A guide to estimating wind speed

Use this guide to estimate wind speed to help you plan to safely spray agricultural chemicals.

Calm wind

Wind speed – 0 km

Visible signs – Smoke rises vertically; spreads out under inversion layer. Flags hang limp.

Spray drift risk – High. Unpredictable vapour and mist drift.

Light air

Wind speed – 1-5 km

Visible signs – Smoke drifts slowly. Flags just begin to move.

Spray drift risk – Moderate–low. Inversion conditions could still be present.

Slight breeze

Wind speed – 6-11 km

Visible signs – Wind felt on face; leaves rustle; flags stir from flagpole.

Spray drift risk – Low

Gentle breeze/wind

Wind speed –12-19 km

Visible signs Leaves and small twigs in constant motion; wind extends a light flag.

Spray drift risk – Moderate–high. Need to carefully monitor conditions.

Moderate wind

Wind speed – 20-29 km

Visible signs – Wind raises dust and loose paper; grasses lay over and small branches are moved; flags flap.

Spray drift risk – High. Droplet drift and vapour drift are almost certain.

Fresh wind

Wind speed – 30-39 km

Visible signs – Small trees in leaf begin to sway; crested wavelets form on inland waters, flags ripple.

Spray drift risk – Very High. Unsuitable for spraying.

Strong wind

Wind speed – 40+ km

Visible signs – Large branches move; whistling heard in wires, flags beat.

Spray drift risk – Extreme. Unsuitable for spraying.


This guide to estimating wind speed is not intended as a substitute for electronic hand held weather meters, which are available from spray equipment suppliers. Wind speed should be measured at spray application height. Some product labels specify a wind speed range for spraying. Check the product label prior to spraying.

Reduce risk from spray drift:

  • Ensure wind direction is safe.
  • Ensure wind speed is safe.
  • Allow adequate downwind buffer zone.
  • Lower boom height
  • Reduce travel speed.
  • Increase droplet size by:
    • Increasing nozzle size;
    • Changing nozzles;
    • Reducing pump pressure.

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Page last updated: 25 May 2022