Grower guides

Delta-T, wind and spray windows: when to spray

Most spray failures aren't the product's fault. They're droplets that evaporated on the way to the canopy, or drifted somewhere they shouldn't have. Two numbers — Delta-T and wind speed — tell you most of what you need to know before you fill the tank.

What Delta-T is

Delta-T is the gap between the air temperature and the wet-bulb temperature, and it's the best single field measure of how fast spray droplets evaporate. Hot, dry air gives a high Delta-T: fine droplets shrink and vanish before they reach the canopy, and what's left is light enough to drift. Cool, humid air gives a low Delta-T: droplets survive, but very low readings flag the still, saturated air that inversions form in. You can read Delta-T off Bureau of Meteorology observations, a weather station, or work it out from temperature and humidity.

The bands

Australian spray guidance bands Delta-T like this:

2–8 — preferred. Droplets survive the trip and conditions suit most spray qualities.
8–10 — marginal. Getting dry; coarser droplets and lower boom heights buy some margin, but the window is closing.
Above 10, or below 2 — unsuitable. Too dry to spray at the top end; at the bottom, the air is humid and still enough that inversion risk takes over.

And wind:

3–15 km/h — preferred. A steady, measurable breeze that shows you which way drift is moving without carrying it off-target.
15–20 km/h — marginal. Drift risk climbing; think hard about what's downwind.
Above 20 km/h — don't spray.
Below 3 km/h — don't be fooled. Dead calm reads as safe and is often the most dangerous condition of all, because of inversions.

Inversions: why calm evenings bite

On clear, still evenings and early mornings, a layer of cool air settles under warmer air and stops mixing. Fine droplets sprayed into that layer don't settle — they hang concentrated in the stable air and can move kilometres before coming down, well after you've packed up. That's how sprays applied in "perfect calm" end up on a neighbour's vines. Smoke or mist hanging low and flat is the classic sign. If the air is dead still, wait for a breeze.

Working the week, not just the day

The same two numbers work ahead of time. Hourly forecasts of temperature, humidity and wind give you Delta-T and drift risk for every hour of the coming week, which turns spraying from "is right now okay" into "Tuesday morning is the window, line the job up for it". That's also what makes the difference when disease pressure is building and there are only a couple of suitable windows before the next rain.

Record the conditions while you're there

Whatever the conditions were, write them down as part of the spray record — temperature, wind speed and direction at least. It's required in a proper spray diary, and if drift is ever alleged, the reading you took at the tank is the evidence that conditions were right when you started.