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Rh Weather Bureau records make a distinction between light frosts and killing frosts, the latter being so called because of their destructive effects. Ground frosts, as a rule, are not killing; the freezing temperature does not extend more than a few inches above the grass. If the freezing temperature extends so high that frost covers the roofs of buildings, the frost is apt to be killing. Records of the dates of the latest killing spring frost and the earliest killing fall frost are highly important from the fact that the number of days intervening constitutes the growing season.

In the latitude of the Great Lakes the growing season is from 110 days to 150 days; in the latitude of Illinois and Missouri it is from 150 days to 200 days; in the belt extending from the northern boundary of Tennessee to the Gulf it is from 200 to 300 days. Florida and Texas, south of the twenty-seventh parallel, are very rarely visited by killing frosts.