Free Delta T calculator for agriculture. Determine optimal spray conditions based on temperature and humidity. Calculate wet bulb temperature, assess evaporation risk, drift risk, and get spraying recommendations.
Poor - Too Humid
Marginal - Humid
Ideal
Marginal - Dry
Poor - Too Dry
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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 capacity of air. It's calculated as the difference between the dry bulb temperature (actual air temperature) and the wet bulb temperature (temperature at 100% humidity). Low Delta T means humid conditions with slow evaporation and high drift risk. High Delta T means dry conditions with rapid evaporation and poor droplet survival.
Delta T Formula
ΔT = T(dry bulb) − T(wet bulb)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.
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.
The ideal Delta T range is 4-8°C (7-14°F). This provides optimal balance between droplet survival and drying time. Avoid spraying when Delta T is below 2°C (too humid) or above 10°C (too dry).
A Delta T of 3°C is marginal but acceptable. Use coarser droplets to reduce drift risk in these humid conditions. Monitor for inversions and be prepared to stop if conditions worsen.
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.