Jarrod Miller, Extension Agronomist, University of Delaware
(edited version from UMD Agronomy News Blog, July 2017)
Pesticide drift and volatility are important considerations this time of year. Drift to adjacent fields is typically due to wind, which is why you select the correct nozzles and check wind speeds. Volatility describes how likely a pesticide will become a gas, allowing it to move off target. The best way to control volatility is to choose less volatile pesticides or apply during cooler weather. While wind speed can be easy to determine, another cause of drift, a temperature inversion, is not.
The earth’s atmosphere is typically warmer at the surface (due to the sun warming your fields), and cools 2-5°F for every 1000 feet you climb. As air warms at the earth’s surface, it expands and rises, then cools and falls, giving the atmosphere circulation. This process can produce cumulus clouds and storms, and move pollutants and smog out of the lower atmosphere. An inversion occurs when a pocket of warm air sits above cooler air at the surface, preventing circulation. On these days, smells of manure application may linger over a region or keep pesticide vapors in the air, allowing them to drift on gentle breezes. Fields affected by drift often have a pattern along the edge of the field, following the prevailing winds at the time of application. Inversions cause broader damage, where smaller herbicide particles remain suspended and cover the entire field.
Cumulus clouds even move across dry landscapes as they warm up. This is a good sign you have normal air circulation.
Prior to application, inversions can be easy to spot where you have fires or smokestacks, as the smoke will only rise so high before flattening out and moving sideways. One of the most common times to have an inversion is on clear nights in the summer. As your fields cool at night (through longwave, infrared radiation), the air near the surface becomes cooler than the air above it. Anytime you see stars at night, and have minimal wind, you can expect to have an inversion early in the morning. As the sun warms your fields back up, the atmosphere should gain circulation again, which will be most obvious when you see cumulus clouds in the sky. These are considered to be short term inversions.
Long term inversions occur when weather patterns sit longer, sometimes for days. If you listen to the weather talk about high/low pressure or cool/warm fronts, you can possibly predict an incoming inversion. At least 30,000 feet in the air is where the jet stream flows, and where these converge together, air is forced down creating a high pressure area. High pressure areas create low winds and clear nights, great conditions for an inversion. There are often high variations in night and daytime temperatures associated with high pressure, another factor in creating inversions. Additionally, air forced down by converging jet streams compresses and warms, creating a pocket of warm air over cooler surface air. Cooler air at the surface does not necessarily mean it feels like fall, this warmer pocket of air may only be 2°F warmer, but is enough to create an inversion and trap pollutants beneath it.
Low pressure areas occur where the jet stream diverges above us, pulling air up and lowering the air pressure at the surface. They are often associated with clouds, precipitation and minimal temperature variation. Without the sun warming the surface, inversions are less likely on days with low pressure. Cold fronts are associated with low pressure. These air masses are cooler and denser, so they can move below warmer air along the surface. As cold fronts move across warmer landscapes, the air rises and condenses into cumulus clouds, and sometimes severe thunderstorms. Inversions are not likely in these cases. However, some cold fronts are shallow, and could sit under warm air and cause an inversion.
Warm fronts are less dense and will move over-top of cooler air masses. As they rise over cooler air, they cause higher pressure at the surface. Slower moving than cold fronts, warm fronts will have more stable air, and longer, less severe rainfall. It is these slower moving warm fronts that may sit longer and cause inversions, which will cause more drift the closer they are to the surface. If you see cirrus clouds high in the atmosphere, it indicates a coming warm front.
Cirrus clouds can be seen on the right side of this photo, indicating an approaching warm front.
To avoid increased drift under an inversion, you can certainly check weather reports on fronts and pressure. Incoming thunderstorms are also a good sign there won’t be an inversion, although you could have high winds. Keeping a few things in mind will help prevent drift onto your neighbors fields and sensitive crops.
Inversions are more likely:
- In the morning, in valleys or near large water bodies after clear nights in the summer (look for cumulus clouds later in the day in indicate good air circulation)
- With shallow cold fronts (typically a cold front with no storms, but I’m not a meteorologist)
- High pressure, warm fronts that linger (Look for incoming cirrus clouds).
Watching for these weather conditions, along with minimal wind and good nozzle section should limit drift off target and minimize stories in the media.
