Delta T for spraying: what it is and why 2 to 8 °C

Delta T is the single most useful number for judging how long a spray droplet will survive between the nozzle and the target. It combines air temperature and relative humidity into one figure that tracks the evaporation rate, which is why it is widely used by agronomists and spray operators, particularly in Australia and Brazil.

How Delta T is calculated

Delta T is the dry-bulb temperature minus the wet-bulb temperature:

Delta T = dry-bulb temperature − wet-bulb temperature

The dry-bulb temperature is ordinary air temperature. The wet-bulb temperature is what a thermometer wrapped in a wet wick reads when air passes over it: water evaporating from the wick cools it. The drier the air, the faster the evaporation and the bigger the gap between the two readings. When the air is saturated nothing evaporates, the two readings are equal and Delta T is zero.

You can measure it with a sling psychrometer or a handheld weather meter, or compute it from temperature and relative humidity. A worked table is in the Delta T chart guide.

Why 2 to 8 °C is the working window

The generally accepted guidance, published by the Australian Bureau of Meteorology and repeated by GRDC and many extension services, is that Delta T should ideally be between 2 and 8 °C, and not greater than 10.

• Below 2 °C the air is close to saturation. Droplets evaporate very slowly, so the smallest ones stay airborne for a long time and can travel far. Low Delta T often coincides with calm, humid conditions around dawn and dusk, when a thermal inversion is likely. Leaves may also be wet with dew, which dilutes the spray.
• 2 to 8 °C is the range in which droplets survive long enough to reach the target without hanging in the air.
• 8 to 10 °C is commonly described as marginal. Evaporation is fast enough that fine droplets shrink quickly, lose their momentum and become more drift-prone. Guidance usually says to work here only with coarser droplets, higher water volumes where the label allows, and extra care.
• Above 10 °C evaporation is rapid, and most guidance says to stop. Water-based droplets can shrink to a fraction of their size before landing, and some products are less effective on heat-stressed plants.

These are guidelines, not legal limits. Where the label sets conditions for temperature, humidity or droplet size, the label prevails.

How Delta T moves through the day

Because it depends on both temperature and humidity, Delta T follows a typical daily curve on a clear day:

1. Before sunrise the air is cool and humid, and Delta T is often below 2.
2. Mid-morning the air warms, relative humidity falls and Delta T climbs into the 2–8 range.
3. Early to mid-afternoon is usually the peak. On hot, dry days Delta T can pass 8 or 10 for several hours.
4. Late afternoon and evening it falls again as the air cools and humidity rises.

The same temperature can give very different Delta T values depending on humidity. At 20 °C and 80 % relative humidity, Delta T is about 2.5; at 30 °C and 40 % it is about 9.6. Checking temperature alone is not enough, and neither is humidity alone — see humidity and dew point and temperature and spraying.

What to do when Delta T is outside the window

• If it is too high, move the job to the morning or evening, switch to a coarser droplet class where the label permits, or wait for a cooler, more humid day.
• If it is too low, check for an inversion and for wet leaves before starting, and wait for the air to warm and mix.

Delta T is one input among several. A perfect Delta T with an inversion or a strong crosswind is still a bad time to spray. See the best time to spray guide for how the factors combine.

Further reading

• Delta T — Australian Bureau of Meteorology
• Best Practices for Effective and Efficient Pesticide Application — Ohio State University Extension

In PulveCast

PulveCast computes Delta T for every forecast hour from temperature and relative humidity and checks it against a default window of 2–8 °C, showing a caution state close to either edge. The band is adjustable in the settings, and each crop preset loads its own.

More guides

• Delta T chart: how to read it
• Thermal inversion and spraying
• Spray drift: causes and how to reduce it
• Wind speed for spraying
• The best time of day to spray
• Humidity and dew point for spraying
• Rain and spraying: rainfast periods and forecasts
• Temperature and spraying
• Droplet size and nozzle selection
• Weather for drone spraying
• Kp index, RTK and GNSS for spray drone operators
• Pre-spray checklist