It is shown that just above a cold black-body surface the effect of radiation is to warm the air, whereas just above a warm surface the effect of radiation is to cool the air. Above a height of the order of one meter the signs of the radiative temperature change are reversed. Since, as Emmons and Montgomery (1947) have pointed out, fog formation usually requires net radiation from the air, it follows that, above a cold surface, fog forms only above a certain critical height whereas above a warm surface fog forms below a critical height. Other processes which produce condensation appear to be less important....