Research aims to take guesswork from spray conditions
GroundCover™ Supplement Issue: 122 | 02 May 2016 | Author: 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).
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.
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).
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.
MRES, 0429 309 508,
GRDC Project Code DAW00231
Was this page helpful?