1912.] 



Night Frosts in Spring. 



933 



protection of the screen is very striking. The differences are 

 not altogether due to enclosure in a screen ; the height of the 

 screen above the ground has something to do with it, because 

 the trickling stream of cold air keeps to the ground, and is 

 often not very thick. 



Distribution of Temperature during Night Frosts. — The 

 following conclusions regarding the distribution of tempera- 

 ture during frosts on calm nights are well established : — 



(a) The frost becomes more severe as one goes from 

 the hills down to the valleys. Hollows on the hillsides 

 are colder than the more exposed parts. 



(b) The frost is most severe at the ground, and becomes 

 less severe at shrub height, still less so at tree height, 

 so that herbage and low shrubs may be destroyed when 

 higher shrubs and trees are spared. 



(c) An overcast sky or a light wind generally prevents 

 ground frosts. 



It is also a well-established meteorological fact that, on the 

 average, wind falls off in the evening, and in settled weather 

 a calm night often follows a day with a good breeze. This is 

 especially the case with an easterly wind. 



Forecasts of Spring Frosts. — Referring to the three causes 

 described above, the first two are easily associated with general 

 meteorological conditions over the country, and to anticipate 

 them forms part of the ordinary duty of weather forecasting. 

 The changes are often very sudden, and while it is, as a rule, 

 possible to anticipate their general character, it is less easy 

 to form an estimate of the intensity of the changes. The 

 difference between the changes which produce only a chill 

 and those which cause a frost is not indicated on the maps 

 used for forecasting. 



The frosts of calm nights are still more difficult to deal 

 with by forecasts issued from a central office. They are 

 subject to the effects of local peculiarities of site and circum- 

 stances of which account can only be taken by those who are 

 on the spot. A light air, hardly strong enough to be called 

 a wind, will keep away a night frost by preventing stagnation ; 

 a calm, on the other hand, favours frost, but from the point 

 of view of the weather forecaster the calm may be an artificial 

 calm due to surrounding trees or buildings, and not at all 

 due to what he would understand by the weather. 



