stratified air masses. The associated dynamics have direct relevance to the atmospheric factors leading to the drift of pesticides. He produced the BoM’s current Manual of Aviation Meteorology (endorsed by the Civil Aviation Safety Authority for pilot training). In 2010, Graeme wrote the article ‘Surface Inversions for Australian Agricultural Regions’ for the Australian Pesticides and Veterinary Medicines Authority (APVMA). The article can be viewed on the APVMA website (www.apvma.gov.au/use_safely/ spray_drift/inversions.php).

First published February 2013 Revised Januarry 2014 ISBN: 978-1-921779-20-6

In submitting this report, the researcher has agreed to the GRDC publishing this material in its edited form.

2 Cover photo by: Bill Gordon, Bill Gordon Consulting

For free copies please contact: Ground Cover Direct Free phone: 1800 11 00 44 Email: [email protected]

or W EA

TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A Ms Maureen Cribb GRDC Publications Manager PO Box 5367 KINGSTON ACT 2604 Phone: 02 6166 4500 Email: [email protected]

Editing: Emma Leonard, AgriKnowHow

Disclaimer

T ION Any recommendations, suggestions or opinions contained in this publication do not Design and production: necessarily represent the policy or views of the Grains Research and Development Coretext, www.coretext.com.au Corporation (GRDC) or its agents. No person should act on the basis of the contents of this publication without first obtaining specific, independent professional advice. The GRDC or its agents will not be liable for any loss, damage, cost or expense incurred or arising by reason of any person using or relying on the information in this publication. Contents 3

Introduction...... 4

Off-target movement of pesticides...... 5 What is likely to drift?...... 5 Common weather guidelines for pesticide application...... 5 Spray when wind is steady and ideally more than 3km/h and up to 15km/h ...... 5 W EA

Avoid spraying in temperatures above 30ºC (see page 16)...... 6 TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A Aim to spray when Delta T is between 2 and 8 and not greater than 10...... 6 Do not spray when inversion conditions exist...... 6 10 steps to minimise spray drift...... 7

1. Atmospheric stability...... 8 Stability ratio...... 8

2. Wind...... 10 Common spray guidelines...... 10 Turbulence...... 10 Synoptic winds...... 11

Local winds...... 11 T ION Measuring local winds...... 11 Wind shear...... 12 Diurnal variation of wind...... 14 Evening and overnight decrease of wind speed and gustiness...... 14 Early morning increase in wind speed and gustiness...... 15

3.Temperature...... 16 Temperature and pesticide application...... 16 Surface to air temperature variations...... 16 Diurnal temperature variations...... 17 Volatilisation – be aware of surface temperature...... 17

4. Evaporation...... 18 Relative humidity...... 18 Delta T...... 18 Relevance for spraying...... 18

5. Surface temperature inversions...... 20 What is a surface temperature inversion?...... 20 Inversion weather...... 20 The hazards of a surface temperature inversion...... 21 When and why does a surface temperature inversion occur?...... 21 1. Radiation inversion – created by radiation cooling...... 21 2. Advection inversion – created by cool or warm air movement...... 22 Transport of airborne pesticides within a surface temperature inversion...... 22 Lifecycle of a surface temperature inversion...... 22 Accumulation of pesticide residues in the atmosphere ...... 23 Recognising a surface temperature inversion...... 23 Visual clues...... 23 Other clues...... 24 Precautions for night spraying...... 24

6. Recording meteorological conditions...... 25 Recording...... 25 Wind...... 25 Temperature and humidity...... 25 Monitoring meteorological conditions...... 25

Useful resources...... 27

References...... 27 Introduction

Typically, there is a short window of opportunity to safely and n local-wind flows, such as drainage winds caused when air efficiently control pests and diseases in agricultural crops. over sloped terrain is cooled by conduction, becomes dense The opportunity may be shortened by unsuitable weather and drains to lower levels, sea breezes and land breezes; conditions. It is therefore essential that spray applicators are n temperature of the air and the surface; and able to identify and react to weather conditions at a local scale. n humidity. Weather Essentials for Pesticide Application aims to help those applying pesticides to understand, observe and Intuitively, the worst weather conditions for spraying are interpret weather conditions at the spray target; especially strong winds, high temperature and low humidity. Research those driven by local microclimates. This knowledge will has found that spray particles are likely to drift further and support the adoption of effective application methods that in higher concentrations when the atmosphere is stable and avoid spray drift. where there is a surface temperature inversion. Particles far smaller than can be seen by the human eye During the night there is high potential for pesticides to can drift and cause damage many kilometres away from the drift at damaging concentrations for long distances in light site of application. winds (less than about 12km/h) and gentle breezes (less The weather factors that are important to the application than about 18km/h) when the temperature is cool, humidity of pesticides can be significantly and critically different to is very high and temperature inversions exist. conditions indicated by forecasts, maps and off-site weather Field tests indicate that the greatest drift deposits observations. This is especially true for conditions overnight and occurred with relatively high wind speeds, coupled with a into mid-morning when surface inversions are likely to exist and temperature inversion and spray in the small droplet spectra local winds develop. Local winds can transport concentrated (about 200µm) (Bird 1995). volumes of pesticides long distances from the target. Applicators and advisers need to be able to recognise The weather factors that need to be considered before and respond to all weather factors so that they can make and while applying pesticides are: timely adjustments to spray practices to minimise the risk n atmospheric stability (including up and down air currents); of spray drift and maximise the potential for the pesticide to n general wind speed, direction and turbulence; reach the appropriate target.

Photo: Peter Cousins Consulting

4 W EA TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A T ION

An indicator of stable conditions – fog. The inversion over this relatively flat area has most likely been enhanced by cool drainage winds off the distant slopes. Off-target movement of pesticides 5

Synoptic weather forecasts provide an overview of FIGURE 1 Spray nozzle classification by droplet temperature, precipitation and wind but they do not provide spectra example reference graph details of what is occurring at the spray target. µm Consequently, it is important for the spray applicator to 1200 W EA understand how current weather conditions interplay with 1000 topography and the field environment to influence off-target TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A movement of pesticides. 1100 It is also important for them to measure and record their 900 own weather data in the paddock (see page 25). 800 What is likely to drift? 700 Droplets smaller than about 200µm tend to be the 1 most drift-prone. These small droplets fall more slowly 600 because they do not have enough weight to overcome air resistance and thermal currents. Even small 500

changes in droplet diameter make a big difference to 400 T ION drift potential. Increasing the size from 150 to 300µm increases weight and volume by eight times. Heavier 300 droplets fall more rapidly and are less affected by air 200 currents, wind and evaporation effects. Products of evaporation – small concentrated droplets, 100 2 vapour and microscopic particles of the active ingredient (usually salts). 0 0 10 20 30 40 50 60 70 80 90 Vapours from volatilisation of pesticides from the Cumulative volume % 3 surface after application. Extra Coarse / Ultra Coarse Aerosols with droplets, vapour and particles attached Very Coarse / Extra Coarse 4 to or absorbed into them. Typical aerosols are tiny Coarse / Very Coarse haze particles, fog droplets, smoke particles, pollens, Medium / Coarse dust and salts. Fine / Medium Therefore, off-target droplet movement is influenced by: Very Fine / Fine n the variation of droplet size, determined by Extra Fine / Very Fine Droplets less than 200µm are prone to drift nozzles,pressure and formulation; n application factors, including boom height, boom stability, All nozzles will produce a percentage of small droplets capable of moving with the wind or evaporating to sizes highly susceptible to being caught up in air forward speed compared to wind speed, boom tip speed currents. The chart shows that for a nozzle that produces a fine to medium and even the mass of the machine and aerodynamic spray quality up to 40 per cent of the spray volume (applied chemical) is in uplift; and droplets smaller than 200µm, which are highly drift prone. If operating n weather conditions. practices are not optimal and/or weather conditions are windy, hot or dry it is possible to lose more than 40% of total spray volume to the atmosphere. The 50% value for all spray qualities shown in above graph is equivalent to Photo: emma leonard the volume median diameter (VMD). A droplet dimension which indicates that half of the spray volume is in droplets smaller than this number and half of the spray volume is in droplets larger than this size. Ultimately the problem of small droplets is the high potential for loss of product, loss of pest control and the potential for excessive drift. SOURCE: ADAPTED WITH PERMISSION FROM ASABE STANDARD S572: SPRAY NOZZLE CLASSIFICATION BY DROPLET SPECTRA (WWW.ASABE.ORG/STANDARDS/IMAGES.ASPX). ST. JOSEPH, MICH.: AMERICAN SOCIETY OF AGRICULTURAL AND BIOLOGICAL ENGINEERS

Spray applicators must consider synoptic and local weather patterns/conditions and the influence of topography on how pesticides may move in the landscape. Common weather guidelines for pesticide application

Spray when wind is steady and ideally more than 3km/h and up to 15km/h (see page 10) Very-light winds (less than about 7km/h) are often inconsistent in strength and direction. They frequently accompany two of the most adverse spraying conditions; strong thermals and strong inversions. n Thermal activity interrupts the general synoptic wind flow. n Thermal updrafts have the potential to lift suspended material into the atmosphere; it may later return to the y l o r

surface in descending air currents. a n As inversions lack turbulence, they can trap and concentrate suspended spray materials in layers. Airborne pesticides can be transported within the layers at high

concentrations for long distances by light winds and P h o t o: F ra nk T gentle breezes. A stable atmosphere with an inversion and Delta T less than 2 – conditions conducive n Winds less than 3km/h are unsuitable for spraying to high concentrations of small droplets remaining airborne for long periods of time. because they are highly likely to be variable in direction and thus the direction of spray drift would be impossible to determine and the safety of downwind receptors compromised. Most common instruments will not accurately detect winds less than 3km/h. Winds of about 6km/h are required for an observer to feel the wind on the face. Winds greater than 15km/h initiate large turbulent eddies that can pick up and disperse sprayed material over a wide area. n even larger droplets may be swept off target and affect nearby environments. n The majority of smaller droplets will tend to be lifted and dispersed into the atmosphere and may travel long distances.

6 Avoid spraying in temperatures

above 30ºC (see page 16) P h o t o: G ra e m Te pp r As temperature increases: An indicator of unstable conditions – cumulus clouds. n droplets evaporate faster; n turbulence intensity generally increases; n volatile pesticides on soil and plant surfaces vaporise n A Delta T less than 2 indicates a very moist atmosphere. faster; and The relative humidity will be greater than 80 per cent at n the potential for thermals to lift airborne pesticides into the temperatures greater than 15ºC. This can extend the W EA atmosphere increases. life of small airborne droplets and increase the potential TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A for fine droplets to drift long distances when surface Always read the product label and follow temperature inversions exist. all instructions. They may differ from the n A Delta T greater than 8 indicates rapid evaporation of the guidelines in this publication. aqueous component of droplets. In such conditions, the amount of product reaching the target is compromised. An operator needs to decide whether to stop spraying or to Aim to spray when Delta T is between 2 and 8 continue using a larger droplet size and higher water rate. and not greater than 10 (see page 18) Delta T is an indicator of the evaporation rate for water Do not spray when inversion conditions droplets. Larger droplets evaporate more slowly than small exist (see page 20)

T ION droplets at the same Delta T. This is because the exposed Temperature inversions lack turbulence. This can lead surface compared to volume is greater for small droplets. to high concentrations of airborne pesticides being Droplet survival reduces as Delta T increases. transported close to the surface in light and variable winds to destinations that cannot usually be predetermined. 7 10 steps to minimise spray drift Operate within the weather guidelines and adopt the following practices to reduce spray drift risk.

Ensure that the spray droplet spectrum is optimised to maximise efficacy and minimise the 1 opportunity for droplets to evaporate down to drift-prone size before reaching the target. W EA Use the coarsest droplets possible and avoid wetters that increase drift potential. TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A Operate machinery at optimum speed to maximise the boom’s stability and minimise the 2 machine’s aerodynamic effect on airflow behind the machine and boom.

Keep the boom height as low as possible to reduce the time that droplets are in the air 3 and to take advantage of lower wind speed normally experienced near the surface.

Plan ahead and be prepared to adjust operations to combat weather variations.

4 T ION 5 Be sure to take into account the local microclimatic conditions, especially at night.

Continually monitor conditions at the site and at a height representative 6 of the spray zone and adjust operating practices for current conditions.

Use on-board weather stations, smoke devices or ribbons attached to booms and fence 7 lines to assist in the detection of variations in wind direction and speed. If using smoke devices, be mindful that the initial smoke rise will be due to the inherent heat of the source. Take extra care when applying pesticides over partially bare ground, 8 which is hot and conducive to rapid evaporation and thermals.

Avoid spraying an hour before sunset if an inversion is likely and for 9 an hour-and-a-half after sunrise if an inversion occurred overnight (variations of wind speed and direction are likely to be unpredictable). 10 Ensure that adequate buffers are maintained to protect sensitive areas. 1. aTmospheric stability

Atmospheric stability describes the atmosphere’s tendency to resist or assist the vertical movement of air in the atmosphere. Stability has a major influence on the movement and concentration of airborne pesticides. There are three common states of atmospheric stability: neutral, unstable and stable (Figure 2, page 9). It is crucial that spray applicators understand and can recognise the three states of stability. Atmospheric stability varies near the surface because the temperature of the air in contact with the surface changes more rapidly than the temperature of the air immediately above. Neutral: Conditions are neutral when there is little difference between the temperature of the air at the surface and that above. Unstable: Conditions are unstable when surface air is considerably warmer than the air above. Stable: They are stable when surface air is considerably cooler than the air above. Generally, stability tends to be neutral in higher wind speeds. However, very stable and very unstable conditions can tolerate considerable wind. In agricultural areas, speeds of 11km/h and higher are not unusual when the atmosphere is stable and surface temperature inversions exist. Unstable conditions tend to be turbulent. Stable conditions are rarely turbulent. Turbulence is the major factor that determines airborne pesticide concentrations. Turbulence refers to the rolling eddies of airflow across the surface and thermals. It is usually suppressed at night and enhanced during the day. 8 Stability ratio Stability ratio (SR) accounts for wind speed effects on stability. Figure 3 shows that stable conditions (inversions)

can exist with wind speeds exceeding 11km/h. The effect ph o t o: Pe e Ni ko lais on ( N Z) of moderately strong winds on airborne droplets and Smoke from home fires trapped by very-stable inversion conditions and transported by particles is evident in the image of smoke being transported strong drainage winds. Note the smoke fumigating to the surface and multiple smoke and concentrated at the surface. This image indicates the sources merging into one concentrated plume. W EA existence of a temperature inversion. Pesticides can drift in TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A the same manner.

T ION FIGURE 2 Three common states of atmospheric stability and their relationship to spray application 9 Neutral atmosphere High potential for small droplets and products of evaporation to drift into the atmosphere. W EA TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A Unstable atmosphere Ideal spray conditions as long as winds are more than 3km/h and up to 15km/h. T ION

Stable atmosphere High potential for high concentrations of air borne pesticides to drift long distances close to the surface.

Fine Medium Coarse

SOURCE: GRAEME TEPPER FIGURE 3 The importance of monitoring weather during spray operations: an estimated stability ratio (SR). In this case, the inversion had eroded to about 70m by 8:30am – a capping inversion with top at 208m remained until about 10:30am. Wind speed (km/h) and Estimated stability ratio (SR) temperature (ºC) (temperature variation with height and wind speed effects) 3.50 19 3.29 3.30 Sunrise 0742 3.10 Sunset 1757 2.90 2.76 17 2.70 2.56 2.50 2.30 15 2.08 2.04 2.04 2.08 2.10 1.90 13 1.70 1.50 >1.2 very stable 1.30 11 0.1 to 1.2 mod. stable 1.17 1.14 1.10 0.98 1.03 0.1 to –0.1 neutral 0.92 0.98 0.90 –0.1 to –1.7 unstable 0.62 0.70 9 0.50 0.30 0.17 0.15 0.10 7 –0.03 –0.02 –0.03–0.05–0.12 –0.12–0.07–0.04 –0.10 Neutral U Ms Very stable Ms Very stable Ms U Neutral –0.30 Midday 1pm 10pm 11pm 1am 10am 11am Midnight 2pm 3pm 4pm 5pm 6pm 7pm 8pm 9pm 5 2am 3am 4am 5am 6am 7am 8am 9am –0.50 Midday

ºC km/h SR Time of day Stability variations and estimated stability ratios over 24 hours. The temperature and wind traces show the relationship between stability, wind speed and temperature at the surface. Left to right neutral conditions initially dominate due to moderately strong winds, followed by a short period of unstable (U) conditions due mainly to a drop in wind speed. This was followed by a short period of neutral through to the transition to moderately stable (Ms) and very stable (inversion) conditions. The inversion weakens to moderately stable conditions due to periodic wind speed increases. By about 3:30am synoptic flow strengthens and stability tends to moderate then neutral, unstable and neutral again. SOURCE: GRAEME TEPPER, SCADDAN/ESPERANCE, 28-29 MAY 2003 2. Wind

Wind is simply air in three-dimensional motion. potential. Many factors lead to spray drift but ultimately it is caused Low wind speed (less than 5km/h) tends to be associated by horizontal and vertical winds that promote the transfer with unpredictable wind direction and very little turbulence of pesticides into the atmosphere. For most purposes, unless thermals exist. horizontal wind can be adequately described as an average Also refer to page 24 – Precautions for night spraying. of the speed and direction of air flowing parallel to the Earth’s surface. Bureau of Meteorology forecasts and Turbulence observations refer to horizontal wind speed and direction Turbulence is the specific feature of wind flow, both averaged over 10 minutes at 10 metres above the ground horizontal and vertical, that determines the concentration (Figure 4). and dilution of airborne pesticides. It consists of rolling When considering pesticide application the most important vertical winds are those associated with thermals. FIGURE 4 Bureau of Meteorology wind chart. Red Vertical winds can significantly impact the volume of lines trace out a continuous and instantaneous wind pesticides swept off site. recording. Wind speed variability often has a gustiness factor (highs and lows) of 40% during Three-dimensional motion determines: sunlight hours (less at night). There can be wind n airborne pesticide concentrations; direction variations of about 30° during sunny n the vertical distribution of airborne pesticides; conditions (less at night). Average values, typical of n direction and distance of pesticide transport; and those reported by observations and forecasts, are n pesticide deposition. depicted as white lines. Gusts Air primarily flows in response to adjacent pressure differences (highs to lows), heating (thermals) and cooling Average wind speed (drainage winds). As air flows over the surface it is deflected by obstacles and slowed by friction. Consequently, near the surface it breaks down into a series of turbulent eddies that lead to rapid fluctuations in wind speed and direction. Winds relevant to pesticide application are: n synoptic winds – horizontal surface winds, initiated by air flow from high to low pressure. Isobar spacing illustrated on weather maps can be used as a general guide to determine wind speed and direction; and 10 n local winds – up, down or horizontal winds that flow over short distances, initiated by unequal heating and cooling of the ground and by the shape and features of the paddock and the regional. Gravity has a major impact Wind direction (average in white) on the destination of air being cooled on slopes. Surface heating has a major influence on the strength of rising-air currents. Local winds cannot be determined from isobar spacing. W EA

TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A Common spray guidelines FIGURE 5 Turbulence n Spray only when wind direction is away from sensitive areas (buffers may vary this requirement). n During the day, aim to spray when wind speed is more Laminar flow than 3km/h and up to 15km/h. n Do not spray if wind is calm or less than 3km/h or at any wind speed where constancy in direction is not assured. High wind speed (above 15 km/h) can lead to high volumes of downwind drift with highest concentrations close to the target and lowest concentrations further downwind. High

T ION wind speeds are the primary cause of loss and waste of Turbulent flow product from the target site. Winds and breezes between about 7km/h and 18km/h, A smoke plume initially illustrates laminar flow before breaking up into turbulent flow. having consistent direction, are generally best for spraying Note the consistent high concentration within the laminar flow and the turbulent eddies as they promote optimal-droplet deposition and least drift and mixing within the turbulent flow. SOURCE: GRAEME TEPPER 11 To the left of this image smoke rises turbulently into the surface temperature inversion layer before being caught up in laminar-flowing winds within the inversion. Note that the smoke particles hang near the surface and travel several kilometres before settling into a low-lying area. W EA TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A

photo: Bill Campbell motions (eddies) in the airflow; larger rolls generate greater Onset and cessation of local winds can be rapid. turbulence and greatest dispersion of airborne pesticides. Therefore, local winds need to be carefully monitored Turbulence intensity is regulated by atmospheric stability to ensure operating practices remain tailored to prevent and is usually suppressed at night and enhanced during the pesticides drifting toward sensitive areas. day (see Atmospheric Stability, page 8). Local winds are usually on a small scale, more pronounced A lack of turbulence leads to airborne pesticides floating after sunset and decline in influence after sunrise. at high concentrations near the surface and high risk of Local wind flows are caused by: adverse drift effects. This is the situation during a surface n on-slope cooling (drainage winds – Figure 6); T ION temperature inversion. n on-slope heating (anabatic winds – Figure 8); Intense turbulence leads to rapid dispersion of airborne n terrain blocking (blocked winds – Figure 9); pesticides both horizontally and vertically, taking larger n differential heating (sea and land breezes – Figures 10 and particles down to the surface and lifting smaller particles 11); high into the atmosphere (Figure 5). Spray drift will occur n cool air falling downslope (downslope winds – Figure 12); but particles will be dispersed, diluting the pesticide n excessive heating (thermals – Figure 13); and concentration and reducing but not eliminating the risk of n density currents (far-inland sea breezes). off-target damage. It is important to note that as turbulence intensity Measuring local winds increases it not only acts to rapidly disperse pesticides it Because local winds are short-lived and variable, frequent also acts to keep airborne pesticides in suspension for long in-paddock observations are required to track variations. periods. While sophisticated instruments can be used to track variations, simple methods are often useful. Ribbons Synoptic winds attached to the end of the boom or on fencelines, smoke Synoptic winds are the winds usually referred to in weather pots or a smoking device fitted to the tractor’s exhaust forecasts (sea breezes being an exception for coastal can all help indicate wind strength and direction. In turn, regions). such observations indicate if the atmosphere has become Synoptic winds are those resulting from air flowing in stable (see Atmospheric Stability, page 8) and the wind has response to pressure-gradient forces that build up between become less turbulent. Handheld wind meters should be areas of high and low pressure over several hundreds or held at the height of the boomspray for wind measurement. thousands of kilometres. The greater the pressure difference, the greater is the pressure-gradient force and hence the Wind shear stronger are the winds. It might be expected that synoptic winds should flow FIGURE 6 Drainage winds – overnight directly-across isobars from high to low pressure but this Potential for drift of fine droplet is rarely the case. Instead, the observed behaviour is that Synoptic wind vapour and particulate matter the synoptic winds flow slightly-across isobars and exhibit day and night (katabatic) anti-clockwise spiralling of wind out of high-pressure Top of the surface inversion systems and clockwise spiralling of wind into low-pressure systems (this is in the Southern Hemisphere, in the Northern Hemisphere winds spiral clockwise out of high-pressure systems and anticlockwise into low-pressure systems). The pressure/wind relationship can be seen by comparing isobar configuration to winds (Figure 7). These charts (minus the arrows) are available from the Bureau of Meteorology. During a surface temperature inversion drainage winds often occur, with air flowing to the lowest point in the catchment. Drainage flow can be deflected by obstacles in a similar fashion to water flowing downhill. These winds can develop over the slightest Local winds of slopes. Drainage winds can carry concentrated airborne pesticides long distances, Local winds frequently flow completely independently of the which can result in widespread pesticide damage many kilometres away from the synoptic flow, with different direction and speed. application site. SOURCE: GRAEME TEPPER Wind shear Applicators should be wary of wind-shear effects from When wind direction and speed change with altitude ‘wind sunrise and up to about an hour-and-a-half after sunrise shear’ exists (Figure 14, page 14). when a surface temperature inversion is progressively Wind shear can lead to airborne pesticides being eroding (Figure 15, page 14). transported in directions other than what the surface To anticipate wind shear, applicators need to compare the direction indicates. overriding synoptic wind flow to that occurring at the surface.

FIGURE 7 The mean sea-level pressure pattern (synoptic chart) on the top generates surface (10 metres above the ground) winds as illustrated by the chart on the bottom MSLP/Precip (03 hourly) ACCESS-REGIONAL mm/3hr Valid 15UTC Sun 05 Feb 2012 t_063 200 95ºE 100ºE 105ºE 110ºE 115ºE 120ºE 125ºE 130ºE 135ºE 140ºE 145ºE 150ºE 155ºE 160ºE 165ºE 170ºE 175ºE

150 5ºS 100 10ºS

50 15ºS

20ºS 20

25ºS 10

30ºS 5

35ºS 2

40ºS 1 45ºS 0.2 50ºS

95ºE 100ºE 105ºE 110ºE 115ºE 120ºE 125ºE 130ºE 135ºE 140ºE 145ºE 150ºE 155ºE 160ºE 165ºE 170ºE 175ºE 12 Speed (kts)

5ºS 120

10ºS 100

15ºS 80

W EA 20ºS 64 TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A 25ºS 48

30ºS 25

35ºS 20

40ºS 15

45ºS 10 T ION 50ºS 5

0 95ºE 100ºE 105ºE 110ºE 115ºE 120ºE 125ºE 130ºE 135ºE 140ºE 145ºE 150ºE 155ºE 160ºE 165ºE 170ºE 175ºE

SOURCE: BUREAU OF METEOROLOGY For example, if the general wind flow indicates westerly to occur when surface temperature inversions and drainage 13 winds but the actual surface winds are easterly then there winds dominate local surface wind flow, while winds above is significant wind shear. If overnight winds have a similar the surface are dominated by the synoptic flow. direction to the general wind flow, wind shear will be minimal (Figure 14, page 14). Diurnal variation of wind The wind shear most relevant to spray applicators is likely Diurnal variations in wind speed and gustiness are related to

FIGURE 8 Anabatic winds (up-slope) FIGURE 9 Blocked winds and contour wind ﬂow W EA

Potential for fine droplets, TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A vapour and particulate matter Synoptic wind to be transported parallel to day and night terrain contours and around obstacles. Potential for fine droplets, Top of the surface inversion vapour and particulate matter to be transported upslope.

Anabatic winds are warm winds that flow up and parallel to steep slopes or mountain sides in response to heating of the surface and of the air in contact with the surface. Blocked winds occur upwind of obstacles when cool dense air cannot flow over them T ION Anabatic winds are most pronounced in the early morning and sometimes in the evening (usually a temperature inversion exists). The winds tend to be very localised due to when the sun is low in the sky. Pesticides can be carried upward through crops vegetation (e.g. tree belts), levy banks and buildings, or they can be more positioned on steep slopes when anabatic winds exist and spray drift occurs. wide-reaching due to hills and major changes in topography. They tend to flow along SOURCE: GRAEME TEPPER contours and can carry concentrated airborne pesticides long distances, which can result in long distant drift and widespread damage. SOURCE: GRAEME TEPPER

FIGURE 10 Sea breeze circulation FIGURE 11 Land breeze circulation Potential for fine Potential exists for droplets, vapour and fine droplets, vapour particulate matter to and particulate matter Warm air rising be transported over to be transported water bodies by inland by sea breezes land breezes and and captured within Warm air rising captured within sea breeze land-breeze Cool air advection circulations. Cool air advection circulations.

Release point

Sea breezes, occurring most often from mid-morning to late afternoon, can override or Land breezes typically occur at night when warmer air over the water surface rises and reinforce synoptic wind flow. Occasionally, sea breezes generate a surge of cold air cooler air from the land is drawn into this space. Land breezes have the potential to that can penetrate well inland (e.g. Renmark, South Australia, and Kalgoorlie, Western carry concentrated airborne pesticides to water surfaces. In combination with sea Australia). These inland incursions may affect applications by inducing rapid changes in breezes, they can recirculate airborne pesticides between land and water surfaces. wind speed and wind direction. They can shift large volumes of pesticides long SOURCE: GRAEME TEPPER distances. SOURCE: GRAEME TEPPER

FIGURE 12 Downslope winds and ‘gusty’ rotors FIGURE 13 Thermal winds

Potential for fine Potential for fine droplets, droplets, vapour and vapour and particulate matter particulate matter to to be transported high in be erratically distributed the atmosphere. by gusty-downslope winds. Afterward, vapour and particulate matter descend to the surface in down-current winds or are washed out by rain.

Downslope winds present as intermittent, strong and gusty winds on the downwind Thermals are drafts of warm air rising from the ground. These air currents begin to slopes of rather steep terrain. They develop when cool air banks up behind an obstacle form as the sun rises. They intensify as the surface heats up during the day and and spills over before running downslope, similar to water periodically running over the weaken as it cools during late afternoon and evening. They are extinguished by sunset, top of a dam. Downstream from the obstacle, turbulent rotors may generate rapid shifts unless the surface retains enough heat to remain hotter than the air above. The upward in wind direction and speed, to several kilometres from the foothills. speed of thermals depends on surface heating and the temperature of the overlying SOURCE: GRAEME TEPPER atmosphere. High surface temperatures with very cool air above lead to vigorous thermals capable of lifting airborne pesticides high into the atmosphere. SOURCE: GRAEME TEPPER temperature and atmospheric stability (Figures 16 and 17). downhill while blocked winds generally flow parallel to topographic contours. Evening and overnight decrease The overnight reduction of wind speed and turbulence is of wind speed and gustiness primarily caused by: In the afternoon and evening, synoptic wind speed tends n rapid cooling of the surface air; to decrease in response to increasing stability of the n a trend to increased stability of the atmosphere; and atmosphere along with a tendency for the synoptic flow n ultimately a strong temperature inversion. to lift off the surface (decouple). As the synoptic wind The reduction in surface wind speed is: decouples from the surface it is frequently replaced by local n often accompanied by an anticlockwise (Southern winds, most commonly cool drainage winds or blocked Hemisphere) directional variation of the surface wind. winds (Figures 6 and 9). Drainage winds generally flow Within an hour or two of sunrise the wind direction usually

FIGURE 14 Synoptic wind direction compared to FIGURE 15 Wind shear local ‘drainage wind’ direction

Top of the Synoptic wind surface inversion No significant Significant direction shear direction shear

The variation between the direction of synoptic winds flowing above the surface and The smoke plume rises because the air is being heated by the fire and becomes buoyant. Smoke-laden air rises into a surface temperature inversion where it is the drainage winds illustrates the difference between a sheared and non-sheared partially trapped. At different altitudes ‘wind shear’ transports smoke particles in environment. SOURCE: GRAEME TEPPER varying directions. SOURCE: GRAEME TEPPER FIGURE 16 Relationship between temperature, wind speed and gustiness over a three-day period, Horsham, Victoria, 24 to 26 May 2007 Wind speed (km/h) and Km/htemperature & ºC (ºC) 55 Sunrise Sunset Sunrise Sunset Sunrise Sunset 50

14 45

25 W EA

TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A 20

0 1am 4am 7am 10am 1pm 4pm 7pm 10pm 1am 4am 7am 10am 1pm 4pm 7pm 10pm 1am 4am 7am 10am 1pm 4pm 7pm 10pm 1am

Wind speed km/h Gusts km/h Temperature T ION Vertical axis indicates km/h and degrees. The horizontal axis indicates time of day. Maximum wind speed and gustiness is directly related to maximum temperature (the most unstable period of the day). Minimum wind speed and gustiness is directly related to minimum temperature (the most stable period of the night). A rough idea of atmospheric turbulence and stability can be gained by observing the difference between actual wind speed and gusts. The greater the difference, the more intense is the turbulence and the more unstable is the atmosphere. SOURCE: GRAEME TEPPER reverts to that expected of the synoptic situation. This 15 turning may be important in planning buffer zones; A WORD ABOUT EQUIPMENT n mirrored by an increase in wind speed and directional SPEED OF TRAVEL variations (wind shear) just above the surface, usually with a maximum speed near the top of the inversion; and From Bill Gordon, spray application specialist n sometimes accompanied by sporadic turbulence and Both faster travel speeds and greater release heights periodic switches in wind direction at the surface. increase the amount of chemical left in the air during spraying operations. This is because turbulent winds behind Early morning increase in wind speed the machine increase. W EA and gustiness If travelling faster or raising the boom above the height TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A Wind speed and gustiness often increase soon after sunrise required for a double overlap of the spray patterns, more due to surface inversion decay (see Surface Temperature consideration must be given to nozzles and other parameters Inversion, page 20) and the return of synoptic winds to the that reduce the airborne fraction of the spray. surface. Reducing the airborne fraction can be achieved by: During the transition, wind speed and direction can be n using a coarser spray quality (the coarsest that will significantly different at the surface compared to just a few provide efficacy); metres above the surface. Airborne pesticides caught up in n nozzles with narrower fan angles; this wind shear can be transported in unexpected directions. n alternating offset nozzles (forward and backwards); n using nozzles with higher exit velocities;

n preventing boom bounce and yawing (whipping); and T ION n selecting adjuvants that do not increase the driftable fraction

FIGURE 17 Surface wind variations between 3pm 5 May 2008 and 3pm 6 May 2008. The large arrow indicates the overriding SSW to W synoptic wind direction for the 24-hour period 0 6 11 St George 11 0 4

15 Goondiwindi 4

6 Boggabilla 2 7 Texas Mungindi

Coolatai Tenterfield 7 9 9 13 Warialda 11 11 Moree 9 7 15 0Inverell 11 Bingara 9 7 Noon 6 0 Walgett 11 9am 7 2 13 0 3am Narrabri

11 3pm 7 Carinda 9 7 3pm 6am

3pm Midnight 3am 6am 9am Noon 3pm While the synoptic wind flow is consistently from the west to south-west the arrows show the changing direction and strength of surface wind over a 24-hour period. Longer arrows represent stronger winds. The variations in surface wind flow are typical of those experienced when inversions form overnight and local winds develop regardless of the overriding synoptic wind flow. SOURCE: GRAEME TEPPER 3. Temperature

For a spray applicator, the temperature of the air into which Surface to air temperature variations pesticides are sprayed and the temperature of the surface Surface temperature variations compared to the air just over which they are applied are important. above the surface can be extreme. With strong sunlight, the Air and surface temperature have direct influence on the surface temperature of bare ground can be more than 20ºC deposition of pesticides and on the rate at which pesticides higher than the air temperature 1.25m above the ground, the volatilise from soil and plant surfaces (see Volatilisation standard height of temperature measurement by the Bureau – be aware of surface temperature, page 17). Increasing of Meteorology. temperature usually decreases the relative humidity of the Heat loss to clear night skies can lead to surface air and thus increases the evaporation rate of the aqueous temperature being several degrees cooler than minimum component of pesticide droplets. temperatures quoted by the Bureau of Meteorology. The difference between the surface temperature and that Besides incident solar radiation, the rate of cooling and of the air immediately above the surface is also very import. heating at the surface is dictated by the nature of surface This is because it dictates the stability of the atmosphere vegetation, soil type, moisture and wind speed (Figure 18). and in turn, affects the potential for airborne pesticides to A common guideline is to spray only when air rise and dilute or remain concentrated near the surface. temperature is less than 30ºC. However, soil temperature, especially in summer with sparse vegetation, can be Temperature and pesticide application considerably hotter than the air. As temperature increases: The total effects of applying herbicides to sparse n droplets evaporate faster because warm air can hold vegetation over hot soils may not yet be fully determined but more moisture than cool air; from a meteorological point of view the concerns include: n turbulence intensity generally increases because the n the possibility of the rapid evaporation of droplets atmosphere becomes more unstable; approaching the surface and becoming highly drift prone, n volatile pesticides on soil and plant surfaces vaporise especially smaller droplets that approach the surface faster because vapour pressure increases as temperature slowly; and increases; and n larger droplets, which would normally be expected to n the potential for thermals to lift airborne pesticides into the reach the surface, being captured and lifted by strong atmosphere increases because increasing the temperature thermals to become spray drift (Figure 19). ordinarily increases the height to which thermals rise. One solution is to use the largest droplets possible

FIGURE 18 Temperature profiles illustrating the effects of vegetation and bare ground on the variability of the vertical temperature profile within the first few metres of the surface. Note that the air temperature at 16 1.25m barely changes while surface temperatures change a great deal in the six-hour period. Height above ground (m) 2.0 Sunrise Midday

1.5

W EA Standard height

TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A for temperature forecasts and observations 1.0

0.5 T ION

0 6 8 10 22 24 26 28 30 32 34 36 Air temperature ºC Irrigated sugarcane Millet Bare ground SOURCE: ADAPTED BY GRAEME TEPPER AFTER LA RAMDAS ET AL FIGURE 19 Droplet fall rate versus thermal uplift This combination of factors makes surface temperature 17 inversion conditions especially hazardous for pesticide application. Volatilisation – be aware of surface 200um Down Up droplet 0.7m/s 0.7m/s temperature Volatilisation occurs when pesticide surface residues change from a solid or liquid to a gas (vapour).

Thermal W EA At normal application temperatures amine and sodium Fall Uplift salt formulations of phenoxy herbicides do not produce TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A volatile vapours, whereas ester formulations always produce Comparison of the typical fall rate of a 200µm droplet and speed of a typical thermal volatile vapours (Victorian DPI, 2010). updraft on a warm day illustrating the power of thermals to lift droplets that would Both high volatile esters (HVE) (ethyl, butyl and isobutyl normally be considered ‘undriftable droplets’. SOURCE: GRAEME TEPPER esters) and low volatile esters (LVE) (hexyl, octyl and so on) are capable of producing volatile vapours. Amine while maintaining efficacy and to operate the machine in a formulations may produce volatile vapours if the temperature configuration and at a speed to minimise the production of rises to 50ºC (Victorian DPI, 2010). drift-prone droplets. Higher temperature increases volatilisation rates and promotes the transfer of vapours into the atmosphere

Diurnal temperature variations from plants and soil. Volatilisation may represent a major T ION The temperature within the first few metres of the surface dissipation pathway for pesticides applied to soil or crops. varies rapidly compared to the air temperature above. This It accounts for up to 90 per cent of the application dose in leads to changes in stability causing: some cases (Bedos, 2002). n during the day – a deep mixed layer to develop into High concentrations of drifting vapour are most likely to which airborne pesticides can dilute (Figure 20); and occur: n at night – wind speed tends to be lower than during the n from sunset to an hour or so after sunrise, when surface day, so little or no mixing occurs in a shallow layer that is inversions and drainage winds restrict dispersion of generally close to the surface in which airborne pesticides vapours at the surface; and tend to be held at high concentrations (Figure 21). n in the early morning, when weak thermals lift vapour into Between sunset and sunrise surface temperature a low-level capping inversion (Figure 27, page 23). inversions often exist. This is where the temperature gradient is reversed so that the temperature at ground level is cooler Vapour lifted by thermals has the potential to be than that of the air above. concentrated by the cap and vapours may later descend to Inversions suppress turbulence, so airborne pesticides the surface some distance from the application site. become concentrated in a layer close to the surface. The concentrated layer of pesticide can drift a long way; distances of up to 40km have been recorded. Movement of the pesticide is likely to be dictated by local winds rather than the synoptic wind.

FIGURE 20 With surface heating, a daytime mixed FIGURE 21 With surface cooling at night an unmixed layer is developed by thermals and ‘anti-thermals’ layer develops, equivalent to the inversion depth

Thermals Anti-thermals

Thermal depth Active mixing Inversion depth Little or no mixing Warm surface Cool surface

The height to which thermal mixing occurs is dependent on the vigour of thermals and The depth of the surface temperature inversion increases from just a few metres at varies from just a few metres in the early morning to up to several thousand metres by around the time of sunset to several hundreds metres just before sunrise. the middle of a hot afternoon before decreasing to a few hundred metres by late SOURCE: GRAEME TEPPER afternoon. Thermal mixing is usually extinguished overland at night due to the onset of stable conditions (inversions). SOURCE: GRAEME TEPPER 4. Evaporation

The rate at which spray droplets evaporate is mainly FIGURE 23 The relationship of Delta T to relative determined by the amount of water vapour that the air humidity and temperature. A common spray can absorb, the droplet volume compared to its exposed guideline is to spray when Delta T is between 2 and surface, chemical formulation and the nature of the airflow 8; with caution below 2 or above 10 Relative humidity (%) Delta over the droplet. 100 T value Fog Very slow evaporation of all droplets Mist 90 Relative humidity Haze 2 Relative humidity (RH) is the ratio of the actual amount of 80 water vapour in the air to the amount it can hold when 4 saturated. The air’s capacity to hold water vapour increases 70 6 as air temperature rises. At 30ºC, the capacity is more than three times that at 10ºC; consequently, while the amount of 60 8 water vapour in the air may be static, the relative humidity 50 decreases as temperature increases (Figure 22). 10 40 12 Delta T A better indicator of the rate at which pesticide droplets 30 evaporate is Delta T. Delta T is the difference between the Very rapid 20 wet and dry bulb temperatures. It combines the effects of evaporation of all droplets temperature and relative humidity (Figure 23). 10 For example, the evaporation potential is about the same for: 0 5 10 15 20 25 30 35 40 Dry temperature ºC n 20ºC and 38 per cent RH and a Delta T of 8; and For the estimation of evaporation potential of the aqueous n 30ºC and 50 per cent RH and a Delta T of 8. component of pesticide droplets the rate can be considered to The amount of water vapour in the air usually only be constant for a given Delta T. changes gradually over days, while temperature changes Preferred Delta T conditions for spraying at a comparatively rapid rate. This leads to higher relative Marginal Delta T conditions for spraying humidity values and lower Delta T values overnight Conditions are marginal for COARSE or greater spray quality compared to during the day. and unsuitable for medium or finer spray quality Unsuitable Delta T conditions for spraying Relevance for spraying NEVER SPRAY DURING A SURFACE TEMPERATURE INVERSION How rapidly a droplet evaporates is dependent on the SOURCE: ADAPTED BY GRAEME TEPPER (2012) ORIGINALLY SOURCED FROM NUFARM’S SPRAYWISE DECISIONS CHART (2012) 18 aqueous component of the droplets because it is only the aqueous component that actually evaporates. Total evaporation of the aqueous component of droplets As spray droplets reduce to less than 150µm diameter before reaching the target leads to airborne gaseous and they become highly susceptible to drift and can be particulate residues of the active component plus impurities transported over long distances by air currents before being and additives floating in the atmosphere. The initial residues deposited (JB Unsworth et al, 1999). of evaporation are highly concentrated forms of the active Partial evaporation of the aqueous component of droplets pesticide. If not adequately dispersed by turbulence, they before reaching the target leads to ever-decreasing droplet can lead to damaging drift for kilometres downwind. W EA size and higher concentrated droplets, slower fall rates and No evaporation of the aqueous component of small TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A high potential for drift. droplets already in suspension can lead to unacceptably long life and long distant drift. FIGURE 22 A depiction of one air parcel undergoing Very small droplets, vapour and particle residues that a temperature change while the amount of moisture remain airborne for a considerable time are cause for remains the same 30ºC concern beyond direct spray drift issues. These residues are finally brought back to the surface in precipitation 20ºC or incorporated in fog where they may remerge into the 10ºC atmosphere after fog dissipation. The primary pathways for actual actual actual the return to the surface of residues are: water vapour water vapour water vapour n interception/impaction (capture by obstacles or raised

T ION terrain); n rainout (washout); RH = 100% RH = 52% RH = 28% Delta T = 0 Delta T ~ 6 Delta T ~ 13 n in rain (within droplets); n fog (droplet settling); and SOURCE: GRAEME TEPPER n air currents (downward flow). 19 W EA TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A T ION

Spraying into a very humid environment photo: Emma Leonard

There is evidence that spraying in foggy conditions relative to reported rainwater concentrations”; and can lead to significant drift issues not yet completely understood. Incidence of major drift incidences have been n Settling of contaminated-fog droplets bound with reported in Australia after application in foggy conditions. pesticides may fall out over wide areas. Scientific studies raise concerns that: According to Seiber (2002): “The scavenging of particle bound pesticides and pesticide vapours into n Small droplets not immediately falling to the surface tend atmospheric moisture (cloud and fogwater, rain and not to evaporate and may act as nuclei for fog droplets; snow), is a potentially major sink for airborne pesticides. Concentrations of some pesticides in fogwater can n Chemical reactions in fog can result in compounds significantly exceed those expected based upon vapour- sometimes more toxic than the original pesticide. water distribution coefficients. Fogwater deposition has Glotfelty et al, 2000, found that “a variety of pesticides been implicated as a source of inadvertent residues to and their toxic alteration products are present in fog, non-target foliage, and of high-risk exposures for raptors and that they occasionally reach high concentrations residing in and around treated areas”. 5. Surface temperature inversions P h o t o: B ill Go rd on

Under a surface temperature inversion air can separate into very stable layers (laminates) that can concentrate and transport airborne pesticides away from the target.

Australian agricultural regions often experience adverse fact that temperature, humidity and wind conditions ease daytime spraying conditions. To avoid them, many to acceptable levels in the evening and overnight often applicators resort to evening, overnight and early morning indicates the onset of a surface temperature inversion. applications because temperature, humidity and winds are Many new product labels specifically forbid often most favourable at such times. However, the very spraying during surface inversions.

FIGURE 24 Typical vertical temperature profiles for What is a surface temperature inversion? a point in time during the night and day. The day A surface temperature inversion consists of layers of the profile typically cools with height and the night atmosphere near the Earth’s surface in which temperature 20 profile typically warms with height to a depth which increases with height. This is the inverse of the normal, constitutes the surface temperature inversion layer. temperature decreasing with height (Figure 24). The point where the temperature stops increasing is When the surface temperature inversion first forms, it is the top of the surface temperature inversion layer. just a few metres deep. With progressive overnight cooling, the depth of the surface temperature inversion rapidly Day Night increases reaching a maximum at about sunrise. At this From the surface, From the surface, time, the inversion depth may be from a few tens of metres temperature temperature up to several hundreds of metres.

W EA decreases increases

TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A with height to the top of the inversion Inversion weather A surface temperature inversion can be compared to a dome of colder air over the ground, which is cut off and largely insulated from the surrounding atmosphere. Being isolated, the surface temperature inversion develops a microclimate in which weather can be significantly different from the broader weather patterns. A surface temperature inversion causes the synoptic winds to decouple from the surface and to flow over the top of the Inversion depth inversion at accelerated speeds.

T ION Calm or very light wind conditions often accompany inversion conditions but winds can be quite strong especially Warm surface Cool surface if drainage or blocked flow develops. For example, in Australia winds in excess of 20km/h have been recorded SOURCE: GRAEME TEPPER when a surface inversion existed. Key elements of weather associated with a surface Research supported by the GRDC is further investigating 21 temperature inversion are: the development of temperature inversions and their n the onset of an inversion often leads to a significant drop implications in relation to spray application. in wind speed as sunset approaches, which signals the decoupling of the general winds from the surface; When and why does a surface n drainage winds can flow down slope and/or around temperature inversion occur? obstacles with directions dictated by topographic features Surface inversions can result from a number of processes rather than the overriding general weather pattern; that cause the air closest to the ground to become cooler n during the night, winds within the inversion turn to flow than the air above. The main reasons experienced in W EA anticlockwise (in the Southern Hemisphere); broadacre agriculture are as follows. TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A n a surface temperature inversion is a prerequisite to fog and frost development; and 1. Radiation surface inversion – created n the decay of an inversion can be accompanied by by radiation cooling winds stronger and gustier than the winds experienced While calm winds and clear skies are optimal conditions for throughout the day. the formation of radiation inversions they can occur anytime during the late afternoon or evening when wind speed is The hazards of a surface less than about 11km/h* and cloud cover does not severely temperature inversion restrict surface cooling. Surface temperature inversion conditions are unsafe for Light winds and clear sky conditions certainly lead to spraying as the potential for spray drift is high. early and rapid onset of inversion conditions which may form T ION Under a surface temperature inversion: before sunset. n air movement tends to be laminar (not turbulent), so the A surface temperature inversion formed by radiation air does not mix in the same way as it does during the cooling is often accompanied by a significant drop in wind day; speed in the evening. The drop is caused by the synoptic n airborne droplets, vapours and particulates can remain wind decoupling from the surface. concentrated in the inversion layer for long periods of time; *Radiation-surface inversions may couple with cool-air n the direction and distance pesticides movement is very advection inversions and withstand wind speeds in excess hard to predict; of 11km/h. n the movement of airborne droplets will vary depending on the landscape; and n droplets or their remnants can move next to the surface and concentrate in low-lying areas or float just above the surface (Figure 25).

FIGURE 25 Possible movement of airborne droplets in surface temperature inversion conditions A: High concentrations of airborne pesticides can float A just above the surface and be carried away by light Blocked winds winds where they may impact on surfaces above the Wind direction flow along hills level of application.

B: Drainage winds can carry high airborne pesticides B downslope into lower lying regions where they may concentrate. Drainage winds

C: When winds are light and a surface inversion is in decay C Base of eroding airborne pesticides can be caught up in a complex thermal capping inversion and wind shear situation causing them to be transported in different directions than indicated by surface winds. Thermals

Legend Fine droplets, Medium droplets Coarse droplets particles and vapour

SOURCE: GRAEME TEPPER After sunset, heat radiates back into space, causing the tend to flow out of the region, but remains in place. Thus, ground to cool. In turn, the air in contact with the ground airborne pesticides tend to float over the immediate area of becomes cooler than the air above. This generates the application unless local winds develop to carry them away. surface temperature inversion. Over gentle-sloping terrain a surface temperature Some soils retain heat after sunset and only cool slowly. inversion also forms when air in contact with the ground The retention of heat, especially over sparsely vegetated is cooled by terrestrial radiation. The cooled layer remains ground, can delay inversion formation. quite shallow over the slope and is typically only 2 to 10m An inversion can occur within 20cm of the soil surface but deep because gravity continually pulls it downward, causing not actually reach the surface due to very small-scale thermals drainage winds. driven by the warm earth. This situation is most likely in the Drainage winds occur when temperature inversions exist early evening in calm conditions. While not well documented or on slopes. Drainage winds moving cool air off-slope and into understood, pesticides could become concentrated and drift in low-lying regions may initiate a surface inversion or intensify this low-level inversion, especially if vegetation cover is sparse. an existing one over flat terrain beyond the slope. Thus, Radiation inversions are the most dangerous for spraying drainage winds can transport airborne pesticides at high operations as they cause airborne droplets, vapours and concentrations for long distances, downhill and also into low particles to remain concentrated at a low level for long lying regions and valleys (Figure 25, B). periods. Winds within the inversion can carry airborne pesticides long distances. Lifecycle of a surface temperature inversion 2. Advection inversion – created by cool A surface temperature inversion typically begins to form just or warm air movement before sunset and is strongest and deepest at the time of Cooler, denser air can move into an area and slide under minimum temperature, which is often just after sunrise. layers of less dense, warm air. This can happen when a Decay typically begins just after sunrise (Figure 27) as cold front moves into an area or a sea breeze pushes cooler the Earth begins to warm and thermals begin to erode the air inland. It can also happen when denser cool air moves inversion from the surface upward, while the higher levels down a slope and slides underneath layers of warm air in of the inversion remain intact as a ‘capping inversion’. In lower parts of the catchment. If this occurs, the intensity of a remnant capping inversion conditions, the potential for a radiation inversion increases. blanket of concentrated pesticide to drift many kilometres Warm air can move over cool surfaces; some of the air is extremely high. If conditions at the end of the decay are closest to the ground becomes cooler while the higher air unstable, pesticide may rise and diffuse into the atmosphere; stays warmer. if they are neutral, the pesticide will settle. Note: Figure 27 depicts a typical lifecycle, exceptions Transport of airborne pesticides within occur when transient cloud and fluctuations of wind speed a surface temperature inversion vary the surface-cooling rate. For example, inversions may To understand the transport of airborne pesticides within begin or cease anytime in the night especially if the synoptic 22 a surface temperature inversion, a distinction needs to be winds are increasing or decreasing as cold fronts approach made between surface temperature inversions forming over or leave an area. flat terrain and those occurring over slopes. Variations in the life of a surface temperature inversion Over flat terrain a surface temperature inversion forms requires spray applicators to continuously monitor conditions when air in contact with the ground is cooled by terrestrial while spraying particularly at night to ensure that a surface radiation. The inversion gradually intensifies and deepens as temperature inversion has not established. the surface cools. Air within the radiation inversion does not

W EA FIGURE 26 Idealised air ﬂow during the day typically increases in speed with height and ﬂows up and over

TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A raised terrain with stronger winds at hilltop (left). When a surface temperature inversion exists, surface winds decouple from the surface and flow over the inversion with winds accelerating (low-level jet) over the top of the inversion. Within the inversion, winds are typically light and often drain down slope, regardless of the overlying wind direction (right). Typical wind flow during the day Typical wind flow at night with an inversion

Synoptic winds Synoptic winds Low-level jet Yellow represents

T ION an inversion layer

Decoupled synoptic winds Drainage winds

SOURCE: GRAEME TEPPER FIGURE 27 An idealised development and decay of a surface temperature inversion with 23 no wind and clear skies. The vertical temperature profile is depicted by the red/blue line. Mid afternoon Sunset Near sunrise Early morning Mid morning ThermalsMid at afternoon Thermals Inversion at Inversion top Inversion fully eroded maximum strengthThermals at extinguished maximum depth. still in place but Thermals well maximum strength by Surface erosion base is well established inversion onset commences. above the surface W EA TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A

SOURCE: GRAEME TEPPER FIGURE 28 Pesticide particles can accumulate in the atmosphere for several days

before being deposited on the surface. T ION Mid afternoon Sunset Just after sunrise 1 to 2 hours Mid morning Thermals lift Airborne pesticides airborne pesticides after sunrise Airborne pesticides airborne remain within the Airborne pesticides either begin to pesticides suspended inversion begin to mix disperse or begin to rise descend to the surface

Figure 28 depicts a summer time fine weather situation where thermals have lofted particles into suspension well above the surface. Spraying at night when an inversion exists caused particles to be suspended near the surface. The period of transition between the night time inversion and the next day’s thermal activity can herald the mixing of both suspensions, after which they may fumigate to the surface at sufficient concentrations to cause adverse effects. SOURCE: GRAEME TEPPER

Accumulation of pesticide residues Recognising a surface in the atmosphere temperature inversion If conditions are considered appropriate for spraying, the While it is reasonable to expect surface temperature likelihood is that many applicators will be working in a region. inversions on most nights between sunset and up to an Consequently, spray droplets from multiple sources could be hour or so after sunrise, forecasting the exact onset and carried into the atmosphere (Figure 28). cessation times is not simple. The difficulty arises because of The result is occasional excessive overloading of pesticide a number of factors, including land/water-surface variations residues in the environment. This can be the cumulative and periodic influence of transient cloud, synoptic and effect of the suspension of pesticides from both day and local wind flow and intermittent turbulence on the variation night spraying events over a few days. This situation is between surface temperature and that of the air above the especially pertinent to sustained periods of summer spraying surface. These variations may cause the inversions to form when thermals are strong and inversions frequent. many hours after sunset. Cessation may be many hours Studies in pollution transport support the theory that before sunrise. Overnight the occurrence of the inversion pesticide residues could accumulate in the overhead air may be sporadic. before being bought back to the surface. The most common scientific method for detecting a surface temperature inversion requires the accurate P h o t o: Te rr e sa A b r k a ne ( N Z)

A smoke plume rises to the capping inversion before drifting in various directions far into the distance. It remains relatively concentrated even in the distance.

measurement of the vertical temperature profile. On-farm, 5. Do not spray when mist or fog are evident and/or Delta T this is usually not practical, so most spray applicators must is less than 2 (Figure 23, page 18). rely on visual and other clues. 6. use wind meters, flags or smoke devices for wind flow and stability reference. Visual clues Surface inversions may exist without visual indicators but *1.5 hours is suggested as a precaution to limit the some visual indicators are: possibility of pesticides being lifted by thermals into the n mist, fog, dew or a frost; eroding and overhead inversion where they may concentrate n smoke or dust hanging in the air near to the surface; and and be transported in unpredictable directions due to wind n cumulus clouds that have built up during the that day shear. collapse towards evening. Demons of night spraying Other clues n Small droplets. A surface temperature inversion is likely to be present if: n Vapours and particulates. n wind speed is constantly less than 11km/h in the evening n Inversion trapping and concentration. and overnight (be mindful that depending on local terrain n Drainage and other local winds. and the development of local winds there may be times n Variable wind speed and direction. when an inversion exists when winds are well in excess of 11km/h); n cool, off-slope or contour flow breezes develop; n distant sounds become clearer and easier to hear; and n aromas become more distinct in the evening.

24 A radiation-surface inversion is unlikely to exist if: n there is continuous overcast weather, with low and heavy cloud; n there is continuous drizzle or rain; n wind speed is greater than 11km/h between sunset and sunrise; n after a clear night, cumulus clouds begin to form; and n temperature rises from the minimum by at least 6OC. W EA

TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A Precautions for night spraying It is not always mandatory to stop spraying when a surface temperature inversion exists. However, operators must always aim to avoid spray drift and follow label requirements.

Guidelines for night spraying 1. CRITICAL – select spray quality and spray practices to eliminate all droplets less than 100µm. 2. Avoid spraying when winds are less than 11km/h between sunset and up to 1.5 hours* after sunrise.

T ION 3. Select operating speeds not conducive to the lifting of spray particles behind the machine. 4. keep boom height low to limit atmospheric exposure and influence on droplets. 6. RECORDING METEOROLOGICAL CONDITIONS 25

Prior to a spray application and just post spray application Temperature and humidity it is mandatory to record: wind speed and direction; Record both temperature and humidity (either relative temperature; relative humidity; and/or Delta T. Any significant humidity or Delta T) representative of the environment into changes occurring during the application should be noted. which sprays are applied. Allow instruments to acclimatise to More frequent recording would be preferable especially for the environmental conditions before taking measurements. W EA applications taking more than a few hours. A convenient While there is no mandatory requirement to do so, it TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A time to record would be at every tank refill. may be wise to check and record a representative ground- Readings should be taken at the pesticide release height surface temperature when pesticides are applied over and as close as possible to the site of application. For sparse vegetation that provides little shade to the surface. application in orchards and vineyards the records should be Infrared thermometers are available for this purpose. conducted outside the canopy on the upwind side. Be aware that Bureau of Meteorology temperature and Automated continuous recording representative of the humidity are recorded at approximately 1.25m above the spray environment would be advantageous. Onboard GPS- ground in a housing that is shaded and ventilated. At that based weather stations provide the closest to continuously height large temperature variations occurring in surface air recording the conditions representative of the spray are already well mixed out. environment. However, although onboard instruments have T ION high accuracy in their own right, some caution needs to be Monitoring meteorological conditions exercised when translating tractor-mounted recording to the Small variations in atmospheric stability, wind, temperature environment at the release height. and humidity can significantly impact spray operations. To effectively monitor them requires more than a casual glance Recording at a forecast and taking a single observation. It requires Wind applicators to be alert to the potential for change and an Record the average speed and direction over a period of at ability to monitor ongoing variations. least a few minutes at the height of pesticide release. Applicators should: Be aware that on a normal sunny day average wind speed has a gustiness factor typically of about 40 per cent Acquire knowledge of the climate of the target area. away from the average, with occasional spikes to about Be aware that available climate data from the Bureau of Meteorology are usually based only on 9:00am and 3:00pm observations with reference to maximums and minimums, although, it does archive up to half-hourly observations for GPS and compass most of its stations. These records are a valuable resource, which can be mined to gain detailed knowledge of diurnal Ultrasonic transducers and hourly variation for the locality of the automatic weather station. Many other authorities and privately set up automatic Wind channel weather stations exist from which detailed climate analysis can be performed. Before relying on data from any source it Temperature, humidity is wise to check the quality of the data. Unfortunately, some and pressure sensors stations carry poor-quality equipment, lack maintenance and have poor exposure to all prevailing conditions. P h o t o: G ra e m Te pp r Simple hand-held meters (above left) can measure temperature, humidity and wind at Mine the knowledge and experience of local a height representative of the spray environment. landholders. Locals often know more about variable Cab-mounted weather stations (right) can be used to measure and record conditions conditions than statistics ever tell. Build a picture from these throughout the spray operation. knowledge sources and project the terrain influences you 100 per cent. For example, a 10km/h wind typically has have learnt from this publication to anticipate conditions you peak gusts of about 14km/h and minimums of 6km/h. need to be alert to while spraying. Wind direction will vary about 30 degrees. For example, an Detailed climate records can be useful to determine: easterly wind will vary rapidly between east-south-east (ESE) n diurnal relationships between temperature, humidity, wind and east-north-east (ENE). flow and stability; Bureau of Meteorology forecasts and observations refer n precursors to local wind flow and typical local wind flows; to wind speed and direction averaged over 10 minutes at a n local wind-flow regimes associated with varying synoptic height of 10m above ground. The wind recorder is separated air mass types; and from any obstruction by 10 times the obstruction’s height. n typical onset and cessation times of temperature inversion For example, if a tree or shed is 5m high then the recorder conditions. must be located at least 50m away. Be familiar with the target field conditions and those of the surrounding area. Consider what weather variations may occur between dryland farming and irrigated areas or cotton crops adjacent to fallow paddocks. Subtle variations can alter wind-flow pattern, vary the stability of the air and subsequently the destination of drift.

Acquire the latest weather forecasts and observations for as near as possible to the target. If you have access to a number of automatic weather stations around the target area then all the better because you will be able to interpret between these to hone in on a specific area of interest.

26 W EA TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A T ION Useful resources References 27

Bureau of Meteorology Interactive maps (www.bom.gov.au/australia/charts/viewer) Bedos, C. (2002). Mass transfer of pesticides into the – these display numerous elements of interest to spray atmosphere by volatilization from soils and plants: review. applicators down to regional level. These Numerical Weather Agronomie 22, INRA, EDP sciences, pp21-23. Prediction (NWP) maps are used to guide the Bureau W EA of Meteorology (BoM) official forecasts. The maps are Bird, S. L. (1995). A compilation of aerial spray drift field TH ER E SS EN T IA LS F OR PE ST I C IDE APP L A produced from computer models. As they contain no input study data for low-flight agricultural application of pesticides. from weather forecasters, they do not include any symbols, In Environmental Fate Studies: State of the Art, 195-207. M. such as cold fronts. L. Leng, E. M. K Loevey, and P. L. Zubkoff. eds. Chelsea, Mich.: Lewis Publishers. Four-day forecast maps (www.bom.gov.au/australia/ charts/4day_col.shtml) – these may differ from interactive Glotfelty D. E. et al. (1987). Pesticides in fog. Nature. 1987 maps as they are manipulated by forecasters after February 12-18;325(6105) pp602-5. consideration of additional data. For weather maps generated by forecasters, view the BoM official weather Gordon, B. (2011). Need to re-visit the Delta T rules, Grains maps for the next four days. Research Update Southern Region, April 2011. T ION

Water and the Land (www.bom.gov.au/watl) – the Gordon, B. (2011). Achieving good pre-emergent spray BoM’s Water and the Land website aims to provide an results, GRDC, February 2011 (www.grdc.com.au/director/ integrated suite of information for people involved in primary events/researchupdates?item_id=7A44A04EB930A310ABA2 production, natural resource management, industry, trade 1B057F32F13F). and commerce. Ramdas, L.A. and Athmanathan, S. (1932). Vertical Aviation Weather Services (www.bom.gov.au/aviation) distribution of air temperature near the ground during night. – although designed for aviators this site has extremely Gerland Beitrago Zur Geophsick, Vol. 37,116. detailed wind information. Seiber, J. B. (2002) Pesticides in Agriculture and the SprayWise Decisions (www.spraywisedecisions.com.au) Environment. CRC Press. – an initiative of Nufarm, this innovative online service helps rural landholders and contractors to better plan and match Unsworth, J. B. et al. (1999). Significance of the long-range the timing of agrochemical sprays to prevailing local weather transport of pesticides in the atmosphere. Pure Appl. conditions. Chem., Vol. 71, No. 7, pp1359–1383.

Willyweather (www.willyweather.com.au) – one of the many Victorian Department of Primary Industries (2010). Volatile weather websites, this one is quite popular with growers. Vapour Drift Risk. Ag Note AG1217. Published: October 2005. Updated: April 2010. Agricast (www.syngenta.com.au) – a tailored weather forecast that includes spray recommendations.

CropMate (http://cropmate.agriculture.nsw.gov.au) – a weather companion for farmers, this website helps growers analyse climate and weather information for their location. It is designed to provide timely and accurate information to help growers make informed planning and management decisions during the crop cycle. GRDC, PO Box 5367, Kingston ACT 2604 T 02 6166 4500 F 02 6166 4599