AREAL DISTRIBUTION AND DIURNAL VARIATION OF WATER VAPOR NEAR THE GROUND IN THE CONTIGUOUS UNITED STATES

Abstract

Abstract : Maps showing the distribution in the United States (except Hawaii and Alaska) of average monthly dew point and its standard deviation, average monthly vapor pressure, and types of diurnal variation and range of dew point are presented and discussed. The maps are based on hourly psychrometric observations at nearly 200 stations for lengths of record of about ten years. The principal features of the maps are evidence of different humidity controls in different areas of the country. The tendency toward east-west alignment of the isopleths in the East, the varied pattern approximating contours in the mountainous areas, and the north-south alignment of the isopleths near the West Coast reflect the differing controls. Four types of diurnal variation of dew point are defined based on the time of occurrence of the average minimum dew point. Type I is indicative of moisture availability at the earth's surface with highest dew points during the day. Type IV is found in dry areas and has lowest dew points during the day. Types II and III are transitional with both a morning and afternoon minimum of dew point in the daily cycle. Areas of the United States with differing humidity regimes are delineated in Part IV of the study. The areas, differentiated on the basis of average annual dew point and the range of average monthly dew points, reflect the dominant control of latitude in the East, altitude in the mountain states, and exposure on the West Coast. This final section serves to summarize the maps presented in the report.