All you need to know about inversion layers.

As you know, the best way to conserve soil moisture and nutrients during summer is to eliminate summer weeds from paddocks. However, summer spraying also comes with the risk of spray drift and inversion layers. So, what are they and how do we avoid them?

What is an inversion layer?

Inversion layers can happen all year round due to a range of causes, but for this topic, we’ll focus on the radiation type inversion or Surface Temperate Inversions which we see typically in summer.
Inversion layers are a layer in the atmosphere above the ground, in which the normal temperate gradient – cool air above, warm air below – is reversed.
Surface Temperature Inversions occur in summer when the air in contact with the soil cools quickly after sunset and becomes cooler than the warm air above it in the atmosphere. This results in a layer of cool air becoming trapped beneath a layer of warm air. This layer of cool trapped air can travel unpredictably and for miles, in a water-like flow. If you’ve ever gone for an evening walk and have felt cool air pool around you, you’ll know what an inversion layer feels like.

What will happen if I spray with an inversion layer present?

If you spray during an inversion (during the night or early morning), or when an inversion is likely to occur (late evening), fine droplets of the chemical can become concentrated in the cool layer near the ground and become isolated from the surrounding weather conditions. These droplets can then become trapped within the inversion layer for long periods and be carried away. The direction and distance in which the droplets move is unpredictable and the chemical can be carried miles off target. Movement can also depend on landscape, as the layered air can tend to drain to lower levels.

How do I know if an inversion layer is present or likely to happen?

There are a few things to be aware of to evaluate whether an inversion layer is likely to be present.

In the morning:
– If mist, fog, dew or a frost have occurred
– Smoke or dust hangs in the air and moves sideways

During the afternoon:
– Cumulus clouds that have built up during the day collapse towards evening
– Distant sounds become clearer and easier to hear
– Scents and aromas become more distinct than during the day

During the evening:
– Wind speed is constantly less than 11km/h in the evening and overnight
– Cool, off-slope breezes develop during the evening or overnight

When can I spray again?

Surface temperature inversions are likely to have dissipated after about two hours after sunrise. Make sure the air temperature has risen by more than 5°C above the overnight minimum, and that wind speed has been constantly above 7km/h for more than 45 minutes after sunrise.

You should always expect a surface temperature inversion has formed at sunset and will continue until after sunrise, unless one or more of the following has occurred:
– Continuous overcast weather with low heavy cloud
– Continuous drizzle or rain
– Wind speed is greater than 11km/h for the whole time between sunset and sunrise
– After a clear night, cumulus clouds begin to form

It’s important to be vigilant and aware of these risks if spraying early morning and late evening. Always check labels to see whether any spray drift restraints apply, especially with 2,4-D products this summer.
Pay attention to weather forecasts and conditions in your area, particularly local wind speed. If spraying in the evening, make sure to check wind speed every 15 to 20 minutes. Use a coarse spray quality and correct nozzles to minimise fine droplets. We urge growers to use common sense and try to mitigate the risk of spray drift as much as possible.

**For a great video demonstrating how fine droplets can spread, watch this youtube clip by Mary O’Brien: https://www.youtube.com/watch