FARM & ACREAGE — Temperature inversions affect pesticide applications

Characteristics of a temperature inversion. (Graph from North Dakota State University)
Characteristics of a temperature inversion. (Graph from North Dakota State University)

By Tyler Williams, Extension Educator

Temperature inversions are a common occurrence in Nebraska and influence pesticide applications and controlled burns.

WHAT IS IT?
A temperature inversion is a condition when the air close to the surface is cooler than the air above it, which is often referred to as “stable.” The conditions are considered stable because cool, dense air is not able to rise upwards into a warmer, less dense atmosphere above. Normal, “unstable” conditions indicate the air temperature decreases with height, which allows the warm air near the surface to rise.

HOW DOES IT FORM?
Temperature inversions form due to changes in incoming and outgoing shortwave and longwave radiation balance. Every object, plant, soil surface, etc. emits longwave radiation in all directions and when the incoming radiation becomes less than emitted radiation, there is a net energy loss and begins to cool. The surrounding air cools, becomes more dense and can no longer rise.

When the incoming solar radiation begins to re-heat the surface and surrounding air (net energy gain), the air begins to rise creating a circulation pattern. As the surface heating becomes greater, the circulation pattern will get larger and create unstable conditions, eliminating the temperature inversion.

Inversions can persist with wind speeds less than 4–5 mph; however, wind speeds greater than that will likely create turbulence and circulations, mixing the air mass. Even though wind speeds are light during an inversion, there may still be horizontal air movement and the direction can be highly variable. Cloud cover during the late evening through the overnight hours will limit or slow inversion formation by reducing radiation from leaving the lower atmosphere, limiting the amount of surface cooling that takes place. The amount of moisture in the air can also impact inversion formation because dry air will cool faster, resulting in faster and stronger inversion development.

WHEN DOES IT FORM?
A typical inversion will begin to form when the surface starts cooling, which is often 1–3 hours before sunset. Inversions below five feet above the surface — or in valleys — may occur earlier. The inversion will typically persist and strengthen overnight where maximum intensity will occur shortly after sunrise, often lasting 1–2 hours after sunrise. Evening inversions pose a greater risk for pesticide movement than morning inversions due to this overnight persistence.

HOW TO DETECT AN INVERSION?
• Topography may indicate a risk of inversion because cold air flows down slopes and into valleys. Inversions usually form first — and are more intense — in low-lying areas.
• Dew or frost often occur during the inversion-forming process and may serve as a warning for an inversion.
• Fog could also indicate an inversion existed prior to fog formation and has intensified.
• Tree lines can cause localized inversions by reducing wind speed, reducing the mixing of air, thus increasing the potential for an inversion.
• Dust or smoke that hangs in the air close to the ground indicates the air is stable and is not able to mix.
• Sounds (train) or smells (feedlot) that carry long distances may indicate an inversion.
• You can measure an inversion by taking temperature readings at two heights (ex. 1 ft and 8–10 ft) using a handheld temperature sensor.

CAN YOU PREDICT AN INVERSION?
Predicting an inversion is quite challenging due to the many localized and small-scale influences mentioned above. The basic, large-scale ingredients for an inversion are clear skies, dry air and low wind speed, which is often associated with high-pressure weather patterns. Forecasts that indicate these ingredients, especially during the evening and overnight, may be a good indicator of ideal, inversion-forming conditions.

The variability in topography and land-use alters the landscape within a county, region or even a field, which heavily influences low-level inversions.

Whether you are planning a herbicide application or prepping for a controlled burn, utilize these potential indicators to best plan your task and minimize unintended impacts.