Spray Weather Window & Go or Wait

It's the band of weather conditions in which spray droplets land on the target and work well — not so hot and dry that they evaporate before reaching the leaf, not so humid and still that they won't dry, and not so windy they drift off-target. This tool reads temperature, humidity and wind to tell you whether you're inside that window now.

Delta-T is the dry-bulb (air) temperature minus the wet-bulb temperature, in °C. It captures how fast water evaporates: a high Delta-T means dry, evaporative air where droplets shrink and drift; a low Delta-T means humid air where they stay wet and don't dry on the leaf. For most ground spraying, Delta-T of about 2–8°C is the sweet spot, with 2–10°C acceptable.

From temperature and relative humidity the tool derives the wet-bulb temperature using a standard psychrometric approximation, then subtracts it from the dry-bulb temperature to give Delta-T. Combined with the wind speed you enter, it returns a go, caution or wait verdict so you can decide whether to spray.

When air is hot and dry the Delta-T is high, so fine droplets lose water and shrink rapidly in flight. Smaller droplets fall slower, evaporate further and are carried away on air currents — meaning more drift, less reaching the target, and poorer efficacy. Spraying in the cooler, more humid parts of the day usually keeps Delta-T in range.

When air is near-saturated the Delta-T is very low (often below 2°C). Droplets stay wet and may not dry on the leaf, so contact products can run off and translaminar uptake suffers. Dead-calm conditions can also bring temperature inversions, where fine droplets hang suspended and drift unpredictably once air moves.

A light, steady breeze of roughly 3–15 km/h is ideal — enough to show wind direction and avoid inversions, but gentle enough to limit drift. Above about 15–20 km/h drift risk rises sharply, especially with fine droplets. Dead calm (under 2–3 km/h) is risky too because of inversions. The tool flags wind that's too high alongside the Delta-T verdict.

Wet-bulb is the lowest temperature reachable by evaporative cooling at the current humidity. Delta-T is dry-bulb minus wet-bulb, the core evaporation indicator. Wind speed is shown against the safe band. Suitability rolls Delta-T and wind into a single go, caution or wait verdict so you know at a glance whether to head out.

No. Delta-T and wind guide timing, but always follow the product label and local regulations for buffer zones, droplet size, maximum wind, temperature limits and inversion rules. Some products have their own weather restrictions that override the general window. Treat this calculator as a planning aid, not a legal authority.

The wet-bulb and Delta-T are good engineering estimates from standard psychrometric formulae, accurate enough for spray planning. Real conditions change quickly across a day and field, so measure at the site with a weather meter, re-check during the job, and stop if conditions move out of the window.

Yes — enter temperature in °C, relative humidity in % and wind speed, and the Delta-T logic holds anywhere. The 2–8°C target is a widely used guideline across regions; adjust to local agronomy advice and label limits for your crop and products.