Research aims to take guesswork from spray conditions

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Weather station

A weather station located at Katanning, WA, used in the inversion research. Installing the kind of weather station required to accurately identify inversion conditions is not always practical for the majority of growers.

PHOTO: Graeme Tepper 

Identifying when a surface temperature inversion is present is difficult, and even when this is possible it may not tell the spray operator if conditions are really suitable for spraying

Most growers are aware of the label spray-drift restraints that prevent them from spraying under surface temperature inversions, but few people have the ability to accurately measure when one is present.

When a surface temperature inversion is present the air temperature close to the ground will be marginally cooler than the temperature measured at a height several metres above the ground. This is the opposite of what normally occurs during the day, when temperature reduces with height, which is why it is called an inversion.

The traditional methods for assessing if a surface temperature inversion is likely to be present include observation of visual clues or measuring the difference in air temperature at two heights using a vertical tower with very sensitive temperature sensors.

It is not practical to set up towers on-farm with the kinds of instruments required to measure very small differences in the vertical temperature profile, so growers tend to rely on visual observations that indicate if a surface temperature inversion is likely to be present (Figure 1).

Smoke spanning the landscape

Figure 1 The movement of smoke across the landscape during surface temperature inversion conditions.

SOURCE: Bill Campbell

Inversion clues

A surface temperature inversion is likely to be present if:

  • mist, fog, dew or a frost has occurred;
  • smoke or dust hangs in the air and moves sideways just above the ground;
  • cumulus clouds that have built up during the day collapse towards evening;
  • wind speed is consistently less than 11 kilometres per hour in the evening and overnight;
  • cool, off-slope breezes develop during the evening or overnight;
  • distant sounds become clearer and easier to hear; and
  • aromas become more distinct during the evening than during the day.
Surface temperature inversions are dangerous conditions for spraying as the potential for spray drift is too high.
Under a surface temperature inversion:
  • air movement tends to be laminar (not turbulent), so the air does not mix in the same way as it does during the day;
  • airborne droplets, vapours and particulates can remain concentrated in the inversion layer for long periods of time;
  • the direction and distance that pesticides may move is very hard to predict;
  • the movement of airborne droplets will vary depending on the landscape; and
  • droplets or their remnants can move next to the surface and concentrate in low-lying areas or float just above the surface (Figure 2).
Cartoon drawing of tractor spraying farm

Figure 2  Possible movement of airborne droplets in surface temperature inversion conditions.

SOURCE: Graeme Tepper

One of the keys to identifying suitable conditions for spraying is to know when the air is moving and mixing enough to improve deposition and to dilute any spray droplets that may remain in the air after release from the sprayer.

While there is heat being radiated from the ground surface and sufficient air movement, the airflows over the surface will tend to be turbulent. This mixes the air closest to the ground and helps to bring airborne droplets back towards the surface.

GRDC-funded research in Western Australia, conducted by MicroMeteorological Research and Education Services (MRES) and managed by the Department of Agriculture and Food, WA, has demonstrated that many automated weather stations can be modified to collect additional information that will help to identify if a surface temperature inversion is present and, more importantly, if it is safe to spray.

While it is still early days with this research and the results need to be further tested in more complex landscapes, the techniques and instrumentation developed show promise for better defining suitable conditions for spraying.

Spray application Grownote e-book on the way

Weather-monitoring equipment and drift-management strategies are just two of the modules in a new GrowNote e-book on spray application for grain growers that has been funded by the GRDC. The e-book, which is scheduled for release later this year, is being compiled by Bill Gordon and is based on contributions from many experts and consultants working in the field of spray application.

The 24 modules include planning spray jobs, improving set-ups and efficiencies, equipment checks, the different spraying systems available, and considerations for upgrading or purchasing new spray equipment.

Each module will be supported by a series of short videos covering tips and tactics relevant to the equipment being discussed.

“We hope the information provided will be clear and concise. It should provide readers with an understanding of how the various types of spraying systems work, and most importantly things to consider when setting them up, so operators can achieve the best outcomes for each spray job,” Bill Gordon says.
More information:
Bill Gordon,
0429 976 565,


More information:

Graeme Tepper,
MRES, 0429 309 508,

Weather essentials for pesticide application

Surface temperature inversions


App provides snappy spray assessment

GRDC Project Code DAW00231

Region West