Spray Weather Window & Go or Wait
Times herbicide
Enter temperature, humidity and wind to get Delta-T and a go/wait verdict— spraying works best at Delta-T 2–8°C with light wind, so droplets neither evaporate nor drift.
Check the spray window
Next: conditions sit in the sweet spot — spray now while Delta-T holds in the 2–8°C band and the wind stays light and steady.
Delta-T uses the Stull wet-bulb approximation. It governs droplet evaporation and uptake; always combine it with label rates, inversion risk and buffer zones for sensitive neighbours.
Spray weather — key facts
- Delta-T
- dry-bulb − wet-bulb (°C)
- Ideal Delta-T
- ≈ 2–8°C
- Acceptable
- 2–10°C
- Ideal wind
- ≈ 3–15 km/h
- Too dry/hot
- droplets evaporate & drift
- Too humid/still
- won't dry, inversion risk
- Too windy
- off-target drift
- Privacy
- Runs in your browser; nothing uploaded
Spray when the droplets land, not when they drift
Getting chemical onto the target is as much about the weather as the nozzle. In hot, dry air with a high Delta-T, fine droplets shrink and evaporate in flight, drifting off-target and wasting product. In near-saturated, still air with a very low Delta-T they stay wet, run off and risk being carried by inversions. The window in between — Delta-T around 2–8°C with a light, steady breeze — is where spraying is efficient and safe.
This tool gives the wet-bulb temperature, Delta-T, a wind check and a single go/wait verdict from the temperature, humidity and wind you enter. Use it to pick the right hours of the day, avoid drift complaints and get more active onto the crop. Pair it with the Spray Drift Buffer, Spray Droplet Density and Sprayer Calibration tools for a full application plan.
Time the spray right
Find the hours inside the Delta-T window.
Cut drift
Avoid hot, dry and windy conditions.
Get more on target
Stop droplets evaporating before the leaf.
Decide fast
One go/wait verdict from live conditions.
Frequently Asked Questions
What is the spray weather window?+
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.
What is Delta-T?+
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.
How is Delta-T calculated here?+
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.
Why does too dry or too hot cause problems?+
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.
Why is too humid or still also bad?+
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.
What wind speed is safe for spraying?+
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.
What do the outputs mean?+
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.
Does Delta-T replace reading the label?+
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.
How accurate are the figures?+
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.
Does it work in any units or climate?+
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.