In the past, we were always told to worry about strong winds when spraying. We now know that low wind conditions can be as bad or worse.
New and reviewed herbicide labels since 2008 state that you must not spray during a surface temperature inversion.

What is a surface temperature inversion?

At sunset, the ground loses heat and under low wind conditions, air close to the ground cools, while the air above is warmer. The air doesn’t mix and is described as being stable. Air temperature increases with height compared with during the day when air temperature most often decreases with height.
When this occurs close to the ground, it is called a surface temperature inversion .
Typical Inversion profiles during night and day - Graeme Tepper

Typical inversion profiles during night and day. In an inversion, the air warms to the top of the inversion. Source: G Tepper
In a surface temperature inversion, the point where the temperature stops increasing and begins to decrease is the top of the inversion layer. This is usually the top level of suspended particles such as dust, smoke and small spray droplets or vapour.
Note that these inversions can act as a barrier to regional winds which go over the top of the inversion preventing mixing of the air. This means that inversions can co-exist with higher wind speeds, which can be greater than 20 kilometres per hour.

Rising smoke from wood heaters moving parrallel Image by AGRONOMO

About 30 minutes after sunrise smoke from wood heaters not mixing and moving parallel to the ground due to a surface temperature inversion. Image: AGRONOMO

What is the big deal with surface temperature inversions?

In November-December 2015, large areas of sensitive crops in New South Wales and Queensland were damaged by spray drift. This was repeated in early 2017 with an estimated 30,000 hectares of cotton damaged by phenoxy herbicide drift .In August 2017, a Swan Hill grape grower successfully sued his neighbour to the tune of $7 million for drifting phenoxy herbicide onto his grapes .Much of this drift damage is by farmers spraying during inversion conditions. Because inversions prevent the mixing of air, any suspended fine droplets and particles, including spray, can move in any direction to be deposited up to 20 kilometres away once the inversion breaks the next day. Cold air drainage within the inversion will cause fine droplets and particles to accumulate in the low parts of the landscape. Spray drift creates a number of major issues including crop damage, damage to sensitive vegetation and contamination of water and produce. Have you signed a declaration saying that you haven’t applied certain chemicals to the crop? You might not have, however contamination could have occurred via drift. Rejection of your product is probably the least of your worries.

Visual clues to indicate the existence of an inversion

  • Formation of mist or fog
  • Smoke or dust hanging in the air and moving horizontally
  • Flattening of clouds as evening approaches inversion signs - fluffy cumulus clouds Image by AGRONOMO

As evening sets in, fluffy (cumulus) clouds flattening is a sign of impending inversion formation. Don’t mix another spray tank. Image: AGRONOMO

Rules of thumb – Conditions necessary for inversions to form

Source: National Working Party on Pesticide Application

  • A reduction in wind speed to below 11 kilometres per hour is necessary for all seasons.
  • In summer, a fall of at least 4°C from maximum temperature recorded on the day, promotes an inversion an hour