On Windy, wind data and other parameter data is data from weather models, NAM, ECMWF or GFS. As far as I know the Windy team does not use a crystal ball as you guess !
Do you know that you can compare observed wind by local weather stations and forecast by weather models on Windy:
Delta T is the difference between the dry bulb temperature and the wet bulb temperature, also called wet bulb depression. It is usually measured with the psychrometer.
It gives a better idea of the water evaporation rate than the relative humidity. It can be calculated from temperature and RH, or from temperature and dew point.
As you are interested by this parameter on your farm, I don’t think that a new layer would be a good way to show it. It would be better to have it in a diagram for a location, may be in a plugin together with other local calculated parameters. By this way you could directly see which are the best days and hours for spraying on your farm.
Concerning Windy Maps which is a different application from Windy, it is planned to give the possibility to display imperial units: