Recognising surface temperature inversions
Understanding complex weather conditions such as surface temperature inversions can help spray applicators avoid spray drift.
What is surface temperature inversion
Surface temperature inversions occur when air temperature increases with height from the ground surface, which is the opposite of what normally happens (i.e., the temperature profile is 'inverted'). This results in a layer of cool, still air being trapped below warmer air. The height above the ground where the temperature stops increasing and begins to decrease is the top of the inversion layer.
If pesticides are sprayed during an inversion, fine droplets of the chemical can be concentrated in the cool layer near the ground and isolated from the surrounding weather conditions (Figure 1). The direction and distance which the droplets will then move becomes unpredictable and the chemical may be transported away from the target area.
Factors such as landscape, slope, surface breezes and how and when the inversion breaks up will determine where the pesticide droplets finally land.
As a result, it is unsafe to spray when conditions favour surface temperature inversions, due to the potential for spray drift.
Recognising a surface temperature inversion
Inversions are most likely to occur between sunset and two hours after sunrise, so any sprays applied during this period are likely to be affected.
Although some inversions can be easily noticed (see photo on right), they may not be visible even during daylight hours, which can make it difficult for chemical users to identify them and reschedule spraying.
The precise measurements needed to confirm the presence of an inversion are usually not practical on the farm, but indicators of an inversion can be provided by conditions in the area.
A surface temperature inversion is likely to be present if:
- mist, fog, dew or a frost have occurred
- smoke or dust hangs in the air and moves sideways, just above the surface
- cumulus clouds that have built up during the day collapse towards evening
- there is a large difference between the observed maximum and overnight minimum temperatures
- wind speed is constantly less than 11km/h in the evening and overnight
- cool, off-slope breezes develop during the evening or overnight
- distant sounds become easier to hear
- aromas are more distinct during the evening than during the day.
Smoke pots and smoking devices fitted to a sprayer's exhaust can help indicate if the atmosphere has become stable or the wind has become less turbulent, which are strong indicators that a surface temp inversion may have formed. Other tools such as on-board weather stations, or flags placed in the line of sight, can indicate if the wind speed has dropped. Spraying should cease if the signs point to an inversion forming.
How long will a surface temperature inversion persist?
Surface temperature inversions are likely to have dissipated after about two hours from sunrise if the air temperature has risen by more than 5°C above the overnight minimum and wind speed has been constantly above 7km/h for more than 45 minutes after sunrise.
However, before spraying after an overnight inversion, it is advisable to check that the conditions listed above are no longer present.
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