The belief that spraying at higher wind speeds causes more drift damage has prompted many farmers to spray at night, when wind speeds tend to be lighter. Although nights are often calmer, more damage can occur as spray drift is affected by temperature inversion, a process that has the potential to occur almost every night.
Normally, air near the ground is warmer than the air above, since the sun’s heat warms the ground. On a sunny day, air near the ground rises and expands. Air parcels fall and rise freely — this vertical mixing is called thermal turbulence and it is very effective at mixing air.
At night, especially clear, cloudless nights, this situation is inverted. The ground cools quickly, and air near the ground becomes cooler. Since air is a poor conductor of heat, air at a higher level remains warmer than the air nearer the ground.
This warmer layer of air acts like a lid, preventing the cooler air nearer the ground from mixing with the air above. As a result, the air is not well mixed.
Under normal daytime conditions, spray drift will be rapidly dispersed through thermal turbulence. Under nighttime conditions with a temperature inversion, spray drift will not disperse and the spray cloud will be more concentrated and potent.
All summer nights have the potential for an inversion, but less so under cloudy and windy conditions. Inversions do not occur during the day, although some very heavy crop canopies can create a small inversion layer due to water evaporation.
Inversion and dispersion
“Under nighttime conditions, if there is an inversion, you have no dispersion of the spray. In fact you have concentration of the spray cloud,” says Thomas Wolf, Research Scientist at Agriculture and Agri-food Canada (AAFC) Saskatoon. “Eventually this spray cloud will move downhill, potentially over large distances, but it will stay concentrated and highly potent. And in that case, if you do hit a sensitive area you will have the potential to create very serious damage.”
Inevitably there are many other factors which contribute to spray drift reduction, and probably the most important one is the operator. “In order to make the correct decisions you’ve got to have an operator who understands all the variables,” says Wolf.
Farmers should have a good handle on the capabilities of their sprayer, whether they can adjust their boom height, the importance of low drift nozzles and the relationship between travel speed, nozzle pressure and spray quality. They also need to know what chemical they are spraying and its potential impact upon different types of plants and organisms and what their neighbours are growing that might be affected. They need to know the potential for environmental damage, particularly if there are sensitive ecological areas nearby. But they also need to understand the atmosphere and what conditions lead to good dispersion and bad dispersion, says Wolf.
“It can get even more complicated because during the day thermal turbulence, and its beneficial impact on dispersion, is suppressed by cloudy conditions,” says Wolf. “At night the inversion can also be suppressed by cloudy conditions and by wind. So the most important part of the sprayer is the person who can understand these things and make the right decisions.” †