Free tool
Delta T calculator
Enter the air temperature and relative humidity to work out Delta T, the gap between the air temperature and the wet-bulb temperature, and see where it sits against the usual spray bands.
Delta T5.0wet bulb 15.0 °C
Optimal: inside the usual limits (3 to 8)
Delta T is one reading among several. Check the wind too, and the product label always prevails.
What Delta T tells you
Delta T is the air temperature minus the wet-bulb temperature. The bigger the gap, the drier the air, and the faster a spray droplet evaporates on its way down.
- Under about 2, the air is near saturation: droplets stop evaporating and stay airborne, the still, humid conditions that also come with inversions.
- Over about 10, droplets shrink fast, and the finest evaporate or drift before they land.
The bands
| AgronomyNote | Bureau chart, as GRDC reproduces it | |
|---|---|---|
| Optimal or preferred | 3 to 8 | 2 to 8 |
| Marginal | 2 to 3, or 8 to 10 | 8 to 10 |
| Outside, or unsuitable | Under 2, or over 10 | Under 2, or over 10 |
AgronomyNote’s optimal band starts a little higher than the chart’s preferred one, so the hours just above saturation read as marginal rather than preferred. Either way, the product label prevails over both.
How it is calculated
The wet-bulb temperature comes from Stull’s 2011 approximation from air temperature and relative humidity at sea-level pressure, which is published for humidity between 5 and 99% and temperatures between −20 and 50 °C and is within about a degree of a measured wet bulb. It is the same function AgronomyNote uses for every hour on its spray conditions card.
Questions agronomists ask
What Delta T is best for spraying?
Published guidance, the Bureau of Meteorology’s chart as GRDC reproduces it, prefers a Delta T of 2 to 8, with 8 to 10 marginal. AgronomyNote treats 3 to 8 as optimal. The product label always prevails.
Does Delta T take the wind into account?
No. Check the wind separately: many labels restrict spraying to between 3 and 20 km/h. AgronomyNote’s spray conditions card reads Delta T, wind, gusts and rain together, every hour.
Can Delta T show an inversion?
Not on its own. A low Delta T often comes with the still, humid air of an inversion, but only a check on site can tell you one is there.
Delta T for every hour of the forecast, without the arithmetic
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