Nine Missouri weather stations recently received updates to help farmers and chemical applicators know when to spray herbicides to avoid off-target movement caused by temperature inversions.

The Extension Commercial Agriculture Program of the University of Missouri operates 24 real-time weather stations throughout the state. The Missouri Soybean Merchandising Council recently funded updates for stations in Monroe City, Vandalia, Albany, Columbia, Green Ridge, Hayward, Lamar, Linneus and Mountain Grove.

MU Plant Sciences researcher Mandy Bish says these weather stations now read air temperatures at three ground heights. MU extension climatologist Pat Guinan and systems administrator John Travlos collaborated with Bish and MU extension weed scientist Kevin Bradley to select and equip stations in multiple cropping districts in the state. Reports from each station feed immediately to the free Missouri Mesonet website. This information indicates whether conditions are right for temperature inversions that contribute to chemical drift.

Bish’s research shows that inversions are common occurrences throughout the growing season. Inversions typically start between 6-7 p.m. during early months and 7-8 p.m. in later summer.

They also occur at times when farmers may have once thought it safe to spray – when skies are clear and the wind is still.

Temperature inversions are stable air masses where cooler air is near the earth’s surface and warmer air is on top.

Off-target movement of herbicides due to temperature inversions is thought to occur when herbicide particles are suspended in the stable air mass. Another possibility is that herbicide droplets may initially land on the intended target but then volatilize, or evaporate, into an inversion, Bish said.

Bish said there are four main indicators of temperature inversion: a clear night sky, low wind speeds of under three miles per hour, presence of dew or frost and a low-lying, horizontal fog. Inversions may occur at different times of the day, but most commonly begin to form at sunset.

Recently, the group released smoke bombs to test whether the smoke could signal that a temperature inversion was occurring.

During the recent MU pest management field day, Bish showed slides of colored smoke bombs being set off and dissipated over ground to show how particles suspend in the air. Dispersion and movement of the color particles correlates to data from weather station data.

Bish noted that inversions are not the only possible component in herbicides moving off target. Volatility and physical drift due to wind, droplet size, sprayer speed and boom height can also contribute to off-target herbicide movement.

The Missouri Mesonet provides real-time weather data from stations across the state. Click on the “Temperature Inversion Potential” option for more information.

More information is also found on the Mizzou Weed Science YouTube channel.