Mean monthly values of precipitable water for about 50 stations in the United States are used to describe the amount and variation of moisture over the area. The average precipitable water over the United States is found to be 1.75 cm and to range from 0.94 cm in February to 2.99 cm in July. Monthly and yearly averages for the period 1946 to 1956 indicate that there is considerable variation on a monthly and sectional basis but that the yearly and overall area amounts of precipitable water are rather stable.

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1 The work reported here was supported by the Office of Naval Research under contract number ONR 2173–02.