The maps on this plate are based on the occurrence of a temperature of 32°F (0°C) or lower in a standard thermometer shelter at a level of four feet above the ground, since these are the only standard representative values available. Frost may occur at grass level when the temperature in the shelter remains above freezing. On the other hand, freezing temperatures in the thermometer shelter are a definite indication of frost at grass level. The maps, showing the mean dates of the first occurrence in the fall and the last occurrence in the spring of a temperature of 32°F (0°C), are representative of different localities in the country. The mean annual frost-free period is the average number of days between the last frost in the spring and the first frost in the fall. This map illustrates the limitations imposed by frost on the length of the growing season.
