Wind speed for spraying

Wind is the factor that most directly decides whether spray lands on target. Too much and droplets are carried away; too little and there may be an inversion that holds fine spray in the air. The goal is a light, steady breeze blowing away from anything sensitive.

The range commonly recommended

Extension guidance commonly cites a wind speed of about 3 to 15 km/h (roughly 2 to 10 mph), measured at the height of the spray release. Some product labels set their own range — for example, some herbicide labels in the United States specify 3 to 10 mph at boom height — and some set a lower maximum near sensitive areas. The label and local regulations always prevail.

Within that range, drift risk still grows as wind increases, so the right droplet class depends on the wind:

• Near the lower end, most nozzles work well.
• Towards the upper end, a coarser droplet class is usually needed — see droplet size and nozzles.
• Above the range, stop, unless the label explicitly allows otherwise.

Why dead calm is also bad

It seems logical that no wind means no drift, but calm conditions, especially around dawn, dusk and at night, often mean a thermal inversion. Without mixing, fine droplets stay suspended and can move off target on the faintest air movement, in a direction that is hard to predict. Calm air also makes wind direction unreliable. A steady light breeze is safer than no breeze.

Gusts

Average wind speed hides gusts. A forecast of 10 km/h with gusts of 25 km/h is a very different day from a steady 10 km/h. Gusts lift and carry droplets unpredictably, disturb boom stability and, for drones, affect flight control and the spray pattern. If gusts regularly exceed your wind limit, treat the period as unsuitable.

Wind direction

Direction matters as much as speed. Before starting, work out where the downwind edge of the field is and what lies beyond it: homes, gardens, susceptible crops, apiaries, water. Plan so the wind blows away from those areas, leave the buffer the label requires, and consider treating the most sensitive edge when the wind is lightest or blowing away from it. See spray drift.

Wind changes with height

Friction with the ground slows the wind near the surface, so wind speed generally increases with height. Weather forecasts usually give wind at 10 m (about 33 ft), which is higher than a ground sprayer's boom and often faster than the wind at boom height. Spray drones fly closer to the canopy than 10 m, but the drone itself is exposed to the wind at its flight height, and rotor downwash interacts with the crosswind. When wind at the surface is light but stronger aloft, droplets released higher travel further. See drone spraying weather.

Because forecasts describe an area, check the wind on site with an anemometer at release height before starting, particularly near tree lines, buildings and slopes.

Further reading

• Effect of Major Variables on Drift Distances of Spray Droplets — Ohio State University Extension
• Introduction to Pesticide Drift — US EPA

In PulveCast

PulveCast checks wind speed and gusts each hour against your equipment's ceiling: 25 km/h (about 15.5 mph) by default, adjustable in the settings, or the manufacturer's limit loaded automatically for supported spray drones. The spray drift card then crosses that wind with your droplet class.

More guides

• Delta T for spraying: what it is and why 2 to 8 °C
• Delta T chart: how to read it
• Thermal inversion and spraying
• Spray drift: causes and how to reduce it
• The best time of day to spray
• Humidity and dew point for spraying
• Rain and spraying: rainfast periods and forecasts
• Temperature and spraying
• Droplet size and nozzle selection
• Weather for drone spraying
• Kp index, RTK and GNSS for spray drone operators
• Pre-spray checklist