ON THE ANNUAL HEAT BALANCE OF THE NORTHERN HEMISPHERE

Abstract

The increased knowledge of atmospheric radiation during the last twenty years has made it desirable to re-compute the heat balance of the northern hemisphere. Insofar as possible the computations are based on observational data and, in particular, no assumption is made regarding the planetary albedo nor the albedo of clouds. The solar radiation reaching the surface is derived largely from the North American pyrheliometric network. These data are extended to the hemisphere on the basis of the differences between the cloud amounts along complete latitude circles and the cloud amounts along the North American segments. Absorption of solar radiation in the atmosphere is computed from the original Smithsonian data. Absorption by clouds is taken into account. The outgoing long-wave radiation is computed on the Elsasser radiation chart, with use of monthly mean data from a large number of individual stations. The planetary albedo is found to be 0.34, with a minimum of 0.28 in the subtropics and a maximum of 0.67 at the pole. About 19 per cent of the incident solar radiation is absorbed in the atmosphere, and 47 per cent is absorbed at the ground. The required poleward flux of heat is computed and is found to reach a maximum of 11.12 ? 1019 cal/day across the 40°N latitude circle. This is 20 to 25 per cent larger than earlier estimates. The hemispheric energy balance between the surface of the earth and the atmosphere indicates that the eddy flux of sensible heat is directed upwards, and that its magnitude is of the order of 10 per cent of the solar radiation incident on the outer atmosphere.