934 



Night Frosts in Spring. [fee-., 



It has already been pointed out that situation, whether on 

 a hill or hillside, or a valley, is also of importance. It is, 

 therefore, necessary that persons interested in protecting their 

 crops from frost should make use of their local knowledge 

 in extension of the information to be obtained from forecasts. 



One of the best aids to the use of local knowledge is the 

 regular study of weather maps. The Meteorological Office 

 issues daily charts of the weather over the British Isles and 

 Xorth-western Europe, which can be had by anyone on pay- 

 ment of the cost of postage and covers (£i per annum), and 

 the conditions for the occurrence of frosts can be watched 

 much more effectively by an examination of the daily map 

 than by the mere consultation of the forecast which is drawn 

 up for a whole district and must be limited to about a dozen 

 words. 



Further information about the probability of a night frost 

 may be got from local observations of the temperature and 

 humidity of the air. From readings of the dry and wet bulb 

 thermometer on any occasion, the so-called "dew point" of 

 the air can be computed. The dew point is the temperature 

 at which dew begins to form. 



It has been supposed that on a calm night the air cannot 

 be cooled far below the temperature of the dew point of the 

 previous evening, but this is not the case. All we can say 

 is that the loss of heat is checked when the temperature falls 

 to that of the dew point. The following cases combine to 

 bring about this result : — (a) While the temperature is above 

 the dew point evaporation is proceeding from the ground and 

 from herbage, and causes a loss of heat from them in addition 

 to that due to exposure. Below the dew point this additional 

 loss of heat does not occur, (b) If exposed surfaces are cooled 

 below the dew point, moisture condenses on them in the form 

 of dew (or hoar frost), and in the process a certain amount 

 of heat is liberated from the condensed moisture, which in part 

 balances the loss of heat due to exposure, (c) The cooling 

 of the air may result in the formation of mist which, acting 

 as a screen, checks a further fall of temperature. Destructive 

 frosts on calm nights are thus more likely when the air is 

 dry, that is, when the dew point is far below the air tempera- 

 ture, or the wet bulb is far below the dry, than on nights when 



