There are several methods of predicting the occurrence of frost (1). Many involve an empirical relationship to the dewpoint temperature Td. For instance, in Victoria (Australia) frost is reckoned possible if Td is less than 6° C, and probable if it is below 0° C. In the latter case, if in addition the wind is southerly, and pressure rising (so that air is subsiding, making the sky clear and the air aloft dry, and therefore transparent to terrestrial longwave radiation), frost is certain, except perhaps along the coast. In Canada he nocturnal minimum temperature is deduced as equal to [Tmax + Td]/2 minus a correction which depends on the wind speed and cloudiness, where Tmax is the highest temperature reached the previous day.
It should be noted that the value of Td is preferably that measured late in the previous day. The variation of Td during the day is mentioned in Section 6.4.
(1) Bagdonas, A., J.C. Georg and J.F. Gerber 1978. Techniques of frost prediction and methods of frost and cold protection. WMO Tech. Note 157 (World Meteor. Organ.) 160pp.