Air temperature is the bulk temperature of the air, not the surface (skin) temperature. A variable whose standard name has the form number_of_days_with_X_below|above_threshold is a count of the number of days on which the condition X_below|above_threshold is satisfied. It must have a coordinate variable or scalar coordinate variable with the a standard name of X to supply the threshold(s). It must have a climatological time variable, and a cell_methods entry for within days which describes the processing of quantity X before the threshold is applied. A number_of_days is an extensive quantity in time, and the cell_methods entry for over days should be "sum".