Spraying weather guides

Short, practical guides to the weather decisions behind every application — for boom sprayers, self-propelled sprayers and spray drones. The product label and your local regulations always come first; these guides explain the weather side of the decision.

Delta T for spraying: what it is and why 2 to 8 °C

What Delta T is, how it is calculated from dry-bulb and wet-bulb temperature, why 2–8 °C is the spraying window and what happens below 2 and above 8.

Delta T chart for spraying: how to read it

Delta T chart by air temperature (5–45 °C) and relative humidity (20–90 %): green 2–8 °C is good spraying weather. Read it in seconds, download it as a PNG.

Thermal inversion and spraying

What a thermal inversion is, when it forms, the signs to look for in the field, why spraying under one moves product off target and how to detect it.

Spray drift: causes and how to reduce it

What spray drift is, the difference between particle and vapour drift, the main causes — droplet size, wind, height, inversion — and practical ways to cut it.

Wind speed for spraying

Spray in a light, steady 3–15 km/h (2–10 mph) breeze blowing away from sensitive areas. Why dead calm is risky, gusts, height and label limits.

The best time of day to spray

Usually mid-morning after the dew and late afternoon before an inversion, when Delta T is 2–8 °C and the wind is 3–15 km/h. When each time of day works.

Humidity and dew point for spraying

How relative humidity and dew point affect spraying: evaporation when the air is dry, dilution and runoff when leaves are wet, and the humidity band to aim for.

Rain and spraying: rainfast periods and forecasts

How rain affects spraying: rainfast periods on the label, reading precipitation probability, heavy dew, and the runoff risk to streams, ponds and water bodies.

Temperature and spraying

How air temperature affects spraying: evaporation, herbicide volatility and crop stress when hot; slow uptake and frost when cold; and the link with Delta T.

Droplet size and nozzle selection

ASABE S572 droplet size classes from Very Fine to Ultra Coarse, the coverage vs drift trade-off, and how to match droplet class to wind and Delta T.

Weather for drone spraying

Weather for spray drones: rotor downwash, flight height and speed, wind limits vs spraying wind, wind at altitude, battery temperature, GNSS/RTK, Kp and rain.

Kp index, RTK and GNSS for spray drone operators

What the Kp index means for spray drone pilots, how geomagnetic storms affect GNSS and RTK accuracy, which Kp values to watch and why visible satellites matter.

Pre-spray checklist

A practical checklist before any pesticide application: label, PPE, calibration, weather window, buffer zones, neighbours and beekeepers, equipment and records.