522 Bulletin 316 



There is always more or less moisture in the air in the form of vapor. The 

 moisture gets into the air because the heat of the sun is always changing the 

 water on the earth into vapor. To change a pint of water into vapor 

 requires a little more heat than would raise the temperature of five pints 

 of water from the freezing to the boiling point. Conversely, the vapor 

 in the air always may be changed again into water if the temperature 

 be sufhciently lowered, and when so changed the heat used originally 

 to evaporate the water is released, or " liberated, " and becomes available to 

 warm the air, just as addition of heat from any outside source may do. 



The dew point is the temperature at which the change from vapor to 

 water begins, and is, therefore, the temperature at which the release of 

 heat begins also. On these facts is based the belief that if the dew point 

 is above freezing, the addition of heat to the air as dew continues to 

 form — for dew is the resvilt of change from vapor to water — will check, 

 or at least retard, the fall in temperature and thus tend to prevent frost. 

 On the other hand, if the dew point is below freezing, no retarding effect 

 can be expected until the formation of frost has taken place. Thus, 

 the dew-point temperature during the early evening, because it shows 

 the temperature at which the release of heat may be expected to begin, 

 has been considered a reliable guide to the lowest temperature of the 

 following night. That its value as a guide in forecasting frost depends 

 on general climatic conditions and varies widely in different parts of the 

 country, seems to be borne out by the experience of a number of observers. 

 O'Gara, in his work on " The Prevention of Frost Injury in the Orchards 

 in the Rogue River Valley " (near Medford, Oreg.), found that there was 

 a very close relation between the afternoon dew-point and the lowest 

 temperature during the ensuing night. On ten clear, quiet nights when 

 frost was likely to occur, the average evening dew-point was 34.7°, while 

 the average of the ensuing minimum temperatures was 34.5°. The great- 

 est difference on any night was 3° and there were several times when 

 the evening dew-point and the following minimum temperature were 

 the same. He considers the afternoon dew-point not only a reliable 

 indication of whether or not frost will occur, but also a valuable guide 

 as to the degree of frost. 



Professor Cox, in his investigation of frost in the cranberry districts 

 of Wisconsin, found that the early evening dew-point afforded " no indi- 

 cation whatever " of the ensuing minimum temperature. At the experi- 

 ment station at Mather, Wis., during the month of September, 1907, 

 the average dew-point temperature at 6 p. m. was 56.3° and the average 

 of the following minima was 46.6° — an average difference of 9.7°. The 

 variation for the different days of the month ranged from 0° to 28°; on 

 thirteen nights the difference was 10° or more and there was only one 



