Free Delta T calculator for spraying. Find optimal conditions from temperature and humidity, with wet bulb, evaporation, and drift risk advice.
Measure at boom height, not at the cab. Wind speed is optional and refines the drift-risk read.
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Delta T (ΔT) is the difference between dry bulb and wet bulb temperature—the most reliable indicator for pesticide spraying conditions. Our calculator helps farmers and applicators determine whether conditions are suitable for spraying by analyzing temperature, humidity, evaporation risk, and drift potential.
Delta T measures the drying power of the air. It is the difference between the dry bulb temperature (a standard thermometer) and the wet bulb temperature — the temperature of a fully evaporatively-cooled surface, here derived from Stull's (2011) formula. The GRDC and Bureau of Meteorology treat 2–8°C as the ideal spraying band: below it the air is too humid (slow drying, high drift), above it too dry (rapid evaporation, poor droplet survival).
Delta T Formula
Critical timing for pre-emergent herbicides requires optimal conditions to ensure product reaches soil and activates properly.
Contact sprays must cover leaf surfaces thoroughly. Poor Delta T conditions cause inadequate coverage.
Fungicides need good droplet spread and drying time. Delta T guides timing for best disease control.
Aircraft applicators use Delta T to determine safe flying windows when droplets will survive the drop to target.
Relative humidity varies with temperature. Delta T accounts for both, giving a more accurate picture of actual evaporation conditions.
Spray droplets must survive from nozzle to target. Delta T predicts whether droplets will reach plants or evaporate mid-flight.
Low Delta T conditions cause fine droplets to drift. The calculator warns you when drift risk is elevated.
Pesticides work best when droplets survive to target and spread properly. Wrong conditions waste expensive chemicals.
The ideal Delta T range is 2–8°C (4–14°F), the band preferred by the GRDC and Bureau of Meteorology. A Delta T of 8–10°C is marginal; below 2°C is too humid and above 10°C is too dry — avoid spraying in both.
Yes. A Delta T of 3°C sits inside the ideal 2–8°C band, so conditions are suitable. Keep an eye out for temperature inversions in the calm, humid air that often accompanies a low Delta T.
Our calculator uses Stull's formula (2011), which is accurate within ±1°C. Different formulas exist with slight variations. The key is consistency—use the same method for comparison.
Use a sling psychrometer (wet and dry bulb thermometer) or a digital weather meter with wet bulb function. Many sprayers have built-in weather stations. Measure at boom height, not at your vehicle.