Atmospheric Moisture-Precipitation Relations

Relationships are established between water vapor inflow, precipitable water depth, and surface precipitation, in Illinois, under average and extreme weather conditions. The water vapor inflow and precipitable water depth normally maximize in summer, but the percentage of the water vapor precipitated as rain or snow is greatest in spring. Normally, nearly 50% of the water vapor is contained in the lower 5,000 ft. Studies of severe rainstorms indicate that an extremely moist atmosphere is not necessary for the development of such storms, although the moisture inflow and concentration is usually above normal. The average precipitation efficiency in winter and summer storms is found to be nearly equal, with heavier summer rainstorms resulting from a greater inflow of water vapor. Analyses of dry periods of five days or longer and of a severe twelve-month drought indicate no distinct trend for large departures from normal depths of precipitable water in such periods.