DOCSLIB.ORG

Some Effects of Vertical Wind Shear on Thunderstorm Structure *

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Some Effects of Vertical Wind Shear on Thunderstorm Structure * 17 0 BULLETIN AMERICAN METEOROLOGICAL SOCIETY Some Effects of Vertical Wind Shear on Thunderstorm Structure * HORACE R. BYERS and Louis J. BATTAN U. S. Weather Bureau, Thunderstorm Project ABSTRACT Observations of thunderclouds obtained with a 3-cm height-finding radar set are used to obtain a description of the vertical shear of thunderclouds. Several photographs are given which show the shearing of the radar clouds. A scattergram of wind shear plotted against echo shear is presented and shows that the two variables are related, with the former exceeding the latter in almost all cases. Scatter-diagrams are given which verify that strong vertical wind shear tends to restrict the growth of thunderstorms. A series of radar cross sections illustrates the displacement of the upper part of a thundercloud which is subjected to wind shear, and the growth of another cloud column from the lower part of the thundercloud. 1. INTRODUCTION 2. SOURCE OF DATA HE primary aim of the Thunderstorm Prior to the utilization of radar for meteorologi- Project has been to obtain a precise de- cal observation, it was difficult to obtain the type Tscription of the thunderstorm. In a recent of cloud observational data which would permit paper Byers and Braham have given a detailed dis- the measurement of the dimensions of most cussion of the structure and circulation of a typical thunderstorms. When isolated cumuliform clouds thunderstorm [1]. The proposed model, however, develop during the daytime, it is possible to take assumes that there is no vertical wind shear. photographs of a particular cloud; however, when there are many low clouds or the visibility is poor, When this simplifying assumption is not made, a photographic techniques using visible light become description of the thunderstorm becomes more unfeasible. This type of data permits qualitative complex. It is the purpose of this paper to demon- determinations of cloud behavior, but quantitative strate some of the effects of wind shear on the calculations are often difiicult and in many cases structure of the thunderstorm. impossible. Since the thundercloud is not a rigid body but By using a radar set with a range-height- rather consists of a volume of small particles (ex- indicator (RHI), it is possible to obtain direct cept when hail is present), having little mass and measurements of the horizontal and vertical extent subject to displacement in the wind field, it is of the thunderstorms as detected by the equip- reasonable to expect that if, for example, the wind ment [9, p. 3-6]. The characteristics of radar as flow in the upper levels is from the north while an instrument for observing clouds are such that that in the lower layers is from the south, the there is closer correspondence between the radar upper part of the cloud will move towards the cloud and the actual cloud during the developing south relative to the lower part and may eventually rather than during the dissipating stages of the blow off if it does not evaporate before this occurs. thunderstorm. This results from the fact that dur- Meteorologists have long recognized that pro- ing the former, a large quantity of water droplets nounced vertical shear often inhibits the growth and frozen particles are being carried to the higher of cumulus into thunderstorms. When the wind portions of the cloud, as well as being held aloft shear varies continuously in magnitude and is con- within the cloud, while in the latter case the ab- stant in direction from the base to the top of the sence of strong updrafts permits the precipitation thunderstorm, the axis of the cloud will have a of the large hydrometeors to the lower levels, leav- tendency to bend or tilt in the direction of the shear ing only the smaller particles in the upper part of the cloud. The progressive depletion of the larger vector [2, p. 7]. If the water content and size scatterers results in a radar echo which tends to distribution of hydrometeors are such as to permit "fade out" at the edges rather than cut off sharply. radar detection of the cloud for a relatively long Since it is improbable that the reflectivity of the period of time, the shearing of the storm can be surface layers of the clouds is sufficiently high to seen on a radar 'scope. produce a detectable echo on the scope of conven- tional "S"- and "X"-band radar sets, the radar * Published Report No. 8 of the Thunderstorm Project. Unauthenticated | Downloaded 10/10/21 04:26 PM UTC VOL. 30.. No. 5, MAY, 1949 169 Thoroughout this study the effects of rain at- tenuation were considered only in a most qualita- tive fashion. Although it is realized that in some cases the magnitude of the attenuation was large, the unavailability of power measurements and rec- ords of the settings of the radar controls made the determination of accurate corrections impossible. 3. QUALITATIVE DEMONSTRATION OF RADAR CLOUD SHEAR When the wind shear is strong along the azi- muth of an RHI cross section through a thunder- storm, the effects of the shear become very evi- dent. FIGURES 2a and 2b are typical examples of such observations. The cross section of FIGURE FIG. 1. Drawing of data available on each photograph 2a was towards an azimuth of 219° while that of of the 'scope of Radar Set AN/TPS-10. The vertical FIGURE 2b was towards 353°; they were photo- lines are 10 miles apart; the nearly horizontal lines are graphed within nine minutes of one another. No 5000 feet apart. upper wind data were available in the immediate vicinity of the thunderstorms; however, the winds cloucl will be smaller than the actual cloud in al- aloft report from Huntington, West Virginia * most all cases even during the developing stages.f (2200Z) indicated that between the levels of 10,- The data used in this study were extracted from 000 feet and 25,000 feet the vertical wind shear the photographic records of the range-height- was towards 170° at 15 mph. Although the re- indicator (RHI) 'scope of the Radar Set AN/ lease was made six to seven hours later than the TPS-10 16]. As the name implies the 'scope pre- radar observations, it is felt that the synoptic situa- sents slant range (straight line distance between tion was such as to permit the use of this wind for radar set and the target) horizontally against a qualitative comparison. It can be seen that the height vertically (FIG. 1). To facilitate the meas- bending of the storms is in the proper direction urement, an overlay was made on which were if the components of the shear along the respective drawn lines of equal horizontal distance from the azimuths are calculated. radar site. From the available RHI 'scope photographs, it was possible to obtain a number of vertical cross t See References [3], [4] and [5] for detailed discus- sions of radar-weather theory. * Approximately 110 miles southeast of the radar site. FIG. 2. Photographs of the RHI 'scope showing the tilting of radar clouds which are located at azimuths from radar site which differ by about 135 degrees. (Fig. 2a, left; Fig. 2b, right.) Unauthenticated | Downloaded 10/10/21 04:26 PM UTC 17 0 BULLETIN AMERICAN METEOROLOGICAL SOCIETY sections through the same thunderstorm. Since azimuth as well as in feet to make it directly com- the vertical "slices" were at intervals of three to parable to the vertical axis. The azimuth of 174° four degrees of azimuth and about two seconds in was selected as the reference from which to lay time, it was possible, by assembling the data ob- off the scale in feet since it was close to the center tained during one azimuthal scan of the radar an- of the storm. tenna through the storm, to construct a three- This thunderstorm occurred on August 25, 1947 dimensional model of the radar cloud. FIGURE 3 and was located about 30 miles from the radar site. presents a series of approximately east-west cross The vertical wind shear computed between the sections obtained by extracting the necessary data levels of 15 and 30 thousand feet at about the from the three-dimensional drawings. The dashed same time as the cloud observations indicated a lines are isolines of equal radar cloud thickness (3 component towards the west of 6 mph. The ef- miles) measured into the plane of the paper. From fects of the wind shear in blowing off the top of the FIGURES given it is impossible to determine the the thunderstorm are strikingly illustrated. An- extent of any portion of the radar cloud on either other interesting feature which is worthy of men- side of the central east-west vertical plane. The tion is that in this case the dissipation of the radar horizontal axis has been labeled in degrees of cloud proceeded most rapidly at the middle and FIG. 3. Series of approximately east-west cross-sections through a radar thundercloud. The azimuth scale is in degrees from north relative to the position of the radar site. The time is indicated in hours, minutes, and seconds. Unauthenticated | Downloaded 10/10/21 04:26 PM UTC VOL. 30.. No. 5, MAY, 1949 171 probably be explained by similar wind shear con- siderations [7, p. 67]. 4. MEASUREMENTS OF VERTICAL CLOUD SHEAR In order to show the relationship between radar cloud and wind shear, the vector difference be- tween the wind velocity at two selected altitudes and the rate of horizontal displacement of the radar cloud at the upper altitude relative to the lower were measured for a number of storms and corresponding values were compared.
Load more

Recommended publications
  • Low Level Wind Shear: Invisible Enemy to Pilots
    Low Level Wind Shear: Invisible Enemy To Pilots On the afternoon of August 2, 1985, a landmark aircraft accident occurred at the Dallas/Fort Worth (DFW) airport. The tragic accident, which killed 137 of the 163 passengers on board Delta Airlines Flight 191, was responsible for making “wind shear” a more commonly known weather phenomenon and implementing many new changes with regard to wind shear detection (Ref. 1). On that day, thunderstorms were in the area of approach to runway 17L at the DFW International Airport, with a thunderstorm rain shaft right in the path of final approach. The crew decided to proceed through the thunderstorm, which turned out to be a critical error. Shortly after entering the storm, turbulence increased and the L1011 aircraft encountered a 26 knot headwind. Just as suddenly, the wind switched to a 46 knot tailwind, resulting in a loss of 72 knots of airspeed. This much of an airspeed loss on final approach, when the jet was only 800 feet above the surface, was unrecoverable and the aircraft eventually crashed short of the runway (Ref. 1). The sudden change in wind speed and direction that the aircraft encountered is called wind shear. Wind shear can occur at many different levels of the atmosphere, however it is most dangerous at the low levels, as a sudden loss of airspeed and altitude can occur. Plenty of altitude is normally needed to recover from the stall produced by the abrupt change in wind speed and direction, which is why pilots need to be aware of the hazards and mitigation of low-level wind shear.
    [Show full text]
  • 3. Electrical Structure of Thunderstorm Clouds
    3. Electrical Structure of Thunderstorm Clouds 1 Cloud Charge Structure and Mechanisms of Cloud Electrification An isolated thundercloud in the central New Mexico, with rudimentary indication of how electric charge is thought to be distributed and around the thundercloud, as inferred from the remote and in situ observations. Adapted from Krehbiel (1986). 2 2 Cloud Charge Structure and Mechanisms of Cloud Electrification A vertical tripole representing the idealized gross charge structure of a thundercloud. The negative screening layer charges at the cloud top and the positive corona space charge produced at ground are ignored here. 3 3 Cloud Charge Structure and Mechanisms of Cloud Electrification E = 2 E (−)cos( 90o−α) Q H = 2 2 3/2 2πεo()H + r sinα = k Q = const R2 ( ) Method of images for finding the electric field due to a negative point charge above a perfectly conducting ground at a field point located at the ground surface. 4 4 Cloud Charge Structure and Mechanisms of Cloud Electrification The electric field at ground due to the vertical tripole, labeled “Total”, as a function of the distance from the axis of the tripole. Also shown are the contributions to the total electric field from the three individual charges of the tripole. An upward directed electric field is defined as positive (according to the physics sign convention). 5 5 Cloud Charge Structure and Mechanisms of Cloud Electrification Electric Field Change Due to Negative Cloud-to-Ground Discharge Electric Field Change Due to a Cloud Discharge Electric Field Change, kV/m Change, Electric Field Electric Field Change, kV/m Change, Electric Field Distance, km Distance, km Electric field change at ground, due to the Electric field change at ground, due to the total removal of the negative charge of the total removal of the negative and upper vertical tripole via a cloud-to-ground positive charges of the vertical tripole via a discharge, as a function of distance from cloud discharge, as a function of distance from the axis of the tripole.
    [Show full text]
  • Weather and Climate: Changing Human Exposures K
    CHAPTER 2 Weather and climate: changing human exposures K. L. Ebi,1 L. O. Mearns,2 B. Nyenzi3 Introduction Research on the potential health effects of weather, climate variability and climate change requires understanding of the exposure of interest. Although often the terms weather and climate are used interchangeably, they actually represent different parts of the same spectrum. Weather is the complex and continuously changing condition of the atmosphere usually considered on a time-scale from minutes to weeks. The atmospheric variables that characterize weather include temperature, precipitation, humidity, pressure, and wind speed and direction. Climate is the average state of the atmosphere, and the associated characteristics of the underlying land or water, in a particular region over a par- ticular time-scale, usually considered over multiple years. Climate variability is the variation around the average climate, including seasonal variations as well as large-scale variations in atmospheric and ocean circulation such as the El Niño/Southern Oscillation (ENSO) or the North Atlantic Oscillation (NAO). Climate change operates over decades or longer time-scales. Research on the health impacts of climate variability and change aims to increase understanding of the potential risks and to identify effective adaptation options. Understanding the potential health consequences of climate change requires the development of empirical knowledge in three areas (1): 1. historical analogue studies to estimate, for specified populations, the risks of climate-sensitive diseases (including understanding the mechanism of effect) and to forecast the potential health effects of comparable exposures either in different geographical regions or in the future; 2. studies seeking early evidence of changes, in either health risk indicators or health status, occurring in response to actual climate change; 3.
    [Show full text]
  • Environmental Systems the Atmosphere and Hydrosphere
    Environmental Systems The atmosphere and hydrosphere THE ATMOSPHERE The atmosphere, the gaseous layer that surrounds the earth, formed over four billion years ago. During the evolution of the solid earth, volcanic eruptions released gases into the developing atmosphere. Assuming the outgassing was similar to that of modern volcanoes, the gases released included: water vapor (H2O), carbon monoxide (CO), carbon dioxide (CO2), hydrochloric acid (HCl), methane (CH4), ammonia (NH3), nitrogen (N2) and sulfur gases. The atmosphere was reducing because there was no free oxygen. Most of the hydrogen and helium that outgassed would have eventually escaped into outer space due to the inability of the earth's gravity to hold on to their small masses. There may have also been significant contributions of volatiles from the massive meteoritic bombardments known to have occurred early in the earth's history. Water vapor in the atmosphere condensed and rained down, of radiant energy in the atmosphere. The sun's radiation spans the eventually forming lakes and oceans. The oceans provided homes infrared, visible and ultraviolet light regions, while the earth's for the earliest organisms which were probably similar to radiation is mostly infrared. cyanobacteria. Oxygen was released into the atmosphere by these early organisms, and carbon became sequestered in sedimentary The vertical temperature profile of the atmosphere is variable and rocks. This led to our current oxidizing atmosphere, which is mostly depends upon the types of radiation that affect each atmospheric comprised of nitrogen (roughly 71 percent) and oxygen (roughly 28 layer. This, in turn, depends upon the chemical composition of that percent).
    [Show full text]
  • Wind Energy Forecasting: a Collaboration of the National Center for Atmospheric Research (NCAR) and Xcel Energy
    Wind Energy Forecasting: A Collaboration of the National Center for Atmospheric Research (NCAR) and Xcel Energy Keith Parks Xcel Energy Denver, Colorado Yih-Huei Wan National Renewable Energy Laboratory Golden, Colorado Gerry Wiener and Yubao Liu University Corporation for Atmospheric Research (UCAR) Boulder, Colorado NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC. S ubcontract Report NREL/SR-5500-52233 October 2011 Contract No. DE-AC36-08GO28308 Wind Energy Forecasting: A Collaboration of the National Center for Atmospheric Research (NCAR) and Xcel Energy Keith Parks Xcel Energy Denver, Colorado Yih-Huei Wan National Renewable Energy Laboratory Golden, Colorado Gerry Wiener and Yubao Liu University Corporation for Atmospheric Research (UCAR) Boulder, Colorado NREL Technical Monitor: Erik Ela Prepared under Subcontract No. AFW-0-99427-01 NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC. National Renewable Energy Laboratory Subcontract Report 1617 Cole Boulevard NREL/SR-5500-52233 Golden, Colorado 80401 October 2011 303-275-3000 • www.nrel.gov Contract No. DE-AC36-08GO28308 This publication received minimal editorial review at NREL. NOTICE This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.
    [Show full text]
  • Wind Characteristics 1 Meteorology of Wind
    Chapter 2—Wind Characteristics 2–1 WIND CHARACTERISTICS The wind blows to the south and goes round to the north:, round and round goes the wind, and on its circuits the wind returns. Ecclesiastes 1:6 The earth’s atmosphere can be modeled as a gigantic heat engine. It extracts energy from one reservoir (the sun) and delivers heat to another reservoir at a lower temperature (space). In the process, work is done on the gases in the atmosphere and upon the earth-atmosphere boundary. There will be regions where the air pressure is temporarily higher or lower than average. This difference in air pressure causes atmospheric gases or wind to flow from the region of higher pressure to that of lower pressure. These regions are typically hundreds of kilometers in diameter. Solar radiation, evaporation of water, cloud cover, and surface roughness all play important roles in determining the conditions of the atmosphere. The study of the interactions between these effects is a complex subject called meteorology, which is covered by many excellent textbooks.[4, 8, 20] Therefore only a brief introduction to that part of meteorology concerning the flow of wind will be given in this text. 1 METEOROLOGY OF WIND The basic driving force of air movement is a difference in air pressure between two regions. This air pressure is described by several physical laws. One of these is Boyle’s law, which states that the product of pressure and volume of a gas at a constant temperature must be a constant, or p1V1 = p2V2 (1) Another law is Charles’ law, which states that, for constant pressure, the volume of a gas varies directly with absolute temperature.
    [Show full text]
  • Climatic Information of Western Sahel F
    Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Clim. Past Discuss., 10, 3877–3900, 2014 www.clim-past-discuss.net/10/3877/2014/ doi:10.5194/cpd-10-3877-2014 CPD © Author(s) 2014. CC Attribution 3.0 License. 10, 3877–3900, 2014 This discussion paper is/has been under review for the journal Climate of the Past (CP). Climatic information Please refer to the corresponding final paper in CP if available. of Western Sahel V. Millán and Climatic information of Western Sahel F. S. Rodrigo (1535–1793 AD) in original documentary sources Title Page Abstract Introduction V. Millán and F. S. Rodrigo Conclusions References Department of Applied Physics, University of Almería, Carretera de San Urbano, s/n, 04120, Almería, Spain Tables Figures Received: 11 September 2014 – Accepted: 12 September 2014 – Published: 26 September J I 2014 Correspondence to: F. S. Rodrigo ([email protected]) J I Published by Copernicus Publications on behalf of the European Geosciences Union. Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion 3877 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract CPD The Sahel is the semi-arid transition zone between arid Sahara and humid tropical Africa, extending approximately 10–20◦ N from Mauritania in the West to Sudan in the 10, 3877–3900, 2014 East. The African continent, one of the most vulnerable regions to climate change, 5 is subject to frequent droughts and famine. One climate challenge research is to iso- Climatic information late those aspects of climate variability that are natural from those that are related of Western Sahel to human influences.
    [Show full text]
  • NWS Unified Surface Analysis Manual
    Unified Surface Analysis Manual Weather Prediction Center Ocean Prediction Center National Hurricane Center Honolulu Forecast Office November 21, 2013 Table of Contents Chapter 1: Surface Analysis – Its History at the Analysis Centers…………….3 Chapter 2: Datasets available for creation of the Unified Analysis………...…..5 Chapter 3: The Unified Surface Analysis and related features.……….……….19 Chapter 4: Creation/Merging of the Unified Surface Analysis………….……..24 Chapter 5: Bibliography………………………………………………….…….30 Appendix A: Unified Graphics Legend showing Ocean Center symbols.….…33 2 Chapter 1: Surface Analysis – Its History at the Analysis Centers 1. INTRODUCTION Since 1942, surface analyses produced by several different offices within the U.S. Weather Bureau (USWB) and the National Oceanic and Atmospheric Administration’s (NOAA’s) National Weather Service (NWS) were generally based on the Norwegian Cyclone Model (Bjerknes 1919) over land, and in recent decades, the Shapiro-Keyser Model over the mid-latitudes of the ocean. The graphic below shows a typical evolution according to both models of cyclone development. Conceptual models of cyclone evolution showing lower-tropospheric (e.g., 850-hPa) geopotential height and fronts (top), and lower-tropospheric potential temperature (bottom). (a) Norwegian cyclone model: (I) incipient frontal cyclone, (II) and (III) narrowing warm sector, (IV) occlusion; (b) Shapiro–Keyser cyclone model: (I) incipient frontal cyclone, (II) frontal fracture, (III) frontal T-bone and bent-back front, (IV) frontal T-bone and warm seclusion. Panel (b) is adapted from Shapiro and Keyser (1990) , their FIG. 10.27 ) to enhance the zonal elongation of the cyclone and fronts and to reflect the continued existence of the frontal T-bone in stage IV.
    [Show full text]
  • Effect of Intense Wind Shear Across the Inversion on Stratocumulus Clouds
    GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L15814, doi:10.1029/2008GL033865, 2008 Click Here for Full Article Effect of intense wind shear across the inversion on stratocumulus clouds Shouping Wang,1 Jean-Christophe Golaz,2 and Qing Wang3 Received 17 March 2008; revised 12 May 2008; accepted 9 July 2008; published 13 August 2008. [1] A large-eddy simulation model is used to examine the mental difference between the strongly sheared and the impact of the intense cross-inversion wind shear on the shear-free stratocumulus convection? stratocumulus cloud structure. The wind shear enhanced [3] A low-level jet near the top of the CTBL is frequently entrainment mixing effectively reduces the cloud water and observed off the California central coast during summer due thickens the inversion layer. It leads to a reduction of the to the topography and land-sea contrast [Strom et al., 2001; turbulence kinetic energy (TKE) production in the cloud Rahn and Parish, 2007]. Figures 1a–1c show soundings layer due to the weakened cloud-top radiative cooling and taken in an aircraft flight in the field experiment of the formation of a turbulent and cloud free sublayer within Development and Evolution of Coastal Stratocumulus the inversion. The thickness of the sublayer increases with the [Kalogiros and Wang, 2002]. The wind speed reaches enhanced wind shear intensity. Under the condition of a maximum 18 msÀ1 just below the base of the inversion; weaker inversion, the enhanced shear mixing within the decreases by 8 msÀ1 across the sharp inversion; and then inversion layer even lowers the cloud-top height and reduces reduces further but more gradually to 7 m sÀ1 at about 730 m.
    [Show full text]
  • Geography 5942 Synoptic Meteorology: Severe Storm Forecasting Spring 2017
    Geography 5942 Synoptic Meteorology: Severe Storm Forecasting Spring 2017 Instructor: Jeff Rogers, Prof. Emeritus Office: Derby 1048 e-mail: [email protected] Phone: 292-0148 Office Hours: Tu, Th 2:10-3:30p.m. Course Prerequisites: Geography 5941, Physics 1250 Class Meetings: Tu, Th, 3:55 – 5:15 p.m. in Db 0140 Access to course lecture materials: http://carmen.osu.edu. Suggested Textbook: Mesoscale Meteorology in Midlatitudes by Paul Markowski and YvetteRichardson. Order through websites such as Amazon, it has not been ordered for the OSU bookstores. Course Objectives: The aim of the course is to introduce students to the methods of analysis and techniques of forecasting thunderstorms and severe weather. The course is divided into five components: 1. Introductory overview of the climatology of severe weather and basic cloud physics, 2. The meteorological ingredients for severe weather and the forecasting of severe weather, 3. Weather radar and satellites as tools in severe weather analysis, 4. Convection and the characteristics and features of mesoscale storms, and 5. Practice in severe weather forecasting through a series of exercises and assignments. The initial course section focuses on the ingredients of, and synoptic setting in which, severe storms develop. The role of instability, moisture, low-level and upper-level synoptic scale uplift will be described as will means by which forecasters identify and categorize the importance of each of these. The subsequent segment of the course describes the ways in which weather radar and geostationary satellite imagery are used in the analysis and forecasting of severe weather. Some theory of radar and satellite imagery is covered but the emphasis is on the usage of these materials in preparing forecasts and in trying to understand the conditions that are ideal for severe weather development.
    [Show full text]
  • Weather & Climate
    Weather & Climate July 2018 “Weather is what you get; Climate is what you expect.” Weather consists of the short-term (minutes to days) variations in the atmosphere. Weather is expressed in terms of temperature, humidity, precipitation, cloudiness, visibility and wind. Climate is the slowly varying aspect of the atmosphere-hydrosphere-land surface system. It is typically characterized in terms of averages of specific states of the atmosphere, ocean, and land, including variables such as temperature (land, ocean, and atmosphere), salinity (oceans), soil moisture (land), wind speed and direction (atmosphere), and current strength and direction (oceans). Example of Weather vs. Climate The actual observed temperatures on any given day are considered weather, whereas long-term averages based on observed temperatures are considered climate. For example, climate averages provide estimates of the maximum and minimum temperatures typical of a given location primarily based on analysis of historical data. Consider the evolution of daily average temperature near Washington DC (40N, 77.5W). The black line is the climatological average for the period 1979-1995. The actual daily temperatures (weather) for 1 January to 31 December 2009 are superposed, with red indicating warmer-than-average and blue indicating cooler-than-average conditions. Departures from the average are generally largest during winter and smallest during summer at this location. Weather Forecasts and Climate Predictions / Projections Weather forecasts are assessments of the future state of the atmosphere with respect to conditions such as precipitation, clouds, temperature, humidity and winds. Climate predictions are usually expressed in probabilistic terms (e.g. probability of warmer or wetter than average conditions) for periods such as weeks, months or seasons.
    [Show full text]
  • ESSENTIALS of METEOROLOGY (7Th Ed.) GLOSSARY
    ESSENTIALS OF METEOROLOGY (7th ed.) GLOSSARY Chapter 1 Aerosols Tiny suspended solid particles (dust, smoke, etc.) or liquid droplets that enter the atmosphere from either natural or human (anthropogenic) sources, such as the burning of fossil fuels. Sulfur-containing fossil fuels, such as coal, produce sulfate aerosols. Air density The ratio of the mass of a substance to the volume occupied by it. Air density is usually expressed as g/cm3 or kg/m3. Also See Density. Air pressure The pressure exerted by the mass of air above a given point, usually expressed in millibars (mb), inches of (atmospheric mercury (Hg) or in hectopascals (hPa). pressure) Atmosphere The envelope of gases that surround a planet and are held to it by the planet's gravitational attraction. The earth's atmosphere is mainly nitrogen and oxygen. Carbon dioxide (CO2) A colorless, odorless gas whose concentration is about 0.039 percent (390 ppm) in a volume of air near sea level. It is a selective absorber of infrared radiation and, consequently, it is important in the earth's atmospheric greenhouse effect. Solid CO2 is called dry ice. Climate The accumulation of daily and seasonal weather events over a long period of time. Front The transition zone between two distinct air masses. Hurricane A tropical cyclone having winds in excess of 64 knots (74 mi/hr). Ionosphere An electrified region of the upper atmosphere where fairly large concentrations of ions and free electrons exist. Lapse rate The rate at which an atmospheric variable (usually temperature) decreases with height. (See Environmental lapse rate.) Mesosphere The atmospheric layer between the stratosphere and the thermosphere.
    [Show full text]