The most effective strategy for conserving moisture during the summer fallow period is by effectively controlling summer weeds. Although there may be some key issues that you will encounter when applying chemicals during this period.

Surface Temperature Inversions

In cool night conditions airborne chemicals concentrate near the surface and winds can move the droplets away from the target.

During the day the soil temperature gradually increases, which allows the air that’s in contact with the soil to be warmed as well. This process caused a surface temperature inversion to be established.  This layer is capable of acting like a barrier which makes spraying conditions unsafe as there is a high risk of spray drift.

inversion layer

Inversion layer during day and night sourced from GRDC

During the night ground loses heat and the low level air cools as shown in the image above. This results in the air temperatures increasing with height and the temperature profile becomes inverted. When this process occurs close to the ground its called a surface temperature inversion. If you take another look at the image above you will see where the surface temperature has impacted upon the air temperature and where they meet is labelled as the inversion layer.

What can cause them?

There are a variety of different causes for temperature inversion layers.

These causes include:

1#Radiation Inversions: Radiation inversions are capable of forming at anytime during the night when the wind speed is less than 11km/h and cloud cover isnt severely restricting surface cooling. When the sun sets the heat radiates back into space, causing the ground to cool, the air in contact with the ground becomes cooler than the air higher up in the atmosphere. This is called a surface temperature inversion. Radiation inversions are the most dangerous when it comes to spray drift.

2# Advection Inversions: Advection inversions are caused when cooler air moves into an area and slides under layers of less dense warm air. This tends to happen when a cold front moves into an area or a sea breeze pushes cool air inland. It may also happen when cool air moves down a slope and slides underneath layers of warm air. This process intensifies the effects of radiation inversions.

inversion layer

3# Vegetation Inversions: Vegetation and crops have the ability to shade the ground under neath them. The air that comes in contact with the ground will stay cooler the adjacent areas where there is less ground cover. This often occurs just after sunrise. It is possible that the air moving above the vegetation of the crop may be significantly warmer than the air moving within the crop. This can allow airborne droplets to move over the vegetation instead of penetrating into it.

4# Transpiration: Transpiration from a dense crop canopy on a hot day can form a cool layer of air just above the crop. Later in the day this layer of cool air can act like an inversion over the crop, making the penetration of small spray droplets quite difficult.

How do I spot surface inversion?

  • Visual Clues
  • A surface temperature inversion is likely to be present if:
  • A mist, fog or dew has recently 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 the evening

Other Clues

  • 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 clearer and easier to hear
  • Aromas become more distinct curing the evening than during the day.

There are a variety of things that you can do to minimise spray drift. Make sure that you read the product labels and that your aware of sensitive areas in a paddock. It will also be beneficial to use larger droplet sizes in combination with an adjuvent that does not increase the amount of drift able fines produced by the nozzle.

If you have any further questions on spray drift make sure that you leave a comment below or you can contact us on facebook and twitter. For some additional information feel free to check out my GRDC reference source by clicking the link below.

Surface Inversions GRDC