TKMPERATURE FACTS 



211 



prevent frost on hillsides when frost forms in valleys or on 

 level lowlands, and on the highlands above. The valley 



FIG. 147. Average date of first killing frost in autumn. 



FIG. 148. Average date of latest killing frost in spring. 



frosts are caused by the continued cooling of the lowland air 

 which cannot drain away, aided perhaps to some extent by 



