v.] SPRING FROSTS 153 



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 

 which there is always a gentle downdraught of cold 

 air from the clear sky above. But the circulation of 

 air in an anti-cyclone is always reversed at a certain 

 elevation, so that as one ascends, the downdraught of 

 cold air becomes less and eventually ceases ; the mean " 

 temperature at the same time rises instead of falling 

 with the elevation. 



Observations of the minimum temperature on the 

 grass made at two stations on a gentle slope of the downs 

 at Wye, about a mile apart and differing in level by 

 100 feet, showed during a period of thirty-six days 

 in February and March 1902 (which was exceptionally 

 calm and mild), a mean difference of i in favour of 

 the upper station ; but on two occasions the lower 

 thermometer fell to 24- 5 and 2i-5, when the upper 

 thermometer was 3Q-5 and 29- 5 respectively. 



