Which weather condition increases vapor drift risk?

• A. Cold weather
• B. Hot weather 85F+
• C. Rainy weather
• D. Nighttime

Answer: (B)

Vapor drift risk is driven by how much pesticide vaporizes into the air after application. Warm temperatures raise the vapor pressure of many pesticides, so they volatilize more readily when it’s hot. On hot days, especially around 85°F and higher, more of the applied chemical becomes vapor and can be carried by air currents to off-target areas, increasing drift risk.

Cold weather lowers vapor pressure, so volatilization is reduced. Rainy conditions add moisture and deposition effects that tend to wash away or settle spray rather than keep it in vapor form. Nighttime is usually cooler with calmer or more stratified air, which generally reduces the tendency for vapor to drift compared with hot daytime conditions.

So the weather condition that increases vapor drift risk is hot weather.