Contained within the 3rd Edition (1957) of the Atlas of Canada is a plate that consists of three maps based on the occurrence of a temperature of 32 degrees F (0 degrees C) or lower in a standard thermometer shelter at a level of four feet above the ground. 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 degrees F (0 degrees 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.