Wet Bulb Potential Temperature

(often abbreviated to WBPT, or 'theta-W') A relatively conservative property within any one air mass that is derived from the temperature and humidity values of a particular air sample for a particular level: usually 850 or 500 hPa. Very warm/very humid southerlies for example (in NW Europe) would have typical 850hPa WBPT values well in excess of 16 degC, and perhaps as high as 20 degC or more; polar maritime air streams would have values typically 5 to 10 degC, but these values would be much lower in the depths of winter.