Best weather conditions for spraying: wind, temperature inversions and drift

Weather decides a large part of how well a crop protection treatment works and how much of the product ends up where it should. Spray in the wrong conditions and you lose efficacy, risk damaging neighboring crops and may break the conditions on the product label. Here is what to watch before you start the sprayer.

Wind: not too strong, not too calm

Wind is the main driver of spray drift, the movement of droplets away from the target area.

• Too much wind carries fine droplets off the field. Many extension services suggest spraying in a steady wind of roughly 3–10 mph (about 1.5–4.5 m/s). Some countries set legal limits, and some product labels set their own maximum.
• Too little wind is not safe either. Dead calm, especially early in the morning or in the evening, often comes with a temperature inversion.
• Wind direction matters as much as speed. Avoid spraying when the wind blows toward sensitive areas: other crops, gardens, water, beehives or houses.

Always check the product label: where it sets wind limits, buffer zones or drift-reduction requirements, those are binding.

Temperature inversions

Normally air gets cooler with height, and warm air near the ground rises and mixes. During an inversion, a layer of cool air is trapped near the ground under warmer air. Fine droplets can hang in that cool layer and then drift a long way when a light breeze picks up.

Inversions are most likely:

• on clear, calm evenings and nights,
• around sunrise, before the sun warms the ground,
• in low-lying areas and valleys.

Signs to look for: mist or fog near the ground, dust or smoke that hangs in a layer instead of rising, and very calm air after a clear night. If you see them, wait until the air starts mixing.

Temperature and humidity

High temperatures and low humidity make droplets evaporate quickly. Smaller droplets drift more easily, and some products become more volatile. Crops under heat or drought stress also take up many products less effectively.

Very low temperatures have their own problems: some herbicides work slowly or poorly when plants are not actively growing, and frost can damage crops that were just treated. The label tells you the temperature range in which a product works best.

Delta T: a quick check of spraying conditions

Delta T is the difference between the dry-bulb and wet-bulb temperature. It combines temperature and humidity into one number that shows how fast water evaporates. Guidance widely used in Australia suggests:

• 2–8 °C: generally good conditions for spraying,
• 8–10 °C: marginal; use coarser droplets and higher water volumes,
• above 10 °C: evaporation is fast and spraying is not recommended,
• below 2 °C: humid conditions that can come with inversions and slow drying.

Rain

Most products need some time on the leaf before rain. Check the rainfast period on the label and the forecast for the next few hours. Rain soon after a treatment can wash the product off and into soil or water.

Record the conditions you actually had

A forecast helps you plan, but your records should show the conditions present during the treatment: temperature, wind speed and direction, humidity and anything unusual. In Agro Notes you can fill the weather in from a forecast for the field and time of the treatment with one tap, then adjust the values to what you saw. Those records help when you compare results later, and they are what you show if anyone asks about drift.