If your chemical label talks about spray conditions, it is talking about Delta-T whether it names it or not. Delta-T is the best on-farm proxy we have for what happens to a droplet between the nozzle and the leaf. Getting it wrong wastes product, risks your neighbour's crop, and can put you on the wrong side of a label.
What Delta-T actually measures
Delta-T is the gap between the dry-bulb temperature, which is the air temperature, and the wet-bulb temperature, which is what a thermometer reads with a wet wick once humidity is accounted for. That gap tells you how fast water evaporates.
A small Delta-T means humid, slow-drying air. A large Delta-T means hot, dry air that strips moisture from a droplet before it can do its job. Both ends of the range cause problems.
Why the extremes hurt you
When Delta-T climbs above about 8, fine droplets lose volume in flight. They become lighter, drift further, and what does land is over-concentrated. You get poor coverage and higher drift risk at the same time.
When Delta-T falls below about 2, the air is so still and humid that droplets hang and drying stalls. Worse, very low Delta-T often lines up with temperature inversions, where cool air is trapped near the ground and your spray can move sideways for kilometres as a concentrated cloud.
A perfect Delta-T reading at dawn can still be the most dangerous time to spray, because that is exactly when inversions form.
Delta-T is necessary, not sufficient
Delta-T is one of three checks, not the whole story. Pair it with two others.
- Wind speed. A steady 3 to 15 km/h is the sweet spot. Below 3, suspect an inversion. Above 15 to 20, drift risk climbs.
- Inversion awareness. Be especially wary around dawn and dusk, on clear still nights, and when smoke or dust hangs low and flat.
Only when all three line up, with Delta-T in range, wind in range, and no inversion, is it genuinely a good window.
When to fill the tank
Spray when Delta-T sits around 2 to 8, wind is a steady 3 to 15 km/h, and there is no inversion. Miss any one of those and the right move is to wait. A missed window costs you a few hours. A bad spray costs you product, efficacy, and possibly a very awkward conversation next door.
Run this yourself
Agrivise maps live Delta-T and wind across twelve cropping regions, updated hourly, so you can see your window before you fill the tank.
Sources
- Bureau of Meteorology: Delta-T and spraying conditions guidance
- GRDC: spray application and reducing spray drift
- APVMA: spray drift management and label instructions
