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.
stations. The information is graphically charted on maps which show the dates of late spring and early fall frosts, the average dates of killing frosts, and the number of days between spring and fall frosts. The following paragraphs apply pretty generally to all parts of the United States: Frost becomes more severe as one goes from hillside to low spots, such as hollows and stream valleys. It is more severe on the grass than at shrub heights. It may form on the grass when the temperature 3 or 4 feet above ground is several degrees above the freezing-point. If the temperature at sunset is not lower than 40° F (5° C) and the sky is overcast, frost is not likely to occur. But if the sky is clear and the wind is at calm, frost is likely. With a brisk wind and a sky either clear or cloudy, frost is not likely to occur unless the temperature falls materially below freezing. If the air is moist at sunset and the temperature is 40° F or lower, frost is likely to occur even with a light wind; but if fog occurs, enough latent heat may be set free to prevent frost. A low-lying fog is a blanket which retards radiation, not only from grass and shrubbery, but from the ground itself.
