Spray application manual
31 January 2025
Module 10: Weather monitoring for spraying operations
10.3 Temperature and humidity
Published 24 January 2025 | Last updated 20 January 2025
Temperature and humidity can affect droplet survival (due to evaporation), as well as the ability of the product to enter or translocate within the target plant (due to stress).
Droplet survival is affected by the rate of evaporation of water from the droplets, which causes them to reduce in size. As droplets reduce in size, their ability to settle towards the ground is greatly reduced and the potential for spray drift to occur increases.
To an extent, selecting large enough droplets (such as a coarse spray quality or larger) and a high enough water rate can help overcome some of the impacts of high evaporation rates.
The evaporation rate of water-based droplets can be estimated by looking at the difference in temperature between a wet bulb thermometer and a dry bulb thermometer, often referred to as ‘Delta T’.
Common Delta T values recommended for spraying are highlighted in Graph 1.
Find out more
For more information on interactions between weeds, the product and the conditions, go to Module 2: Product requirements
The relationship of Delta T to relative humidity
Where to measure temperature and humidity
How and where you should measure temperature and humidity will depend on what you require the information for: is it purely for record-keeping, or is it also for efficacy and decision-making?
For record-keeping purposes
For record-keeping purposes, temperature and humidity should be measured in a shaded position at 1.25 metres above the ground at the site of application.
Tips
If using a hand-held weather meter in the paddock, be sure the meter has had enough time to acclimatise to the situation (especially if it has been in a vehicle).
If mixing at the application site, leave a weather meter hanging in a shaded position so it is already acclimatised when you want to use it.
Ensure that temperature and humidity sensors are not receiving direct radiation from the sun, as this may affect the accuracy of the measurements. It may be useful to turn the weather meter ‘side on’ to the direction of the sun, so the sensors are shaded.
For assessing condition of the target and decision-making
Air temperature and relative humidity at 1.25m above the ground will give an indication of droplet survival of airborne droplets. Under wind conditions suitable for spraying, applying a Coarse or larger spray quality should result in more than
90 per cent of droplets reaching the target or the soil. Assessing Delta T at the target may assist in understanding conditions that may affect pesticide leaf uptake.
Tips
When using a coarse spray quality or larger, if Delta T values in the air approach values of about 10 to 12, it is time to assess the condition of the target plants or weeds.
If the operator compares the Delta T value measured in the air with a measurement at the target plants or weeds, this can provide information about the condition of the weeds and about droplet survival.
Delta T values measured lower at the target than those measured in the air can indicate that plants may be transpiring, or that there is good soil moisture present, which suggest plants are not likely to be heat- stressed and droplet survival should be suitable.