Understanding temperature inversions to avoid spray drift
By Bruce Barker
It’s 5 a.m. on a calm, sunny morning in June. Perfect time to spray? Not so fast. A temperature inversion is likely, which could result in small spray droplets remaining suspended in the air and moving off-target.
“I think it is a safe assumption to say that a temperature inversion on the Prairies will occur every night from one hour before sunset to one hour after sunrise unless there is cloud cover or wind,” says Tom Wolf, a sprayer technology specialist with Agrimetrix Research and Training in Saskatoon, who can also be found at sprayers101.com and also on Twitter @Nozzle_Guy.
Most farmers have directly observed the signs of an inversion. Dust or smoke may rise slowly and hang in the air, often slowly drifting away at the same level, or strong odours or distant sounds that aren’t normally observed are also indicative of an inversion.
Inversions develop in a 24-hour cycle due to the differences of warming and cooling of the air near the soil surface. As nighttime sets in, the soil cools. The air near the soil surface becomes colder and denser than the air above it. This is a temperature inversion – essentially preventing air from mixing. As long as skies remain clear, the soil surface temperature continues to cool the air near it as the night progresses. The height of the inversion grows as the surface air cools, and can reach upwards of 100 metres. Maximum inversion intensity and height will occur shortly before the sun begins to heat the soil surface in the morning.
In the early morning without cloud cover, the sun’s solar radiation warms the soil, which in turn warms the air near the soil surface. As the surface air warms, the warm air rises and is subsequently displaced by cooler air that sinks to the ground. This sets up what’s called a thermal turbulence cycle, where warm air is moving upwards and the inversion begins to break up.
By late morning, mixing and air turbulence between the warmer air and the cooler air often causes light, but gusty, variable-direction winds near the surface. The formation of late-morning or afternoon cumulus clouds indicates that this convection cycle is occurring. This condition helps mix adjacent air layers, diluting the dust, smoke, or drift they contain. The opportunity to spray will depend on how turbulent the winds are. By mid-afternoon, as the wind rises, the wind turbulence mixes warm and cool air near the ground to equalize the air temperatures.
The risk of spraying during an inversion is that small spray droplets do not disperse and dilute to harmless levels with moderate winds. Rather, air trapped by an inversion may flow downhill or move with very light breeze and carry the small droplets, which remain very concentrated, off-target to other crops where drift damage may occur.
Cloud cover can prevent inversions from forming. Generally, greater cloud cover causes slower surface cooling and slower inversion formation in the late afternoon or evening. When skies are completely overcast, the surface will cool very slowing and inversion formation is unlikely. This is why nighttime temperatures are usually warmer under cloudy conditions than clear skies.
The intensity of an inversion can be measured by comparing the air temperature at two different heights above the soil surface. Subtract the air temperature measured at six to 12 inches above the soil surface from the air temperature at eight to 10 feet. If that number is positive, the atmosphere is inverted. The greater the difference in temperature, the more intense the inversion is.
Wolf says measuring the air temperature differences can be difficult with conventional thermometers because the temperature differences over short distances are small, often only one degree or less. North Dakota State University (NDSU) has published a guide that says other observations can provide an indication of a temperature inversion:
- Large temperature swings between daytime and the previous night
- Calm (e.g. less than three km/h wind) and clear conditions when the sun is low
- Intense high-pressure systems (usually associated with clear skies) and low humidity where you intend to spray
- Dew or frost indicating cooler air near the ground (fog may be too late).
- Smoke or dust hanging in the air or moving laterally
- Odours travelling large distances and seeming more intense
- Daytime cumulus clouds collapse toward the evening
- Overnight cloud cover is 25 per cent or less
Avoid spraying in inversions
Even with low drift nozzles, spraying is not recommended during an inversion. Low-drift nozzles still produce some fine droplets less than 200 microns in diameter. While larger droplets with greater velocity will hit the target quickly, fine droplets remain suspended and move with low wind speeds and float for long distances. The high humidity conditions associated with clear nights prevent these droplets from evaporating.
Early morning shortly after sunrise when skies are clear and wind is low is usually one of the worst times to spray. Under these conditions, waiting until solar radiation starts to heat up the ground will weaken the inversion. A thermal turbulence cycle will develop, and once the inversion is broken, spraying can begin, as long as winds do not become too strong to cause drift of larger spray droplets.
“A good rule of thumb for inversions is to wait a couple hours after sunrise before spraying to allow the inversion cap to disperse,” Wolf says.
Information from NDSU indicates that evening inversions pose a greater risk for spray drift than morning inversions because evening inversions are very persistent and can remain until shortly after sunrise if skies remain clear, providing a longer time frame for spray droplets to move off-target. Only windy or cloudy conditions will disrupt the inversion.
In its comprehensive document on temperature inversions, NDSU reports that low wind speeds three-to-four hours before sunset under clear sky conditions may appear to be ideal conditions for pesticide application, but these conditions can be deceiving, and are actually ideal for rapid inversion formation.
For a comprehensive look at temperature inversions, refer to NDSU Extension’s factsheet, “Air Temperature Inversions”