Delta T explained — the spraying number that isn't on your weather app
Delta T is dry bulb minus wet bulb — one number for how fast the air will pull the water out of a spray droplet. The GRDC ideal band is 2–8. Above 10, don't spray. Here's how to read it, work it out, and watch it move across the day.
Try your conditions
Delta T calculator
Slide to your paddock's temperature and humidity. The calculator uses the Stull (2011) wet-bulb formula — the same one CropWindow runs on every forecast hour — and gives you the call against the GRDC bands.
Delta T
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A calculated figure is for planning. On the day, confirm with a measured reading at the boom — a handheld spray meter or a live station reads your actual air, not a model's.
What Delta T actually is
Delta T is the dry bulb temperature minus the wet bulb temperature. The dry bulb is the ordinary air temperature. The wet bulb is what a thermometer reads with a wet wick over it — evaporation off the wick cools the bulb, and the drier the air, the harder it evaporates and the bigger the gap.
That gap is the whole point. Delta T is a measure of the air's evaporation power — how thirsty it is. A spray droplet is a small parcel of water on a short trip from nozzle to leaf, and Delta T tells you how much of it survives the journey. That makes it the most useful number on a spray meter, and the one no consumer weather app shows you.
Why it decides whether your spray works
When Delta T is high, droplets shrink in flight — and a shrinking droplet fails twice. First, coverage collapses: if the droplet evaporates before it lands, the chemical you paid for ends up in the atmosphere instead of on the leaf, even though the boom looks like it's putting out plenty. Second, drift climbs: smaller droplets are lighter, fall slower, stay airborne longer and travel further on any breeze. A hot, dry afternoon can turn a routine job into off-target damage on the neighbour's crop without the wind ever picking up.
Very low Delta T is a different trap. Below 2, droplets survive fine — but air that still and humid frequently means a temperature inversion: a layer of cool air pinned under warmer air, where fine droplets hang, concentrate and can be carried kilometres before they land. The GRDC treats hazardous surface inversions as one of the highest drift risks in Australian broadacre spraying. A low reading is a prompt to check, not a green light.
The GRDC bands
The whole picture, straight from GRDC spray-application guidance:
| Delta T | Call | What it means |
|---|---|---|
| Below 2 | Caution — usually wait | Too humid. Droplets stay wet too long, and still conditions like this often signal an inversion. Rule it out before you go. |
| 2–8 | Ideal | Droplets survive the trip to the leaf. This is your spray window. |
| 8–10 | Marginal — with care | Workable only with coarser droplets and higher water rates. Watch coverage and the trend. |
| Above 10 | Do not spray | Evaporation is too aggressive — droplets can vanish before they land. Stop. |
Delta T is not the whole call. Wind matters just as much — the working band is roughly 3–15 km/h at boom height — and a perfect Delta T with gusts over your limit is still a no-go. The spray-window guide shows how wind, Delta T and inversion risk stack into one decision.
How to read it across a day
Delta T follows the shape of the day: low and humid in the morning, peaking mid-afternoon, easing at dusk. At sunrise it often sits below 2 — too humid, and possibly still under last night's inversion. As the sun burns the dew off, it climbs into the 2–8 band: this is the classic morning window. On a hot, dry day it keeps climbing straight through 8 and past 10 by early afternoon, and the window slams shut until the cool change or the evening.
The evening dip is the dangerous one. As temperature falls and humidity rises, the meter drifts back into the ideal band and then below it — right as the next inversion is forming. A good-looking Delta T at 10pm almost always has an inversion sitting behind it, which is why night spraying is treated as high-risk regardless of the number, and why many labels restrict it. A rough rule: spray from about 1.5 hours after sunrise, and wind back two hours before sunset as the inversion risk builds — unless you can actively confirm there's no inversion.
Working it out from temperature and humidity
If you know air temperature and relative humidity, you can calculate the wet bulb with Stull's (2011) approximation and subtract. That's what the calculator above does, and what CropWindow does for every forecast hour. For the record, with temperature T in °C and relative humidity RH as a percentage:
Tw = T·atan(0.151977·√(RH + 8.313659)) + atan(T + RH) − atan(RH − 1.676331) + 0.00391838·RH1.5·atan(0.023101·RH) − 4.686035, and Delta T = T − Tw.
Measured still beats modelled. A whirling hygrometer or a handheld spray weather meter reads your actual air, at your height, right now — use the calculation to plan the day and the measurement to start the job.
Where Delta T is published — and where it isn't
The Bureau of Meteorology publishes Delta T on its land and agriculture charts, but general weather apps don't carry it. Consumer apps report what the sky is doing — temperature, humidity, rain chance — not what your spray droplets will do. The inputs are all there; the sum is left to you. That gap is why sprayers end up doing mental arithmetic in the ute at 6am, and it's the gap this page and the app exist to close.
How CropWindow watches it for you
CropWindow computes Delta T for every hour of your local forecast and checks it against each spray job's limits — alongside wind, rain and inversion risk. You get the best window per job with a red, amber or green confidence call and the reasons behind it, and if the forecast turns, it tells you before you've filled the tank. The GRDC bands are the defaults; every threshold is yours to edit.
Before you fill the tank. Delta T is planning guidance, not a green light. Confirm the reading at the boom, check wind and inversion risk as well, and follow the product label and your state's regulations — the label is the law. A forecast Delta T tells you when to plan the job; a measured one tells you whether to start it.
Fair questions
Delta T, asked and answered
What is a good Delta T for spraying?
The GRDC ideal band is 2 to 8. Between 8 and 10 is workable only with coarser droplets and higher water rates, and above 10 you should not spray — droplets can evaporate before they reach the target. Below 2, droplets stay wet too long and very still, humid mornings often signal an inversion.
How do I calculate Delta T?
Delta T is dry-bulb temperature minus wet-bulb temperature. The wet-bulb can be computed from temperature and relative humidity with Stull's 2011 formula — that's what the calculator on this page does, and what CropWindow does for every forecast hour.
Why isn't Delta T on my weather app?
Consumer weather apps report what the sky is doing, not what your spray droplets will do. Delta T is an agricultural number — the Bureau of Meteorology publishes it on its land and agriculture charts, but it's missing from general forecast apps, which is why sprayers end up doing the sums themselves.
Sources
- GRDC Spray Application Manual — Module 10.3: Temperature and humidity (Delta T) and Module 10.5: Using forecast information.
- GRDC — Hazardous surface temperature inversions factsheet.
- Sprayers 101 — What is Delta T and why is it important for spraying?
- Bureau of Meteorology — Delta T guidance on its land and agriculture charts (bom.gov.au). The app's forecasts come from Open-Meteo, not BoM.
- Stull, R. (2011), "Wet-bulb temperature from relative humidity and air temperature," Journal of Applied Meteorology and Climatology — the wet-bulb approximation used on this page.
Stop doing the wet-bulb sums in the ute
CropWindow reads Delta T, wind and inversion risk across your week and hands you the window. Free in early access, Android now.
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