500 PROTECTION AGAINST FROST. 



descent of heavy, cold air from above, which collects like 

 a lake over the low ground and causes sharply-defined 

 frost-limits. 



ii. Woods on north-easterly, easterly, south-easterly or 

 southerly aspects suffer most from frost; on the two 

 former aspects, because, unless sheltered by a hill, they are 

 exposed to cold frosty winds, whilst the sudden exposure to 

 the sun's rays after sunrise increases the danger' of easterly 

 aspects. On south-easterly and southerly aspects growth 

 begins earlier in the spring than on colder aspects, prolonging 

 the period of exposure. On south aspects the variations of 

 temperature are also greater than on cooler aspects. Northerly 

 and westerly aspects are least exposed to injury by frost. 



iii. High plateaux are more heated by the sun than low- 

 lands, where the atmosphere is denser ; vegetation on them is 

 therefore more precocious. But the radiation of heat at 

 night is greater than in lowlands, and consequently there is a 

 greater range of temperature, while woody plants stand sudden 

 changes from heat to cold worse than intense cold. 



As a rule, valleys, lowlands, and plateaux suffer more from 

 frost than hills and mountain sides. 



iv. Wet impermeable soils, such as cold clays, induce low 

 air-temperature, and frost holds out longer there. The richer 

 and deeper the soil, the more quickly do plants recover from 

 freezing and grow out of the reach of frost. 



<j. Nature of Soil-covering. 



A dense and high growth of grass and herbage increases the 

 danger from frost, as it prevents the soil from being heated, 

 diminishes the circulation of the air, and reduces the tempera- 

 ture by transpiring moisture and radiating heat. From 

 observations made at Viernheim in Hesse, the temperature 

 over an area covered with grass may be 16 F. lower than 

 on a plot of similar land bare of vegetation. On the other 

 hand, a lightly shading growth of birch, sallow, thorns, broom, 

 etc., will reduce radiation and thus preserve from frost plants 

 of valuable species which may be growing among the woody 

 undergrowth. Under conditions otherwise the same, frost 

 penetrates more slowly and less deeply into a soil covered with 



