By Graeme Tepper - Grains Research & Development Corporation

Weather Essentials for Pesticide Application | 3 | GRDCContentsIntroduction----------------------------------------------------------------------------------------------- 4Determining and recording local weather factors------------------------------------------------ 4Regional climate --------------------------------------------------------------------------------- 4Topography--------------------------------------------------------------------------------------- 5Downwind receptors ---------------------------------------------------------------------------- 5Field conditions and surrounding environments---------------------------------------------- 5Common weather guidelines for pesticide application------------------------------------------ 610 steps to minimise spray drift------------------------------------------------------------------- 71. Atmospheric stability--------------------------------------------------------------------------------- 8Stability ratio (SR)----------------------------------------------------------------------------------- 92. Wind--------------------------------------------------------------------------------------------------10Surface winds--------------------------------------------------------------------------------------10Synoptic winds-------------------------------------------------------------------------------------11Local winds-----------------------------------------------------------------------------------------11Wind shear------------------------------------------------------------------------------------------13Diurnal variation of wind---------------------------------------------------------------------------13Evening and overnight decrease of wind speed and gustiness ----------------------------14Early morning increase in wind speed and gustiness---------------------------------------143. Temperature------------------------------------------------------------------------------------------15Temperature and pesticide application ----------------------------------------------------------15Surface to air temperature variations ------------------------------------------------------------15Diurnal temperature variations--------------------------------------------------------------------164. Relative humidity------------------------------------------------------------------------------------17Relevance for spraying----------------------------------------------------------------------------175. Surface temperature inversions--------------------------------------------------------------------18What is a surface temperature inversion?--------------------------------------------------------18Inversion weather-----------------------------------------------------------------------------------18The hazards of a surface temperature inversion------------------------------------------------18When and why does a surface temperature inversion occur?---------------------------------201. Radiation inversion – created by radiation cooling----------------------------------------202. Advection inversion – created by cool or warm air movement--------------------------203. Vegetation inversion-------------------------------------------------------------------------20Transport of airborne pesticides within a surface temperature inversion---------------------20Lifecycle of a surface temperature inversion----------------------------------------------------21Recognising a surface temperature inversion---------------------------------------------------21Visual clues--------------------------------------------------------------------------------------21Other clues--------------------------------------------------------------------------------------21Precautions for night spraying--------------------------------------------------------------------21Useful resources----------------------------------------------------------------------------------------23References-----------------------------------------------------------------------------------------------23

Weather Essentials for Pesticide Application | 7 | GRDC10 steps to minimise spray driftOperate within the weather guidelines and adopt the following practices toreduce spray drift risk.1Ensure2Operate3Keep4Plan5When6Continually7In8Take9Avoid10that the spray droplet spectrum is optimised to maximise efficacyand minimise the opportunity for droplets to evaporate down to drift-pronesize before reaching the target. Use the coarsest droplets possible and avoidwetters that increase drift potential.machinery at optimum speed to maximise the boom’s stability and minimisethe machine’s effect on airflow around and behind the boom.the boom height as low as possible to reduce the time dropletsare in the air.ahead and be prepared to adjust operations or stop if weatherconditions change.making weather-related decisions be sure to take into accountthe local microclimatic conditions, especially at night.monitor conditions at the site and at a height representative ofthe spray zone and adjust operating practices for current conditions.light winds, use smoke devices or streamers for wind flow reference (be mindful thatthe initial smoke rise will be due to the inherent heat of the source).extra care – or avoid – applying pesticides over partially bare ground, which is hotand conducive to rapid evaporation and thermals.spraying an hour before sunset if an inversion is likely and for an hour-and-a-halfafter sunrise if an inversion occurred overnight (variations of wind speed and directonare likely to rapid and unpredictable).Ensure that adequate buffers are maintained to protect sensitive areas.An indicator of stable conditions – fog. The inversion over this relatively flat area has most likely been enhancedby cool drainage winds off the distant slopes.Photo: Peter Cousins Consulting

GRDC | 8 |Weather Essentials for Pesticide Application1. Atmospheric stabilityAtmospheric stability describes the atmosphere’s tendencyto resist or assist the vertical movement of air in theatmosphere. This has a major influence on the movementand concentration of airborne pesticides.There are three common states of atmospheric stability:neutral, unstable and stable (Figure 1).It is crucial that spray applicators understand and canrecognise the three states of stability.FIGURE 1 Three common states of atmospheric stability and their relationship to spray applicationNeutral atmosphereLight windsIdeal spray conditions as long as windsare more than 3 and up to 15km/h.The atmospheric stability is ‘neutral’ when there is little difference between the temperature of air at the surface and air in the lower atmosphere.In neutral atmospheric conditions with a light wind, larger droplets soon fall to the surface while fine droplets tend to disperse in a ‘cone-like’ patternwith maximum concentrations through the centreline.Unstable atmosphereLight windsHigh potential for small dropletsto drift and dilute into the atmosphere.Unstable conditions exist when layers of cooler air sit above warmer air. Typically, unstable conditions are experienced from mid-morning to mid-afternoon.The air freely rises and is agitated by thermals, which act to dilute airborne pesticides and carry airborne pesticides higher into the atmosphere.Stable atmosphereLight windsHigh potential for low-level,long distance and concentrated spray drift.In stable conditions, layers of cool air lie under warm air. These conditions are termed a temperature inversion (see page 18). Strongly stable conditions aretypically experienced overnight and up to an hour or so after sunrise. In stable conditions, the air lacks turbulence, which leads to fine droplets and vapoursremaining at high concentrations near the surface.LEGENDFine Medium Coarse

Weather Essentials for Pesticide Application | 9 | GRDCAtmospheric stability varies near the surface because thetemperature of the air in contact with the surface changesmore rapidly than the temperature of the air immediatelyabove.Conditions are neutral when there is little differencebetween the temperature of the air at the surface andthat above it. Conditions are unstable when surface air isconsiderably warmer than the air above. They are stablewhen surface air is considerably cooler than the air above.Generally, stability tends to be neutral in higher windspeeds. However, very stable and very unstable conditionscan tolerate considerable wind. In agricultural areas, speedsof up to 11km/h have been recorded with very stableconditions when surface temperature inversions occur.Unstable conditions tend to be turbulent. Stableconditions are rarely turbulent. Turbulence is the major factorthat determines airborne pesticide concentrations.Turbulence refers to the rolling eddies of airflow acrossthe surface. It is usually suppressed at night and enhancedduring the day.Stability ratio (SR)SR accounts for wind speed effects on stability. Figure 2below shows that stable conditions (inversions) can existwith wind speeds of up to 11km/h. The effect of suchwind speed is evident in the image above of smoke beingtransported and concentrated at the surface when aninversion exists. Pesticides can drift in the same manner.Photo: Pete Nikolaison (NZ)Smoke from home fires trapped by very-stable inversionconditions and transported by strong drainage winds. Note thesmoke fumigating to the surface and multiple smoke sourcesmerging into one concentrated plume.FIGURE 2 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) andEstimated stability ratio (SR)temperature (ºC)(temperature variation with height and wind speed effects)3.50193.293.303.10Sunset 1757Sunrise 07422.902.76172.702.562.502.30152.08 2.04 2.042.082.101.90131.701.50>1.2 very stable1.30110.1 to 1.2 mod. stable1.17 1.141.100.1 to –0.1 neutral 0.981.030.920.980.90–0.1 to –1.7 unstable0.620.7090.500.300.170.1570.10–0.03–0.03–0.05 –0.12 –0.12 –0.07–0.04 –0.02–0.10Neutral U Ms Very stable Ms Very stable Ms U Neutral–0.305–0.50Midday1pm2pm3pm4pm5pm6pm7pm8pm9pm10pm11pmMidnightºC km/h SRTime of dayStability variations and estimated stability ratios over 24 hours. The temperature and wind traces show the relationship between stability, wind speed andtemperature 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 dropin 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 inversionweakens to moderately stable conditions due to periodic wind speed increases. By about 3:30am synoptic flow strengthens and stability tends to moderatethen neutral, unstable and neutral again.1am2am3am4am5am6am7am8am9am10am11amMiddaySOURCE: GRAEME TEPPER, SCADDAN/ESPERANCE, 28-29 MAY 2003

GRDC | 10 |Weather Essentials for Pesticide Application2. WindWind is simply air in three-dimensional motion.Surface windsMany factors lead to spray drift but, ultimately, it is causedby horizontal and vertical winds. For most purposes,horizontal wind can be adequately described as an averageof the speed and direction of air flowing parallel to theEarth’s surface. According to the Bureau of Meteorology,FIGURE 3 Bureau of Meteorology wind chart. Red linestrace out a continuous and instantaneous wind recording.Wind speed variability often has a gustiness factor (highsand lows) of 40% during sunlight hours (less at night).There can be wind direction variations of about 30°during sunny conditions (less at night). Average values,typical of those reported by observations and forecasts,are depicted as white lines.Average wind speedGustsWind direction (average in white)horizontal wind speed and direction describe a 10 minuteaverage at 10 metres above the ground (Figure 3).Three-dimensional motion of the air has a direct influence on:n airborne pesticide concentrations;n droplet deposition; andn direction and distance of droplet transport.Air primarily moves in response to adjacent pressuredifferences (highs to lows), heating (thermals) and cooling(drainage winds). As air flows over the surface it is deflectedby obstacles and slowed by friction. Consequently, at lowlevels it breaks down into a series of eddies that lead torapid fluctuations in wind speed and direction.Winds most relevant to pesticide application are:n synoptic winds – horizontal surface winds, initiated by airflow from high to low pressure;Common spray guidelinesn Spray only when wind direction is away fromsensitive areas (buffers may vary this requirement).n During the day, aim to spray when wind speed ismore than 3 and up to 15km/h.n Do not spray if wind is calm (less than 3km/h), orlight and variable.High wind speed (above 15 km/h) can lead to highvolumes of downwind drift with highest concentrationsclose to the target and lowest concentrations furtherdownwind. High wind speeds are the primary cause ofloss and waste of product from the target site.Moderate wind speed (5 to 15km/h) leads to optimaldroplet deposition to target and least drift potential.Low wind speed (less than 5km/h) tends to beassociated with unpredictable wind direction and verylittle turbulence unless thermals exist.Also refer to page 22 – Precautions for night spraying.FIGURE 4 The mean sea-level pressure pattern (synoptic chart) on the left generates surface(10 metres above the ground) winds as illustrated by the chart on the rightSOURCE: BUREAU OF METEOROLOGY

Weather Essentials for Pesticide Application | 11 | GRDCn local winds – induced by topographic features. Localwinds include katabatic (drainage winds), anabatic (uphillwinds), sea and land breezes, and winds that flow parallelto topographic barriers due to blocking or deflection byobstacles; andn vertical winds – thermals and anti-thermals.FIGURE 5 Drainage windsKatabatic windFIGURE 6 USynoptic windsWind forecasts are primarily based on pressure gradientsindicated by isobars. Isobar orientation and spacing reflectwind direction and speed. Closer isobars indicate strongerwinds; isobars far apart indicate weak winds (Figure 4).Air outside tropical regions rarely flows directly from highto low pressure but spirals anticlockwise out of highs andclockwise into lows (southern hemisphere, reverse in thenorthern hemisphere).The pressure/wind relationship can be seen by comparingisobar configuration to winds (Figure 4). These charts areavailable from the Bureau of Meteorology.FIGURE 5 Drainage windsLocal windsLocal winds refer to any variation from winds expected of thegeneral weather pattern. Katabatic Local wind wind flows are caused by:n changes in terrain and landscape – katabatic (drainagewinds), anabatic (uphill winds) and blocked winds (raisedterrain halts and deflects wind flow) (Figures 5, 6 and 7);n variation in surface temperature – sea breezes, landbreezes and thermals (Figures 8, 9 and 10); andn turbulence (Figure 11).In stable conditions, generally experienced from eveningto mid-morning, local winds often dominate and can bequite different in speed and direction to the synoptic winds.During a surface temperature inversion, katabatic or drainage winds often occur,With the exception of sea breezes, local wind flows arewith air flowing to the lowest point in the catchment. Drainage flow can bedeflected usually by least obstacles pronounced in a similar in fashion unstable to water conditions. flowing downhill. Thesewinds Local can develop winds over are the usually slightest on of slopes. a small scale, are moreDrainage pronounced winds can after carry sunset concentrated and are airborne short pesticides lived in long comparisondistances,which to synoptic can result winds.widespread pesticide damage considerable distances fromthe application site.Not blockedBlocked flowBlocked flowSOURCE: GRAEME TEPPERFIGURE 7 Blocked windsMeasuring local windsSpray applicators need to identify variations of windflow in order to protect downwind receptors. Becauselocal winds are short-lived and variable, frequentin-paddock measurements are required to trackvariations.While sophisticated instruments can be used totrack variations, simple methods are often useful.Ribbons attached to the end of the boom or onfencelines, smoke pots or a smoking device fitted tothe tractor’s exhaust can all help indicate wind strengthand direction. In turn, such observations indicate ifBlockedthewindsatmosphereoccur upwindhasofbecomeobstacles whenstablecool(seedenseAtmosphericair cannot flow overthem (usually a temperature inversion exists). The winds tend to be very localisedStability) and the wind has become less turbulent.due to vegetation (e.g. tree belts), levy banks and buildings, or they can be morewide-reaching Handheld due wind to hills meters and major should changes be in held topography. at the They height tend of to flowalong the contours boomspray and can carry for wind concentrated measurement. airborne pesticides long distances,which can result in long distant drift and widespread damage.FIGURE 9 Land breezesSOURCE: GRAEME TEPPERDuring a surface temperature inversion, katabatic or drainage winds often occur,with air flowing to the lowest point in the catchment. Drainage flow can bedeflected by obstacles in a similar fashion to water flowing downhill. Thesewinds can develop over the slightest of slopes.Drainage winds can carry concentrated airborne pesticides long distances,FIGURE which can 5 result Drainage widespread winds pesticide damage considerable distances fromthe application site.FIGURE 76 Blocked Uphill winds windsNot blockedBlocked flowAnabatic windBlocked flowSOURCE: GRAEME TEPPERDuring a surface temperature inversion, katabatic or drainage winds often occur,with air flowing to the lowest point in the catchment. Drainage flow can bedeflected by obstacles in a similar fashion to water flowing downhill. TheseIn winds the early can develop morning over and the evening, slightest when of slopes.absorb maximum sunshine andare Blocked Drainage considerably winds winds occur can warmer carry upwind than concentrated of adjacent obstacles airborne air, when anabatic cool pesticides dense winds air long develop cannot distances, that flow hug overthem which slope (usually can and result a flow temperature in uphill. widespread Pesticide inversion pesticide clouds exists). damage captured The winds considerable the tend anabatic to distances be very winds localised from canaffect due the application to vegetation site. on (e.g. the tree upward belts), slope. levy banks and buildings, or SOURCE: they can GRAEME be moreTEPPERwide-reaching due to hills and major changes in topography. They tend to flowFIGURE along contours 7 Blocked and can carry winds concentrated airborne pesticides long distances,which can result in long distant drift and widespread damage.SOURCE: GRAEME TEPPERIn the early morare considerablthe slope and flaffect vegetatioFIGURE 8 SIn the early morare considerablthe Sea slope breezes, and wflaffect override vegetatio or reina surge of coldand Kalgoorlie,applications bycan shift large vFIGURE 98 Land Sea breezesFIGURE 10Land breeze circulationBlocked winds occur upwind of obstacles when cool dense air cannot flow overSea them breezes, (usually which a temperature occur most inversion often from exists). mid-morning The winds tend to late to afternoon, be very localised canoverrideLanddue tobreezesvegetation or reinforcetypically(e.g. synoptic treeoccurbelts),atwindnight.levy flow.Theybanks Occasionally,occurand buildings,whenseawarmerbreezes or theyaircanovergenerate bethemoreawaterwide-reaching surgesurfaceof coldrisesdue air that toandhills can‘landand penetratebreezes’major changes wellmoveinlandin totopography. (e.g.replaceRenmark,it, settingThey South tendup ato Australia, flowandcirculation.along Kalgoorlie, contoursLandand Westernbreezescan carry Australia).haveconcentratedthe potentialThese inland airborneto carryincursions pesticidesconcentratedmay long affectairbornedistances, sprayapplicationspesticideswhich cantoresult bywaterinducing longsurfaces.distant rapidInwind driftcombinationand speed widespread andwithwindseadamage. directionbreezes,changes.they canTheycanrecirculateshift largeairbornevolumespesticidesof airbornebetweenpesticidesland andlongwaterdistances. SOURCE: GRAEME TEPPERsurfaces.Thermals a surge of are cold drto and form Kalgoorlie, as the sand applications weaken by asby can sunset, shift large unles vabove. The upwtemperature ofcool air above lehigh into the atmFIGURE 9 Land breezes SOURCE: GRAEME TEPPERFIGURE 10FIGURE 10 ThermalsKatabatic windNot blockedBlocked flowBlocked flowSee breeze circulationFIGURE 6 UFIGURE 8 SSea breezes, woverride or rein

ds often occur,can beill. Theseistances,tances fromE: GRAEME TEPPERDuring a surface temperature inversion, katabatic or drainage winds often occur, In the early morning and evening, when slopes absorb maximum sunshine andIn with the air early flowing morning to the and lowest evening, point when in the slopes catchment. absorb Drainage maximum flow sunshine can be and are considerably warmer than adjacent air, anabatic winds develop that hugare deflected considerably by obstacles warmer in a than Weather similar adjacent fashion air, Essentials to anabatic water flowing winds downhill. for develop Pesticide These that hug Applicationthe slope and flow uphill. Pesticide clouds captured in the anabatic winds canthe winds slope can and develop flow over uphill. the Pesticide slightest clouds of slopes. captured in the anabatic winds can affect vegetation on the upward slope.SOURCE: GRAEME TEPPERaffect Drainage vegetation winds can on carry the upward concentrated slope. airborne pesticides long SOURCE: distances, GRAEME TEPPERwhich can result in widespread pesticide damage considerable distances fromthe application site.GRDC | 12 |FIGUREFIGURE76BlockedUphill windswindsFIGURE 8 Sea breezesNot blockedAnabatic windBlocked flowSee breeze circulationSOURCE: GRAEME TEPPERFIGURE 8 Sea breezesFIGURE 11 TurbulenceLaminar flow(no turbulence,no mixing)Turbulent flow(erratic motionand mixing)See breeze circulationTurbulenceIt consiststurbulenceEither frictithe air flowdirection ofBlocked flowWindTurbulencenight and eds often occur,w can benot ill. These flow overe very localisedistances, can be morestances tend to flow fromg E: distances, GRAEME TEPPERE: GRAEME TEPPERIn Sea the breezes, early morning which occur and evening, most often when from slopes mid-morning absorb maximum to late afternoon, sunshine can andBlockedare override considerablywinds or reinforce occurwarmer synoptic upwindthanof wind adjacentobstacles flow. air, Occasionally, whenanabaticcool densewinds sea air breezes developcannot generate thatflowhugoverthemthe a surge slope(usually of and cold aflow air temperature that uphill. can Pesticide penetrate inversionclouds well exists). inland capturedThe (e.g. windsin Renmark, thetendanabaticto be South verywinds Australia, localisedcanand Kalgoorlie, Western Australia). These inland incursions may affect sprayFIGURE dueaffecttovegetationvegetationon11 Turbulence (e.g.thetreeupwardbelts),slope.levy banks buildings, or SOURCE: they can GRAEME be more TEPPERwide-reaching applications by due inducing to hills rapid and major wind speed changes and in wind topography. direction They changes. tend to They flowalong can shift contours large volumes and can Turbulent carry of airborne concentrated flow pesticides airborne long distances. pesticides long distances,which can result in long (erratic distant motion drift and widespread damage. SOURCE: GRAEME TEPPERLaminar flowand mixing)SOURCE: GRAEME TEPPER(no turbulence,FIGURE FIGUREno mixing) 98 Land Sea breezes breezesFIGURE 10 ThermalsPossible ‘erratic’ pathSea breezes, of a small which droplet occur or most particle often from mid-morning to late afternoon, canoverride or reinforce synoptic wind flow. Occasionally, sea breezes generatea surge Eddies of cold of air varying that can size penetrate do the mixing well inland (e.g. Renmark, South Australia,and Kalgoorlie, Western Australia). These inland incursions may affect sprayapplications by inducing rapid wind speed and wind direction changes. Theycan shift large volumes of airborne pesticides long distances.Turbulence determines the concentration and dilution of airborne pesticides.SOURCE: GRAEME TEPPERIt consists of rolling motions (eddies) in the airflow; larger rolls give greaterturbulence and greatest dispersion of airborne pesticides.FIGURE 10 ThermalsEither friction or thermals cause turbulence. Friction at the surface retardsthe air flow, causing mechanical turbulence; rising thermals interrupt the generaldirection of air flow causing thermal turbulence.A lack of tunear the suThe risk ofIntense turhorizontallysmaller parbe disperseeliminatingWindTurbulence is regulated by atmospheric stability and is usually suppressed atnight and enhanced during the day (see Atmospheric Stability, page 8).irnotoverflowtheoveregveryup alocalisedyairbornecan be moreytendcanto flowg distances,E: GRAEME TEPPERE: GRAEME TEPPERPossible ‘erratic’ path See breeze circulationof a small droplet or particleEddies of varying size do the mixingLand breeze circulationThermals Sea breezes, are which drafts of occur warm most air often rising from from mid-morning the ground. These to late air afternoon, currents begin canto override form as or the reinforce sun rises. synoptic They wind intensify flow. as Occasionally, the surface heats sea breezes up during generateLand the dayand a surgebreezesweaken of coldtypicallyas air it cools thatoccurcan during penetrateat night. Theylate afternoon well inlandoccur when evening. (e.g. Renmark,warmer airThey are Southoverextinguished Australia,thewaterby and sunset, Kalgoorlie,surface risesunless Westernand ‘landthe surface Australia).breezes’retains Thesemoveenough inlandin toheat incursionsreplace it, setting up acirculation. Land breezes have the potential to carry to concentrated remain may hotter affectairborne than spray the airpesticides above. applications The to upward by inducingwater surfaces. speed rapid of thermals wind speedIn combination depends and windwith on sea surface directionbreezes, heating changes.they and can the Theyrecirculate temperature can shift largeairborne of volumes the overlying of airbornepesticides atmosphere. pesticidesbetween land High longand surface distances.water temperatures surfaces. with verycool air above leads to vigorous thermals capable of lifting airborne pesticideshigh into the atmosphere.FIGURE 10 ThermalsSOURCE: GRAEME TEPPERSOURCE: GRAEME TEPPERSOURCE: GRAEME TEPPERA lack of turbulence leads to airborne pesticides floating at high concentrationsnear the surface. This is the situation during a surface temperature inversion.The risk of spray drift is high.Intense turbulence leads to rapid dispersion of airborne pesticides bothhorizontally and vertically, taking larger particles down to the surface and liftingsmallerThermalsparticlesare draftshighofintowarmtheairatmosphere.rising fromSpraythe ground.drift willTheseoccurairbutcurrentsparticlesbeginwillbeto formdispersed,as thedilutingsun rises.theTheypesticideintensifyconcentrationas the surfaceand reducingheats upbutduringnotthe dayeliminatingand weakentheasriskit coolsof off-targetduring latedamage.afternoon and evening. They are extinguishedSOURCE: GRAEME TEPPERby sunset, unless the surface retains enough heat to remain hotter than the airabove. The upward speed of thermals depends on surface heating and thetemperature of the overlying atmosphere. High surface temperatures with verycool air above leads to vigorous thermals capable of lifting airborne pesticideshigh into the atmosphere.SOURCE: GRAEME TEPPERir over theg up aairborney canE: GRAEME TEPPERThermals are drafts of warm air rising from the ground. These air currents beginto form as the sun rises. They intensify as the surface heats up during the dayand weaken as it cools during late afternoon and evening. They are extinguishedby sunset, unless the surface retains enough heat to remain hotter than the airabove. The upward speed of thermals depends on surface heating and thetemperature of the overlying atmosphere. High surface temperatures with verycool air above leads to vigorous thermals capable of lifting airborne pesticideshigh into the atmosphere.SOURCE: GRAEME TEPPERSmoke initially with laminar flow before breaking into turbulent fow.

Weather Essentials for Pesticide Application | 15 | GRDC3. TemperatureTemperature has a direct influence on the uptake ofpesticides and on the rate at which pesticides volatilise fromsoil and plant surfaces (see Volatilisation, page 16). It alsohas an indirect influence on the evaporation of the aqueouscomponent of pesticide droplets.Temperature variation with altitude dictates the stabilityof the atmosphere. This, in turn, affects the potential forairborne pesticides to dilute or remain concentrated in theatmosphere.Temperature and pesticide applicationAs temperature increases:n droplets evaporate faster;n turbulence intensity generally increases;n volatile pesticides on soil and plant surfaces vaporisefaster; andn the potential for thermals to lift airborne pesticides into theatmosphere increases.Surface to air temperature variationsSurface temperature variations compared to the air justabove can be extreme. With strong sunlight, the surfacetemperature of bare ground can be more than 20ºC higherthan the air temperature 1.25m above the ground, thestandard height of temperature measurement by the Bureauof Meteorology.Radiation to clear skies at night can lead to surfacetemperature being several degrees cooler than averagetemperatures quoted by the Bureau of Meteorology.Besides incident solar radiation, the rate of cooling andheating at the surface is dictated by the nature of surfacevegetation, soil type, moisture and wind speed.A common guideline is to spray only when airtemperature is less than 30ºC. However, soil temperature,especially in summer with sparse vegetation, can beconsiderably hotter than the air.The total effects of applying herbicides to sparsevegetation over hot soils may not yet be fully determined butfrom a meteorological point of view the concerns include:n the possibility of the rapid evaporation of dropletsapproaching the surface and becoming highly drift prone,especially smaller droplets that approach the surfaceslowly; andn larger droplets normally expected to reach the surfacebeing captured and lifted by strong thermals, leading tolarger droplets than expected becoming drift prone.One solution is to use the largest droplets possiblewhile maintaining efficacy and to operate the machine in aconfiguration and at a speed to minimise the production ofdrift-prone droplets.FIGURE 14 Temperature profiles illustrating the effects of vegetation and bare ground on the variability of the verticaltemperature profile within the first few metres of the surface. Note that the air temperature at 1.25m barely changeswhile surface temperatures change a great deal in the six-hour period.Height above ground (m)2.0SunriseMidday1.51.0Standard heightfor temperatureforecasts andobservations0.506 8 10 22 24 26Air temperature ºCIrrigated sugarcane Millet Bare ground28 30 32 34 36SOURCE: ADAPTED BY GRAEME TEPPER AFTER LA RAMDAS et al

Weather Essentials for Pesticide Application | 17 | GRDC4. Relative humidityThe rate at which spray droplets evaporate is determinedby the amount of water vapour that the air can absorb.The absorption potential is dictated by the amount of watervapour already in the air and the temperature of the air.Relative humidity (RH) is the ratio of the actual amountof water vapour in the air to the amount it could hold whensaturated. The air’s capacity to hold water vapour increasesas air temperature increases. At 30ºC, the capacity is morethan three times that at 10ºC; consequently, while theamount of water vapour in the air may be static, the relativehumidity decreases as temperature increases (Figure 17).A better indicator of the rate at which pesticide dropletsevaporate is Delta T. Delta T is the difference between thewet and dry bulb temperatures. It combines the effects oftemperature and relative humidity (Figure 18).FIGURE 17 A depiction of one air parcel undergoinga temperature change while the amount of moistureremains the same30ºC10ºCactualwater vapour20ºCactualwater vapouractualwater vapourFor example, the evaporation potential is about thesame for:n 20ºC and 38% RH and a Delta T of 8; andn 30ºC and 50% RH and a Delta T of 8.The amount of water vapour in the air usually onlychanges gradually over days, while temperature changesat a comparatively rapid rate. This leads to relative humidityvalues being higher and Delta T values lower overnight thanduring the day.Relevance for sprayingHow rapidly the aqueous component of pesticide dropletsevaporate influences the potential for spray drift.If the spray droplets reduce to less than about 150µmdiameter they can be transported over long distances by aircurrents before being deposited (JB Unsworth et al, 1999).Partial evaporation of the aqueous component ofdroplets before reaching the target leads to smaller andmore concentrated droplets, slower fall rates and highpotential for drift.Total evaporation of the aqueous component of dropletsbefore reaching the target leads to airborne gaseous andparticulate residues of the active component (and additives)floating in the atmosphere. The initial residues are highlyconcentrated forms of the active pesticide. If not adequatelydispersed by turbulence, they can lead to damaging drift.No evaporation of the aqueous component of smalldroplets can lead to unacceptably long life and longdistant drift.Very small droplets, vapour and particle residuesRH = 100% RH = 52%RH = 28%Delta T = 0 Delta T ~ 6Delta T ~ 13A depiction of one air parcel undergoing a temperature change while theamount of moisture remains the same.SOURCE: GRAEME TEPPERremaining airborne for aconsiderable time are causeFIGURE 18 The relationship of Delta T to relative humidity and temperature. A commonspray guideline is to spray when Delta T is between 2 and 8; with caution to 10 for concern beyond immediatespray drift issues. TheseRelative humidity (%)100Delta T value residues are finally bought backto the surface or incorporated902 in fog where they may remerge80into the atmosphere after4fog dissipation. The primary706 pathways for the return to the60surface of residues are:8n interception/impaction5010 (capture by obstacles or4012raised terrain);n rainout (washout);30n in rain (within droplets);20n fog (droplet settling); andn air currents (downward flow).100 5 10 15 20Dry temperature ºC25 30 35 40Preferred Delta T conditions for spraying Marginal Delta T conditions for sprayingUnsuitable Delta T conditions for spraying Conditions are marginal for COARSE or greater spray qualityand unsuitable for medium or finer spray qualitySOURCE: Adapted by Graeme Tepper from Nufarm’s SprayWise Decisions chart (2012)

GRDC | 18 |Weather Essentials for Pesticide Application5. Surface temperature inversionsAustralian agricultural regions often experience adversedaytime spraying conditions. To avoid them, manyapplicators resort to evening, overnight and early morningapplications because temperature, humidity and winds areoften most favourable at such times. However, the veryfact that temperature, humidity and wind conditions easeto acceptable levels in the evening and overnight oftenindicates the onset of a surface temperature inversion.Many new product labels specifically forbid sprayingduring surface inversions.What is a surface temperatureinversion?A surface temperature inversion consists of layers of theatmosphere near the Earth’s surface in which temperatureincreases with height. This is the inverse of the normaltemperature decrease with height (Figure 19).When the surface temperature inversion first forms, it isjust a few metres deep. With progressive overnight cooling,the depth of the surface temperature inversion rapidlyincreases to be at its maximum at about sunrise. At thistime, the inversion depth may be from a few tens of metresup to several hundreds of metres.Inversion weatherA surface temperature inversion can be likened to a domeof colder air over the ground, which is cut off and largelyinsulated from the surrounding atmosphere.Being isolated, the surface temperature inversiondevelops a micro-climate in which weather can besignificantly different from what might be expected from thebroader weather patterns. A surface temperature inversioncauses the synoptic winds to decouple from the surface;they continue to flow over the top of the inversion. Within theinversion, winds reduce to calm or develop as local winds –usually drainage winds.Key elements of weather within a surface temperatureinversion are:n the onset of an inversion often leads to a significant dropin wind speed as sunset approaches, which signals thedecoupling of the general winds from the surface;n drainage winds can flow downslope and/or aroundobstacles with directions dictated by topographic featuresrather than the overriding general weather pattern;n during the night, winds within the inversion turn to flowanticlockwise (in the southern hemisphere);n a surface temperature inversion is a prerequisite to fogand frost development; andn the decay of an inversion can be accompanied bywinds stronger and gustier than the winds experiencedthroughout the day.The hazards of a surfacetemperature inversionSurface temperature inversion conditions are unsafe forspraying as the potential for spray drift is high.Under a surface temperature inversion:n air movement tends to be laminar (not turbulent), so theUnder a surface temperature inversion air can separate into very stable layers (laminates) that can concentrateand transport airborne pesticides away from the target.Photo: Bill Gordon

Weather Essentials for Pesticide Application | 19 | GRDCair does not mix in the same way as during the day;n airborne droplets, vapours and particulates can remainconcentrated in the inversion layer for long periods of time;n the direction and distance pesticides movement is veryhard to predict;n the movement of airborne droplets will vary depending onthe landscape; andn droplets or their remnants can move next to the surfaceand concentrate in low lying areas or float just above thesurface (Figure 20).Research supported by the GRDC is further investigatingthe development and implications of temperature inversionsin relation to spray application.FIGURE 19 Typical vertical temperature profiles for a point in time during the night and day. The day profile typically coolswith height and the night profile typically warms with height to a depth which constitutes the surface temperatureinversion layer. The point where the temperature stops increasing is the top of the surface temperature inversion layer.Daytime temperature profileNight-time temperature profileCooling with height(about –1ºC/100m)Inversion topHeightWarmingwithheightDepth of surfacetemperatureinversionInversion baseCoolWarmSOURCE: GRAEME TEPPERFIGURE 20 Possible movement of airborne droplets in surface temperature inversion conditionsALight windsA: High concentrations of airborne pesticides can float justabove the surface and be carried away by light winds wherethey may impact on surfaces above the level of application.BDrainage windsB: Drainage winds can carry high airborne pesticidesdownslope into lower lying regions where they mayconcentrate.CLight windsBase pf erpdomg omversopmC: When winds are light and a surface inversion is in decayairborne pesticides can be caught up in a complex thermaland wind shear situation causing them to be transported indifferent directions than indicated by surface winds.ThermalsLight windsLegendFine droplets, particles and vapour Medium droplets Coarse dropletsSOURCE: GRAEME TEPPER

GRDC | 20 |Weather Essentials for Pesticide ApplicationWhen and why does a surfacetemperature inversion occur?A surface temperature inversion usually develops early in theevening, growing overnight and decaying after sunrise (seeLifecycle, page 21).It can result from a number of processes that cause theair closest to the ground to become cooler than the airabove. The three main reasons experienced in broadacreagriculture are as follows.1. Radiation inversion – created byradiation coolingA radiation inversion can form at any time during the nightwhen wind speed is less than 11km/h and cloud cover doesnot severely restrict surface cooling. In calm and clear skyconditions, they may form just before sunset. Formation isoften accompanied by a significant drop in wind speed inthe evening. The drop is caused by the synoptic wind beingdecoupled from the surface wind.After sunset, heat radiates back into space, causing theground to cool. In turn, the air in contact with the groundbecomes cooler than the air higher up. This generates thesurface temperature inversion.Some soils retain heat after sunset and only cool slowly.The retention of heat, especially over sparsely vegetatedground, can complicate inversion formation. An inversioncan occur within 20cm of the soil surface but not actuallyreach the surface due to very small-scale thermals drivenby the warm earth. This situation is most likely in the earlyevening in calm conditions. While not well documented orunderstood, pesticides could become concentrated and driftin this low-level inversion, especially if vegetation cover issparse.Radiation inversions are the most dangerous for sprayingoperations as they cause airborne droplets to remainconcentrated at a low level for long periods. Winds withinthe inversion can carry airborne pesticides long distances.2. Advection inversion – created by cool or warmair movementCooler, denser air can move into an area and slide underlayers of less dense, warm air.This can happen when a cold front moves into anarea or a sea breeze pushes cooler air inland. It can alsohappen when denser cool air moves down a slope andslides underneath layers of warm air in lower parts ofthe catchment. If this occurs, the intensity of a radiationinversion increases.Warm air can move over cool surfaces; some of the airclosest to the ground becomes cooler while the higher airstays warmer.3. Vegetation inversionVegetation and crops can shade the ground underneaththem. The air in contact with the ground will stay coolerthan adjacent areas where there is less ground cover.This often occurs just after sunrise. The air moving abovethe vegetation or crop may be warmer than the air in thevegetation. This can allow airborne droplets to travel over,rather than through, vegetation.Transpiration from a dense crop canopy on a hot day canform a cool layer of air just above the crop. Later in the day(when wind speeds tend to reduce) this layer of cooler aircan act like an inversion over the crop, making penetrationof smaller spray droplets into the canopy difficult andincreasing the risk of off-target movement.Transport of airborne pesticides within asurface temperature inversionTo understand the transport of airborne pesticides withina surface temperature inversion, a distinction needs to bemade between surface temperature inversions forming overflat terrain and those occurring over slopes.Over flat terrain a surface temperature inversion formswhen air in contact with the ground is cooled by terrestrialFIGURE 21 Idealised air flow during the day typically increases in speed with height and flows up and over raisedterrain with stronger winds at hill top (left). When a surface temperature inversion exists, surface winds decouplefrom the surface and flow over the inversion with winds increasing 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 daytime wind profileTypical night-time wind profileHeightLonger arrowsrepresent greaterwind speedYellow area representsan inversion layerDrainage winds

Weather Essentials for Pesticide Application | 21 | GRDCradiation. The inversion gradually intensifies and deepens asthe surface cools. Air within the radiation inversion does nottend to flow out of the region but remains in place. Thus,airborne pesticides tend to float over the immediate area ofapplication unless local winds develop to carry them away.Over gentle-sloping terrain a surface temperatureinversion also forms when air in contact with the groundis cooled by terrestrial radiation. The cooled layer remainsquite shallow over the slope and is typically only 2 to 10mdeep because gravity continually pulls it downward, causingdrainage winds. Drainage winds moving cool air awayfrom the slope and over or into lower lying regions mayinitiate a ‘drainage inversion’ or intensify an existing surfacetemperature inversion. Drainage inversions, once formed,have similar attributes to surface temperature inversions.Airborne pesticides can be transported long distancesdownhill, over flat terrain toward the lowest lying regionsand into valleys by drainage winds (Figure 21).Lifecycle of a surface temperatureinversionA surface temperature inversion typically begins to form justbefore sunset and is strongest and deepest at the time ofminimum temperature, which is often just after sunrise.Decay typically begins just after sunrise (Figure 22) asthe Earth begins to warm and thermals begin to erode theinversion from the surface upward while the higher levelsof the inversion remain intact as a ‘capping inversion’. Inremnant capping inversion conditions, the potential for ablanket of concentrated pesticide to drift many kilometresis extremely high. If conditions at the end of the decay areunstable, pesticide may rise and diffuse into the atmosphere;if they are neutral, the pesticide will settle.Note: Figure 22 depicts a typical lifecycle, exceptionsoccur when transient cloud and fluctuations of wind speedvary the surface-cooling rate. For example, inversions maybegin or cease anytime in the night especially if the synopticFIGURE 22 An idealised development and decay of a surface temperature inversion with no wind and clear skies.The vertical temperature profile is depicted by the red line.3 pm5:15 pm 10 pm 6 am 7 am 9 am 10 am 11 amHigherSunset5:23 pm Sunrise6:31 amInversiontopCoolFigure 22InversionInversion capMixingWarmlayerAn idealised development and decay of a surface temperature inversion with no wind and clear skies. The vertical temperature profile is depicted by the red line.From left Reading to right: the sequence from left to right:3pm3pm in surface wind The speed observed. temperature The drop is profile mirrored by cools a compensating with increase height, in wind indicating speed near the a top freely of the inversion. mixing environment to some height well above the10pm to 6am The surface temperature inversion continues to deepen through the night.6.31am At sunrise as the surface. sun begins to heat the surface and the inversion begins to erode from the surface upwards by the action of thermals which are limited by the inversion cap. The often observed increase inwind speed during the early hours of the morning is related to the decay of the surface temperature inversion and the recoupling of the synoptic winds to the surface.5.15pm 7am to 10am As the base of the A surface little temperature before inversion sunset, lifts a the temperature depth of mixing increases inversion but remains forms capped by at the the a thinning surfaceand overhead inversion. a drop in surface wind speed observed.Post 10am The inversion no longer exists and the profile tends toward the previous day, assuming there have been no changes in the general weather system.The drop is mirrored by a compensating increase in wind speed near the top of the inversion.10pm to 6am The surface temperature inversion continues to deepen through the night.6.31am At sunrise as the sun begins to heat the surface and the inversion begins to erode from the surface upwards bythe action of thermals which are limited by the inversion cap. The often observed increase in wind speed duringthe early hours of the morning is related to the decay of the surface temperature inversion and the recoupling ofthe synoptic winds to the surface.7am to 10am As the base of the surface temperature inversion lifts the depth of mixing increases but remains capped by the athinning overhead inversion.Post 10am The inversion no longer exists and the profile tends toward the previous day, assuming there have been nochanges in the general weather system.The temperature profile cools with height, indicating a freely mixing environment to some height well above the surface. 5.15pm A little before sunset, a temperature inversion forms at the surfaceand a dropSOURCE: Graeme TepperSOURCE: Graeme TepperA smoke plume rises to the capping inversion before drifting in various directions farinto the distance. It remains relatively concentrated even in the distance.Photo: Terresa Aberkane (NZ)

GRDC | 22 |Weather Essentials for Pesticide Applicationwinds are increasing or decreasing as cold fronts approachor leave an area.Variations in the life of a surface temperature inversionrequires spray applicators to continuously monitor conditionswhile spraying at night to ensure that a surface temperatureinversion has not established.Recognising a surfacetemperature inversionWhile it is reasonable to expect surface temperatureinversions on most nights between sunset and up to an houror so after sunrise, forecasting the exact onset and cessationtimes is not simple. The difficulty arises because of a numberof factors, including land/water-surface variations and periodicinfluence of transient cloud, synoptic and local wind flowand intermittent turbulence on the variation between surfacetemperature and that of the air above the surface.The scientific method for detecting a surface temperatureinversion requires the accurate measurement of the verticaltemperature profile. On-farm, this is usually not practical, somost spray applicators must rely on visual and other clues.Visual cluesSurface inversions may exist without visual indicators butsome visual indicators are:n there is mist, fog, dew or a frost;n smoke or dust hangs in the air and moves sideways,just above the surface; andn cumulus clouds that have built up during the daycollapse towards evening.Precautions for night sprayingIt is not always mandatory to stop spraying when surfacetemperature inversions exist, however operators mustalways aim to avoid spray drift and follow label requirements.If spraying at night it is suggested these guidelines befollowed.1. CRITICAL – select spray quality and spray practices toeiminate all droplets less than 100µm.2. Do not spray in winds less than 11km/h between sunsetto 1.5 hours after sunrise.3. Select operating speeds not conducive to the lifting ofspray particles behind the machine.4. Keep boom height low to limit atmospheric exposure andinfluence on droplets.5. Do not spray when mist or fog are evident and/or Delta Tis less than 2.6. Use wind meters, flags or smoke devices for wind flowand stability reference.Demons of night sprayingn Small droplets.n Vapours and particulates.n Inversion trapping and concentration.n Drainage and other local winds.n Variable wind speed and direction.Other cluesA surface temperature inversion is likely to be present if:n wind speed is constantly less than 11km/h in the eveningand overnight;n cool, off-slope breezes develop during the evening orovernight;n distant sounds become clearer and easier to hear; andn aromas become more distinct during the evening thanduring the day.Spray applicators should always expect that a surfacetemperature inversion is most likely to have formed at sunsetand will persist for sometime after sunrise UNLESS one ormore of the following has occurred:n continuous overcast weather, with low and heavy cloud;n continuous rain;n wind greater than 10km/h for the whole time betweensunset and sunrise; andn after a clear night, cumulus clouds begin to form.

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