THE EFFECT OF MOISTURE ON LAYER THICKNESSES USED TO MONITOR GLOBAL TEMPERATURES

Mean layer virtual temperature estimates, based on geopotential height measurements, form the basis for one approach being used to monitor c hanges in upper-air temperature. However, virtual temperature is a fun ction of atmospheric moisture content as well as temperature. This pap er investigates the impact of real or apparent changes in atmospheric moisture on changes in mean layer virtual temperature. Real changes in mean layer specific humidity of up to 50% would cause changes in mean layer virtual temperature of less than 1-degrees-C, except in the tro pical boundary layer, where the high moisture content would lead to la rger virtual temperature changes. The effect of humidity changes is ne gligible in polar regions and most pronounced in the tropics, which co uld influence the interpretation of the latitudinal gradient of virtua l temperature trend estimates. Improvements in radiosonde humidity sen sors since 1958 have led to an apparent decrease in atmospheric humidi ty. On global average, for the 850-300-mb layer, such changes are esti mated to contribute to an apparent cooling of between 0.05-degrees and 0.1-degrees-C, or about 10% to 20% of the observed warming trend sinc e 1958.