Use the SD Spray Tool for Inversion Detection and Weather for Pesticide Application

May is a busy month as fieldwork progresses and pesticide applications are a critical component for the management of most farm operations. Weeds, insects, and diseases are all best treated early in their development before there is significant damage or yield loss.

The SD Mesonet Spray Tool provides real-time weather data for pesticide applicators. This dedicated website for pesticide applicators uses the SD Mesonet weather data, which is updated every five minutes.

In 2018, the network of weather stations was augmented with additional thermometers specifically for measuring temperature inversions. This information, in addition to wind speed and direction, temperature, humidity, and an abbreviated weather forecast, are all posted on the Spray Tool website. The website is designed for both desktop and mobile devices.

Temperature inversions are dangerous for pesticide applicators, as inversions trap droplets in the atmosphere. The droplets can remain airborne for several hours, maybe as much as 12 or more during overnight hours. Then, there is a risk of drift and damage to neighboring areas when the air or wind moves the droplets away from the intended target area. Pesticide application during an inversion should be avoided. Some pesticide labels strictly forbid application during an inversion.

The Spray Tool can inform an applicator if there is an inversion present or not, but can also provide much of the other required weather information for recordkeeping. As of the 2019 spray season, SD Department of Agriculture will accept SD Mesonet data as one of a few acceptable sources for weather recordkeeping.

For applicators who are spraying over-the-top formulations of dicamba in soybeans, the Spray Tool will also provide an estimate of boom-height wind speed. This uses the wind speed measurement at 10’ height, and an EPA calculation to estimate 40” wind speed for the boom height.

To use the Spray Tool for inversion detection and wind speed, there are three colors to indicate levels of risk. Similar to a stoplight, green, yellow, and red colors indicate an increasing level of risk or unfavorable conditions for pesticide application.

Inversion Color Codes

For inversions, the color codes are described below:

The reported strength of inversion is the temperature (°F) of the air at 10 ft minus the temperature of the air at 40 in.

Inversion Detected (Red)

• Positive number
• Cooler air under warmer air
• Occurs most often at night and in the first and last hours of daylight
• Unsuitable for spraying

No Inversion, Marginal (Yellow)

• Zero
• No detectable temperature difference
• Can occur anytime, but often during the first two and last two hours of daylight
• Marginal for spraying

Inversion Not Detected (Green)

• Negative number
• Warmer air under cooler air
• Rarely at night and the first and last hours of daylight
• Preferred for spraying

Wind Speed Color Codes

For wind speed, the color codes are described here:

Wind Speed

Wind direction, speed and gust (mph) are measured at the agricultural weather standard of 10 feet. Label restrictions for wind speeds vary, so check your label.

40-in (Boom Height) Speed

Wind speed at a height of 40 inches is calculated from 10-foot wind speed using EPA methods. This should only be used for labels that specify a boom height wind (i.e., XtendiMax, FeXapan, Engenia).

Visit the Mesonet Spray Tool

The SD Mesonet Spray Tool is funded in part by SD Soybean Research and Promotion Council.

Written collaboratively by Laura Edwards and Nathan Edwards.

Published in the Summer 2021 Soybean Leader magazine.