Ground Cover Supplement : GC Supplement - Spray application 2016
22 Surface temperature inversions Research aims to take guesswork from spray conditions 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 By Graeme Tepper 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). 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 PHOTO: GRAEME TEPPER 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.
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