Weed & Pest
Air turbulence plays a role in pesticide dispersion when spraying fieldsWritten by Natasha Wheeler
“Usually, farmers will get up early in the morning to go out and spray their fields as soon as they can see enough. But, what we really ought to do is wait until about two hours after sunrise,” remarks Tom Wolf of Agrimetrix Research and Training.
Wolf recommends spraying after the air temperature has risen two degrees Celsius above the temperature at sunrise, the coldest time of the night.
“By then, the sun has warmed up the soil, the air has warmed up a little bit, and we start to get a little bit of breeze so there is more turbulent mixing,” he explains.
Many producers want to spray in calm conditions to avoid off-target movement, but calm conditions can also be associated with inversions, which are unfavorable for spraying.
“An inversion is defined by a temperature difference between two heights,” he explains, describing how there can be different temperature layers in the air.
In sunny conditions, the sun heats up the soil, and the ground becomes warmer than the air above it.
“The higher up we go from the ground, the more the air cools. That’s a normal daytime condition – air near the ground is warmer than the air above it,” Wolf says.
Even without the presence of the sun, air is typically cooler higher up, as illustrated by snowy mountaintops.
“There is less air pressure there. The rate of cooling with elevation is related to air pressure, and it’s called the adiabatic lapse rate. It means, if we take a parcel or air and simply elevate it, it will expand a little bit because there is less air pressure the higher we go,” he comments.
“That expansion takes work. The molecules have to physically move further apart and that work is temperature related,” he adds.
Typically, the rate of cooling is approximately one degree Celsius per 100 meters.
“When it’s sunny out, that rate of cooling is actually much faster because the soil has warmed the air near the ground, and as that dissipates, it causes the air to cool faster,” Wolf describes.
A parcel of air that is elevated from the ground on a sunny day, despite cooling as it rises, will still be warmer than the air around it, and therefore, it will continue to rise.
“Once that parcel of air rises, it’s gone,” he states. “Likewise, if we displace a parcel of air from above and move it down, the opposite happens.”
A cold parcel of air that sinks is still colder than the air around it when it nears the warm soil, so it is pushed all the way to the ground.
“What we have is very high turbulence – lots of parcels rising and falling. That’s called turbulent conditions, and it leads to dispersion of spray,” he explains.
A spray cloud released into turbulent air moves across the currents and disperses out over the field until it’s gone.
“If I was to release a spray cloud a few meters to my right, by the time it was a few meters to my left, it would probably be about five meters high. It would just go up, and by the time it reaches a little bit further downwind, there’s almost nothing left of it. It’s gone into the atmosphere. That’s why spraying under sunny daytime conditions is favored,” Wolf says.
On a day without sun, the ground can be much cooler than the air above it.
“If we take that same parcel of air and make it rise, it will warm a little bit as it rises, but it will still be colder than the air around it. As a result of that, it will fall back down to the ground and fall to exactly the same place where it came from,” he continues.
A parcel of air from above that is displaced downward will likewise return to where it started. Therefore, no mixing takes place in the air, creating two distinct layers.
“The parcels of air cannot be displaced,” states Wolf. “We see this in the evening fog sometimes, for example when the fog is hanging just at windshield height,” describes Wolf.
These distinct temperature layers are known as an inversion, and inversions are most common early in the morning and throughout the night.
“The reason it’s dangerous to spray in an inversion is because if the spray cloud doesn’t mix up, it will stay very concentrated, and it will cause a great deal of damage where it sits,” he remarks.
Large spray particles fall to the ground, but smaller particles remain buoyant in the air and can be carried for miles without dispersing.
“The small particles rely on atmospheric turbulence to disperse themselves. That’s what the sun will do for us,” he notes.
Before spraying, Wolf recommends, “We should take the temperature at sunrise and wait until we get about one or two degrees Celsius above that. Then, we’ll know the sun has done its work.”