Contained within the 3rd Edition (1957) of the Atlas of Canada is a plate that shows four maps, one for each of January, April, July and October showing mean sea-level pressure in millibars. These maps were prepared from atmospheric pressure readings taken daily at 00, 06, 12 and 18 hours G.M.T. during the period 1940 to 1949 inclusive, except for the area north of 72.5 degrees N, where the period was 1948 to 1953 inclusive (and for which the results were adjusted to the 1940 to 1949 period). Ten mean sea-level pressure maps were drawn for each of the selected months in these periods using averages of the observed pressure. Then a grid system was superimposed over these maps and the pressure at 200 points read off. From these figures the mean maps reproduced here were drawn.