The climatology maps on this page show the percent of the annual normal precipitation that falls for each of the twelve calendar months (January to December), for each of the twelve 3-month seasons (January-March to December-February), and for each of the twelve 6-month seasons (January-June to December-May). If precipitation were evenly distributed throughout the year, then the value would be 8.5% (1 month divided by 12 months) for each monthly map, 25% (3/12) for each 3-month map, and 50% (6/12) for each 6-month map. These maps illustrate the geographical distribution of the wet and dry seasons across the North American continent.
For further information on the data presented here, contact:
Climate Services and Monitoring Division
http://www.ncdc.noaa.gov/oa/climate/monitoring/drought/nadm/climo.php