Delta T Calculator and Chart

What is Delta T?

Delta T is the difference between the dry bulb temperature (the normal air temperature) and the wet bulb temperature. A wet bulb thermometer has a wet wick around it, so it cools as the water evaporates. The drier the air, the faster water evaporates and the bigger the gap. The Bureau of Meteorology describes Delta T as “indicative of evaporation rate and droplet lifetime”, which is why it is used to judge spray conditions.

Delta T joins temperature and humidity in one number, which humidity alone cannot do. At 30% humidity, Delta T is 7.6 at 15 °C but 13.3 at 35 °C.

Delta T ranges for spraying

Delta T is one check, not the only one. The Bureau’s leaflet also says to avoid spraying above 28 °C, and that humidity above 45% is often recommended; some labels set their own top temperature, often 30 °C. None of the current labels we read set a Delta T limit: they set the wind range, the droplet size and the inversion rule, so those come first.

Sources: the Bureau of Meteorology’s Delta T page and its leaflet Weather for Pesticide Spraying, which carries Nufarm’s Delta T chart. The low Delta T signs are from the GRDC article Paddock practices: a guide to reducing off target spray damage. The coarse spray range is from Nufarm’s 24 hour spray risk profile, and the advice past 8 from the Cotton Pest Management Guide 2025 to 26 (CRDC and CottonInfo).

How to work out Delta T

Delta T = dry bulb temperature minus wet bulb temperature.

With a sling or fan psychrometer (two thermometers, one with a wet wick), whirl or fan it until the wet bulb stops falling, read both at once and take one from the other. The wet bulb starts to rise as soon as the air stops moving past it, so a slow reading shows too low a Delta T (WMO guide to instruments, chapter 4). Hand held weather instruments work out the wet bulb from temperature, humidity and air pressure; keep the sensor out of direct sun and let it settle. A dry bulb of 26 °C and a wet bulb of 19 °C is a Delta T of 7.0, which is about 51% humidity.

From air temperature and relative humidity, the calculator works out the wet bulb with the psychrometric equation at sea level pressure (1013.25 hPa):

• Vapour pressure in the air = humidity ÷ 100 × saturation vapour pressure at the air temperature
• Wet bulb = the temperature where saturation vapour pressure, less 0.00066 × (1 + 0.00115 × wet bulb) × 1013.25 × (air temperature minus wet bulb), equals that vapour pressure
• Saturation vapour pressure (hPa) = 6.1121 × e^{(18.678 − T ÷ 234.5) × T ÷ (257.14 + T)} (Buck, 1996)

The calculator solves this in small steps until the two sides match, then rounds Delta T to one decimal place. We checked it against the Delta T chart in the Bureau’s leaflet at 39 points from 10 to 45 °C: on average it is within 0.1 °C of the printed curves. Higher up, air pressure is lower and Delta T reads a little higher: at about 1,000 m it is roughly 4% higher than the chart shows. Enter your height above sea level in the calculator to allow for it.

The Bureau’s latest observations pages list a Delta T too, but they work it out for a wet bulb thermometer inside a weather screen, where little air moves past it, using a larger coefficient (7.866 × 10⁻⁴, from the Bureau’s note Calculation of Relative Humidity). In dry air that reads lower than this calculator, a hand held instrument or the Bureau’s own spraying chart: at 20 °C and 20% humidity this calculator gives 10.6, while the observations pages show about 9.7. A spray droplet moving through the air is well ventilated, so it behaves like the ventilated wet bulb this calculator uses. The two figures agree closely when Delta T is low, and the observations come from a screen that may be a long way from your paddock.

Worked examples

1. Early morning, dew on the grass: 12.0 °C and 95% humidity is a Delta T of 0.4. With dew and a wind of 2 km/h, an inversion is likely: no go. Wait until the air has warmed and a steady breeze has come up.
2. Mid morning: 22.0 °C and 55% humidity is a Delta T of 5.8. With a wind of 9 km/h and a label band of 3 to 15 km/h, it is a go.
3. Hot afternoon: 34.0 °C and 20% humidity is a Delta T of 15.5. Fine droplets will dry out and drift: no go until it cools down.

The record line for the mid morning example reads: 17/10/2026 9:30 am, temperature 22.0 °C, RH 55%, Delta T 5.8, wind 9 km/h from the SE.

Delta T chart

The graph shows where each Delta T falls by air temperature and humidity. The table below gives the numbers, ready to print.

Print this chart. For readings between the rows and columns, or above 90% humidity on a dewy morning, use the calculator above.

Signs of a surface temperature inversion

On a clear, calm night the ground cools and chills the air just above it, with warmer air sitting on top like a lid. Fine droplets can hang in that cool layer and move a long way sideways, then land well away from where you sprayed. Labels say do not apply during surface temperature inversion conditions.

GRDC’s fact sheet on inversions says one is likely if:

• mist, fog, dew or a frost have occurred
• smoke or dust hangs in the air and moves sideways, just above the ground
• cumulus clouds that built up during the day collapse towards evening
• wind speed stays below 11 km/h in the evening and overnight
• cool breezes flow down slopes in the evening or overnight
• distant sounds are clearer and smells are stronger than during the day

The 2,4-D label instructions say these conditions “exist most evenings one to two hours before sunset and persist until one to two hours after sunrise” (Nufarm 2,4-D spray drift instructions, which also list the signs above). GRDC adds that a Delta T below 2 with low overnight wind and dew or frost points to a likely inversion. A Delta T inside 2 to 8 does not rule one out: a CRDC field study in the Macquarie Valley found inversions with Delta T in the suitable range (project CGA1704 report). Where no recognised inversion monitoring covers the site, current labels treat every surface temperature inversion as hazardous. An inversion has probably cleared once the air has warmed more than 5 °C above the overnight low and the wind has stayed above 7 km/h for 45 minutes after sunrise.

In the grain and cotton regions of NSW and Queensland, the WAND network of 102 inversion towers, from CRDC and GRDC with Goanna Ag, shows whether a hazardous inversion is present and is updated every 10 minutes.

Wind and your product label

Each label sets its own wind band, under the spray drift restraints. Many current labels read “DO NOT apply unless the wind speed is between 3 and 20 kilometres per hour at the application site during the time of application” (for example Nufarm Dropzone, 2026), and some say 3 and 15 (Nufarm 2,4-D spray drift instructions). Type the band from your own label into the calculator. It checks your wind speed against it, but it never supplies a band.

Measure the wind where you are spraying. The Bureau notes that the wind in its forecasts and observations is the 10 minute average at 10 metres up, and that wind at crop level may be different. Calm air is not safe either: the Bureau’s leaflet says to avoid calm conditions because small droplets may stay in the air for a long time.

For spot spraying weeds with a knapsack, a coarser nozzle, lower pressure and a shield all cut drift. See our guide to knapsack sprayers for weed control, the Gloria weed killer spraying shield and our agriculture and horticulture sprayers. Check your sprayer’s seals suit the chemical in the Chemical Compatibility Guide.

What to write in your spray record

State rules ask for the weather on spray records. For example, NSW asks people spraying outdoors to record the wind strength and direction and any big weather changes (NSW EPA), and Victoria’s pest control record rules ask for the temperature, wind direction and wind speed for outdoor work (Victorian Department of Health, technical note 3). Labels can ask for more: one current herbicide label says to record the wind speed and direction and the “air temperature and relative humidity during application” within 24 hours, and keep the record for 2 years (APVMA approved label). Check the rules for your state, your work and your label.

The calculator’s record line gives the date, time, temperature, humidity, Delta T, and wind speed and direction. Tap Copy for spray record and paste it into your record.

Frequently asked questions