Contained within the 3rd Edition (1957) of the Atlas of Canada is a plate that shows four maps of certain climatic aspects of the growing seasons. The first map shows the mean annual length of the growing season throughout Canada, assuming the growing season to be that part of the year when the mean daily temperature is above 42 degrees F (5.6 degrees C). The map was constructed from data for the period 1921-1950 inclusive and is based on the mean annual number of days on which the mean temperature, as estimated from the smoothed annual course of temperature, was above 42 degrees F (5.6 degrees C). The number of degree-days above 42 degrees F (5.6 degrees C) is the difference between the daily mean temperature and 42 degrees F (5.6 degrees C) when the former is more than 42 degrees F (5.6 degrees C). The annual number of degree-days, as shown on the second map, is the sum of such daily values during the year. The mean growing season precipitation map shows the average precipitation during the growing season for the period 1921 to 1950 inclusive. For this purpose, the growing season for all of Canada was arbitrarily assumed to be the period from April 1st to August 31st inclusive. There is also a map showing the variability of growing season precipitation. The variability is expressed in terms of the coefficient of variation. For any one place it would be the standard deviation of growing season precipitation divided by the mean growing season precipitation. The result is multiplied by 100 so that it can be shown as a percentage.