Annual Cycles of Tropospheric Water Vapor

To understand better the annual cycles of atmospheric humidity, radiosonde data were used to create climatologies of temperature, dew point, relative humidity, and precipitable water in the lower troposphere for 56 locations around the world for the period 1973-1990. On the basis of the annual ranges of relative humidity at the surface and at the 850, 700, and 500 mbar levels and the ratio of the annual maximum to minimum surface to 500-mbar precipitable water, we have defined five humidity regimes: (1) middleand high-latitude continental, (2) middleand high-latitude oceanic, (3) midlatitude monsoon, (4) tropical oceanic, and (5) tropical monsoon. For each regime we describe the annual cycles of temperature and humidity variables and discuss phase relationships among them. Relative humidity ranges are small in the first two regimes, where precipitable water and temperature vary in phase. Relative humidity ranges in the other three regimes are moderate to large, and in the tropics the annual march of horizontal moisture advection and vertical convection, not temperature, controls seasonal humidity variations. These results suggest that the assumption of constant relative humidity made in some climate models is not always justified and that precipitable water is not a strong function of temperature in the tropics.