The mean maps show average geopotential metric heights for the 850-, 700-, 500-, and 300-mb pressure surfaces at 000 and 1200 GMT for each month of the year. The maps were derived by computer from objectively analyzed grid-point data for November 1961 through January 1969. The use of these maps is briefly described to obtain the average monthly height for any day and 12-hour period

Map of mean diurnal (12-hourly) and median (midway between 0000 and 1200 GMT) monthly height changes at the 850-, 700-, and 500-mb levels over North America and vicinity from January through December are presented, using grid-point data from an earlier study. The height changes were objectively derived and analyzed by computer. The use of these charts as an aid to weather forecasting is briefly described