Delta T chart: how to read it
A Delta T chart by air temperature (5–45 °C) and relative humidity (20–90 %), with the colour bands used for spraying and how to read it in the field.
A Delta T chart lets you read the spraying condition directly from two numbers you can get from any weather station or forecast: air temperature and relative humidity. The table below is computed for air temperatures from 5 to 45 °C (41 to 113 °F) in 5 °C steps, and relative humidity from 20 to 90 % in 10 % steps.
- Below 2 °C: droplets stay airborne and drift
- 2–8 °C: ideal for spraying
- 8–10 °C: marginal, use coarser droplets
- Above 10 °C: high evaporation, avoid
| Air temperature ↓ · Relative humidity → | 20 % | 30 % | 40 % | 50 % | 60 % | 70 % | 80 % | 90 % |
|---|---|---|---|---|---|---|---|---|
| 5 °C (41 °F) | 6.1 | 5.5 | 4.9 | 4.2 | 3.5 | 2.7 | 1.9 | 1 |
| 10 °C (50 °F) | 7.7 | 6.7 | 5.8 | 4.9 | 4 | 3 | 2.1 | 1.1 |
| 15 °C (59 °F) | 9.3 | 8 | 6.7 | 5.6 | 4.5 | 3.4 | 2.3 | 1.2 |
| 20 °C (68 °F) | 10.9 | 9.2 | 7.7 | 6.3 | 5 | 3.7 | 2.5 | 1.2 |
| 25 °C (77 °F) | 12.5 | 10.4 | 8.6 | 7 | 5.5 | 4.1 | 2.7 | 1.3 |
| 30 °C (86 °F) | 14.1 | 11.6 | 9.6 | 7.7 | 6 | 4.4 | 2.9 | 1.4 |
| 35 °C (95 °F) | 15.7 | 12.9 | 10.5 | 8.4 | 6.5 | 4.7 | 3.1 | 1.5 |
| 40 °C (104 °F) | 17.3 | 14.1 | 11.4 | 9.1 | 7 | 5.1 | 3.3 | 1.5 |
| 45 °C (113 °F) | 18.9 | 15.3 | 12.4 | 9.8 | 7.5 | 5.4 | 3.5 | 1.6 |
How to read the table
- Find the row for the current air temperature (dry bulb).
- Find the column for the current relative humidity.
- The cell where they meet is Delta T, in °C: the dry-bulb temperature minus the wet-bulb temperature.
For example, at 20 °C and 80 % relative humidity Delta T is about 2.5, inside the working window. At the same humidity but 30 °C it is higher; at 30 °C and 40 % it is about 9.6, marginal. If your reading falls between two rows or columns, use the nearer one or estimate between them — the values change smoothly.
What the colours mean
The cells are coloured by the bands most often cited in spraying guidance, such as the Australian Bureau of Meteorology's recommendation that Delta T be ideally between 2 and 8 and not greater than 10:
- Below 2 °C — air close to saturation. Droplets survive a long time and fine ones can drift far; inversions and wet leaves are likely. Avoid, or take particular care.
- 2 to 8 °C — the ideal working range for most applications.
- 8 to 10 °C — marginal. Evaporation is fast; use coarser droplets and follow the label.
- Above 10 °C — avoid. Droplets evaporate quickly and much of the spray may never reach the target.
The label always takes precedence over any chart. Some labels set their own limits for temperature, humidity or droplet size.
Things the chart does not tell you
- It is computed at sea-level pressure. Wet-bulb temperature depends slightly on air pressure. At high altitude the true Delta T is a little higher than the table shows for the same temperature and humidity, so treat values near a band edge with caution on high plateaus.
- It is only as good as your readings. Measure temperature and humidity in the shade, at roughly the height of the spray, away from vehicles and hot surfaces. Forecast values describe an area, not your exact field.
- It says nothing about wind or stability. A green cell under a thermal inversion or in a strong wind is still a bad time to spray. Check wind speed and spray drift risk as well.
Reading the pattern
The table shows why the afternoon is usually the problem period: as temperature rises and humidity falls, you move down and to the left into higher Delta T. It also shows why cool, very humid mornings can sit below 2. A day that starts at 15 °C and 90 % humidity and reaches 32 °C and 35 % will cross every band. For what Delta T is and why the thresholds exist, read the Delta T guide.
Further reading
In PulveCast
PulveCast calculates Delta T hour by hour from the forecast temperature and relative humidity, so you do not have to look it up, and colours each hour against your own Delta T limits (2–8 °C by default).
More guides
- Delta T for spraying: what it is and why 2 to 8 °C
- 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
Frequently asked questions
How do I read a Delta T chart?
Find the row for air temperature and the column for relative humidity. The value where they cross is Delta T in °C. Values from 2 to 8 are generally considered ideal for spraying, 8 to 10 marginal, and above 10 or below 2 unsuitable.
Does altitude change Delta T?
Slightly. Charts are usually computed at sea-level pressure. At lower pressure, higher up, the wet-bulb temperature is a little lower, so the real Delta T is slightly higher than the chart shows.
Is a green Delta T enough to start spraying?
No. Delta T only describes evaporation. You also need suitable wind, no thermal inversion, no rain about to fall, and conditions that meet the product label.