Low lying fog in fields may indicate a temperature inversion. (Photo credit: Dwight Lingenfelter
For those who have planted Xtend soybeans and plan to apply an approved dicamba-containing product (Xtendimax, Engenia and FeXapan) at burndown or over-the-top of your crop, be careful of drift associated with temperature inversion affects. I think most applicators are familiar with the drift-reducing guidelines such as prescribed nozzle types, boom heights, sprayer speed, wind speed and field buffers that can help limit droplet or physical drift but temperature inversions can cause problems too with herbicide movement. During a temperature inversion, small droplets can remain suspended in the air and move long distances horizontally during the inversion period.
What is a temperature inversion? According to Kyle Imhoff, our State Climatologist here at Penn State, here are basic details about nocturnal temperature inversions. Under most circumstances, temperature decreases with height in the atmosphere. An inversion occurs when the opposite occurs – temperatures increase with height. These most frequently occur as a nocturnal inversion (nighttime inversion) and they most often occur at the surface (but not always). It all boils down to radiation. During the day, the sun warms the ground which in turn warms the layers of air above it and then those layers warm the layers above them, and so on. At night, the surface is shut off from its heat source (the sun). The surface of the earth cools more rapidly than the air above it thus leading to cold surface layer with relatively warmer layers above it.
When do inversions occur? Inversions begin to set up around the time of sunset. These typically occur during clear, calm weather conditions (high pressure). They happen during clear calm nights because no clouds are present to help emit longwave radiation back to the surface of the earth. Therefore, when it is cloudy overnight, no inversion occurs because the surface does not cool. Also, winds produce eddies which help to mix the air near the surface and stops an inversion from forming. So, having no clouds or wind allows for rapid cooling at the surface and thus a temperature inversion forms.
Furthermore, cold air drainage can influence movement of very small herbicide droplets too. Cool air can drain into lower areas during the night. This scenario usually is associated with some wind flow in the drainage zone (usually less than 10 mph) and thus leads to an inversion.
How to determine the presence of a temperature inversion? Here are a few indicators:
- Clear night
- Calm (winds <3 mph)
- Dew or frost present
- Horizontal smoke patterns
- Ground fog in low lying areas
- Distant sounds becoming more easily audible
In addition, consult a reliable weather forecast to determine if high pressure and light wind is forecasted. However, keep in mind, inversion conditions can be highly localized. Inversions will be deepest right around sunrise after a calm, clear night. Most inversion occur between 2 hours before sunset until 1 hour after sunrise. In some studies, inversions have occurred 15-20 times or more each month during the growing season. So, plan your spray applications accordingly.