Wax temp ratings are based upon snow temperatures (correct?). Is there a rule of them to correlate air temp to snow temp?

Air and snow temps correlate fairly closely. When snow temps are taken by wax techs they measure the top 1/4 to 1/2 inch of snow. In the absence of sun the snow lags behind the air temp, in bright sun it will exceed the air temp. In practical terms wax a touch warmer in the afternoon and a touch cooler in the morning (in most instances the air temp and snow temp will be within the range of the wax). As snow temps approach the melt point waxing becomes a crap shoot, best to use no wax skis or skate at those temps.