134 THE TEMPERATURE OF THE SOIL [chap. 



sea- level, yet it is well known that the severest frosts 

 occur locally at the bottom of valleys and hollow places. 

 This is particularly noticeable in the sudden night frosts, 

 characteristic of early autumn and late spring, which 

 are so dangerous to vegetation ; it is usual to find the 

 tenderer plants of our gardens, such as dahlias, cut 

 down by frost on the lower levels long before the 

 gardens on the hill are affected. Spring frosts, again, 

 will often nip the early potatoes in the valleys when 

 the higher lands are untouched. Fruit plantations 

 should not be set in the valleys, for no crop suffers 

 more from these unseasonable snaps of cold ; so clearly 

 is this fact recognised, that in some fruit-growing 

 districts only land above a certain elevation is regarded 

 as suitable for fruit, and commands a higher rent in 

 consequence. Two causes co-operate in producing the 

 excess of cold at the lower levels. The night frosts 

 in question are always the result of excessive radiation 

 when the sky is clear and the air still. The ground 

 surface loses heat rapidly and cools the layer of air 

 above; the cold air thus produced is denser, and pro- 

 ceeds to flow downhill and accumulate at the lower 

 levels. There is thus a renewal of the air above the 

 higher slopes, and the effect of radiation is mitigated 

 by the inflow of warmer air ; at the bottom no change 

 of air is produced and the radiation proceeds to its 

 full effect. 



At the same time the vegetation in the valley 

 is generally more susceptible to a frost; the greater 

 warmth by day, together with the extra moisture and 

 shelter, induce an earlier and a softer growth. 



The other cause that operates to produce severer 

 frosts in the valleys arises from the fact that frosts, 

 and radiation weather generally, accompany the dis- 

 tribution of pressure known as an " anti-cyclone," during 



