Delta T is used by the agricultural industry. It is an important indicator for acceptable spraying conditions. It is indicative of evaporation rate and droplet lifetime. Delta T is calculated by subtracting the wet bulb temperature from the dry bulb temperature. When applying pesticides, Delta T should ideally be between 2 and 8, and not greater than 10. Select the right Delta T to determine the best weather conditions for spraying(PDF 593 Kb).
The diagram below relates air temperature and relative humidity to values of Delta T.
This detail comes from the Australian Bureau of Meteorology, and Delta T values are generally available on many site pages, especially where regular automatic data collection is occurring.
Considered a very useful tool if you are spraying agrochemicals, with a guide being that if in the range of 3-7, it is okay to be spraying [ a 2-8 range is about the safe limit as above hence a small safety margin for security].
You can check for previous conditions as a check for likely conditions the next day in many areas where weather is reasonably stable.
In the Top End, evening and early morning are generally safest, with the Delta -T value within a suitable range, but......not always.
If you can, plan ahead and check just before spraying. Warm temperatures will drive it up, but there is often a guide for many chemicals to use below 30C, sometimes a bit difficult to achieve, so less effectiveness might be expected.
It may be possible to adjust spray parameters - pressure, droplet size and spray direction for example to compensate for poor conditions if spray events are critical, but often it may be prudent to wait a little time for better conditions eg early am or evening.
In the dry, but cooler dry season in the north, it is quite common for the Delta T to be well above 10 in the afternoon, when both above 30C is common and RH is very low [less than 30%] - obviously avoid this time for spraying!!
There are commercial services available from a number of agribusiness firms, some of which do add some additional options. However, with the Delta T values and graph available, you can quickly check yourself too for no real cost, except your time to check. It is simple, but a desirable check before spraying.
There are commercial services available from a number of agribusiness firms, some of which do add some additional options. However, with the Delta T values and graph available, you can quickly check yourself too for no real cost, except your time to check. It is simple, but a desirable check before spraying.
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