The effect of cloudiness on a greenhouse model of the Venus atmosphere

In previous models of the greenhouse effect in the Venus atmosphere it has been assumed that infrared-absorbing atmospheric gases provide the sole contribution to the infrared opacity of the Venus atmosphere. In the present study the influence of an extensive cloud cover, opaque to infrared radiation, is also included in the greenhouse model. The magnitude of the greenhouse effect, which is defined here as the ratio of the surface temperature produced by the greenhouse to the surface temperature of an atmosphereless Venus, is computed as a function of infrared opacity of the atmosphere, and amount and height (actually ratio of cloud-top pressure to surface pressure) of clouds. It is assumed that the Venus atmosphere is gray, that the absorbing gas has a constant mixing ratio, and that the temperature variation with altitude is linear. Calculations are made for two temperature lapse rates: the adiabatic lapse rate and nine-tenths of the adiabatic lapse rate. The adiabatic lapse rate maximizes the greenhouse effect; for this case estimates of the minimum infrared opacity required to maintain the observed surface temperature can be determined. For a surface temperature of 700°K, 99 per cent cloudiness, and cloud-top temperature of 240°K, the minimum required infrared opacity is 6. Uncertainties and questionable side effects of the model are discussed. The model is quite general and can be applied to other planetary atmospheres; sample calculations are shown for the earth and Mars.