What is Delta T and When Can I Spray?

The Simple Version

Delta T is the difference between the dry bulb temperature and the wet bulb temperature of the air. In plain English, it is a measure of how actively the air wants to evaporate water. When Delta T is high, the air is dry and hot and droplets will evaporate before they land. When Delta T is low, the air is cool and humid and droplets stay wetter for longer — but that also creates its own problems.

For most herbicide, fungicide, and insecticide applications in Australian conditions, the target Delta T range is between 2 and 8. That is the sweet spot where droplet integrity is maintained, chemical efficacy is optimised, and the risk of spray drift is manageable. Outside that range you are working against the conditions, not with them.

Why It Matters More Than You Might Think

Australian farmers have been relying on wind speed as their primary spray indicator for a long time. Wind speed matters — you should not be spraying in 30-kilometre winds regardless of anything else — but it is not the only thing that determines whether your spray job will work. A calm, 35-degree afternoon in January might have a Delta T of 12 or 13. The air is so dry and hot that fine droplets are evaporating before they reach the target. You are wasting chemical, wasting time, and potentially creating vapour that drifts.

Conversely, a still morning with heavy dew and fog might have a Delta T of less than 2. The air is almost saturated. Droplets stay wet and large, which means they run off leaves rather than absorbing into the leaf surface. Fungicide applications in these conditions are particularly poor value because the wet-surface conditions that encouraged you to spray are the same conditions that prevent the chemistry from doing its job properly.

Where the Numbers Come From

Delta T cannot be measured directly with a single sensor. It is a calculated value derived from dry bulb temperature and relative humidity. The formula uses these two readings to calculate what the wet bulb temperature would be, and the difference between dry bulb and wet bulb is your Delta T.

This is why you need a weather station that logs both temperature and humidity to track Delta T properly. BOM forecasts give you a rough guide, but conditions in your paddock — particularly in cropped areas — can differ significantly from the nearest weather station, which might be 50 kilometres away and sitting over bare ground rather than a crop canopy. Your own on-farm data is always more accurate for spray decision-making than regional forecasts.

The Three Zones and What They Mean for Spraying

Delta T below 2: Conditions are too humid for most spray applications. Droplets stay wet for too long, run-off from leaf surfaces is likely, and the temperature conditions that produce low Delta T — typically early morning with heavy dew — also mean the plant stomata may not yet be open. Hold off.

Delta T 2 to 8: This is your working window. Conditions are suitable for most agricultural spray applications. At the lower end of this range, conditions are better for fine droplet sizes and products that need leaf absorption. At the upper end, move to coarser nozzle settings to reduce drift risk.

Delta T above 8: Evaporation is too high. Fine droplets are being lost before they land, particularly with contact herbicides and fungicides that need sustained contact with the target. You can compensate partially with very coarse droplet sizes and high carrier volumes, but the honest advice is that above 10 you are usually better to wait for better conditions.

Delta T above 10 or 12 in high-temperature conditions is also when temperature inversion risk increases significantly. Inversions trap fine spray droplets in a shallow layer of air near the ground and can carry them kilometres from the application site. This is a major concern for dicamba and other volatile herbicides, and increasingly it is a legal liability issue, not just a practical one.

When Does the Window Typically Open in Australian Conditions?

Across most of southern and eastern Australia, the Delta T sweet spot in summer is typically found early in the morning — roughly between 7am and 10am — before temperatures climb and relative humidity drops. In some districts during heatwave conditions, there may be no suitable window at all during the day and you need to weigh up whether to delay or work at first light.

In winter-dominant rainfall zones, the spring season often provides more consistent Delta T windows in the mid-morning to early afternoon range, particularly after cool overnight temperatures have moderated to above 10 degrees. Delta T typically rises through the day and may push above 8 or 9 by early afternoon before cooling back into range late afternoon.

The key insight that most farmers who start logging Delta T data discover is that the window moves more than they expected. A day that looked like it would be fine for morning spraying based on the forecast can shut down by 9am if temperature climbs faster than expected. Real-time monitoring lets you make the call based on what is actually happening in your paddock, not what a forecast said last night.

Setting Alerts on Your Weather Station

The most practical way to use Delta T data is to set alerts on your weather station monitoring platform so you get a notification when conditions enter your spray window. Rather than checking data every half hour during busy periods, you receive a message when Delta T drops into range and another when it climbs out. That gives you a real working window rather than a guessed one, and it means you can make confident decisions about whether to start the tractor, keep going, or pull up for the day.

BushLinx® weather stations log temperature and humidity on-farm and display calculated Delta T in real time through the platform. You can set upper and lower alerts so you know exactly when your spray window opens and closes without having to watch the numbers yourself.