Excessive wind leads to off-target chemical applications. Right?
While that might be conventional wisdom, you’ll get an argument from Andrew Thostenson. In fact, the opposite may be true. “There has been far more crop damage to drift from low-wind situations than high wind,” says the North Dakota State University pesticide program specialist.
That’s due to a weather anomaly called a temperature inversion. On a normal sunny day, sunlight heats the ground and every object on it. At night, that heat buildup radiates back into the sky.
“You get a stratification of temperature at night, with it now colder at the ground level and warmer as you move up,” says Thostenson. “It’s just the opposite of the day.”
This situation is most pronounced on clear and calm days. In the late afternoon and then all evening, temperatures are as much as 10°F. to 15°F. cooler at ground level compared with 10 feet above. That’s the inversion.
Low-level summertime fog is a good visual image of an inversion. Moisture condenses in the cool and heavy lower air and is trapped near ground level. Wind speed in excess of 3 mph (or possibly less) will usually mix the air and demolish the inversion, says Thostenson.
Two problems exist, though.
This cool lower air isn’t being pushed by wind, but it can flow by gravity. Because it is heavy, it flows downhill like water and tends to accumulate in valleys and low spots in the night and early mornings. With no sunlight to break it up, the inversion and downhill drift can go on all night.
The second problem is that a pesticide sprayed late in the day may be trapped in the cool air and move through the calm night.
“The pesticide can hang like a fog, drift along, and move off target to a nearby home, a shelterbelt, or neighboring crop,” says Thostenson. “With just a little more wind, it would break up that inversion, disperse the chemical, and not be a problem. No wind is the worst thing. That’s why I can say that no wind is almost worse than spraying in wind.”
So what can you do?
First, minimize the number of fine droplets in your spray.
“Too many fines are never good, but they are especially bad in an air temperature inversion,” says Thostenson.
Read the pesticide label. If spraying in high temperatures is not recommended, the chemical may be prone to vaporization.
While many operators like to spray late in the day when winds tend to die down, it’s a mistake in an inversion situation, says Thostenson.
Inversions tend to last all through the night, and then they break up in the morning with more wind and sunlight. Chemicals caught in the inversion have several hours of drift potential.
Here are three inversion signs:
- Smoke from a fire hanging near ground level that does not rise.
- An evening wavy mirage as you look across the countryside.
- Dust from a gravel road that hangs near ground level.
You can even hear an inversion. The refraction effect makes sound carry long distances on the still, stratified air.
One of the problems in detecting an inversion is the way the public weather observers report wind speed. In some cases, they measure it at 30 or more feet above the ground, while the speed at ground application level can be 20% to 25% lower.
Thostenson advises that chemical applicators do their own wind speed checks at application height.
If you suspect an inversion is developing, the best thing you can do is quit spraying for the evening. Resume spraying the next morning, when the inversion breaks apart.
(Source – http://www.agriculture.com/crops/pesticides/offtarget-chemical-applications_177-ar46911)