Delta-T and Spray Windows, Explained

What Delta-T actually is

Delta-T is the gap between two temperatures: the dry-bulb temperature (what a normal thermometer reads) and the wet-bulb temperature (what that thermometer reads with a wet sock on it, cooled by evaporation). Dry air evaporates water fast, so the wet bulb runs much cooler and the gap is wide. Humid air barely evaporates anything, so the two read nearly the same.

That gap is a direct measure of the air’s drying power — which is exactly what a spray droplet cares about. A droplet leaves the nozzle and starts evaporating immediately; the question is whether it reaches the leaf as a droplet or shrinks into a fine particle that goes wherever the air goes. Delta-T answers that question in one number, which is why it’s printed on spray charts and weather apps across the industry.

The ranges that matter

Guidance from agricultural extension services and chemical manufacturers lands on broadly the same bands. Product labels come first — but as a rule of thumb:

• Below about 4 °F (2 °C): too humid. Droplets barely evaporate; fine ones stay airborne and drift long distances. These conditions — still, damp, early morning or evening — are also when temperature inversions form, which carry suspended spray somewhere you didn’t aim it.
• About 4–14 °F (2–8 °C): the preferred window for most spraying. Droplets survive the flight, deposit, and dry at a reasonable rate.
• About 14–18 °F (8–10 °C): marginal. Evaporation is eating your finer droplets; coarser setups and larger droplet sizes buy some margin.
• Above about 18 °F (10 °C): the window is generally closed for most products — a meaningful share of what leaves the boom won’t arrive as spray.

The frustrating property of Delta-T is how fast it moves. A morning that starts in the window at 8 am can be out of it by 11 as the day heats and dries — the window isn’t a day, it’s hours.

Why the number has to be local

Delta-T is built from temperature and humidity, and both are stubbornly local — a river bottom, an irrigated circle, and a dry bench a few miles apart can be in three different spray windows at the same moment. A regional forecast or an airport station forty miles away tells you what conditions were like somewhere else. The droplets fly through the air over your field.

That’s the case for reading it off your own ground. A weather station on-site reports live Delta-T alongside the other two numbers that govern the decision — wind speed and direction — so “spray or wait” gets made on actual conditions, from the truck, before the tank is filled. And because it’s logged hourly, you also get the record: what conditions were when the job went on, which is worth having if a result ever gets questioned.

Reading the whole panel

Delta-T is the evaporation gauge, not the whole dashboard. The working checklist stays: label first (rates, buffers, and conditions on the product label override everything), wind within the label’s range and blowing away from what’s sensitive, no inversion — be wary of the still, cool edges of the day, and Delta-T in the window for the droplet size you’re running. An on-site station covers the measurable three from one pole; the label and your judgment cover the rest.