Delta T is the difference between wet bulb and dry bulb, and it is indicative of evaporation rate. The higher the Delta T the higher the evap rate. When spraying pesticides/herbicides, if the Delta T is too high the evaporation rate will also be too high, reducing the droplet life and therefore reducing the effectiveness of the spray. The preferred conditions for spraying are with a Delta T between 2 and 8 - if it is higher then for the same effectiveness you may need to increase application rates - with the resultant cost as well as potential environmental side effects. A short intro on weather effects for spraying is at
http://www.bom.gov.au/info/leaflets/Pesticide-Spraying.pdf.
In the "old days" DT was easy to measure by comparing the two std thermometers. These days with instruments reading dry bulb and RH, you need to reverse engineer an estimate of wet bulb to calculate DT. The NOAA algorithm uses an iterative process to estimate wet bulb based on dry bulb, RH and atm pressure (the latter parameter needed, I think, due to vapour pressure changing with pressure)
DT becomes critical here as often the times suitable for spraying to prepare for next years crop also coincide with the onset of warmer temps etc. For example the other day, once I cleared the paddock it was around 1200 and the Delta T was around 12 (temp about 30C and RH around 40 from memory) - making it too hot/dry to spray. Of course since then the weather turned (DT is currently around 4 and has been sitting around 2 for the last couple of days) but the wind also has picked up too much and there has been intermittent showers! On the bright side, the weather here has, in general, has been better here than in CA by all accounts!!!
Regards
Mike
