Solar Radiation and Planetary Atmospheres

Abstract

I ADMIT that I have asserted too absolutely the principle that for a radiating planet fed by radiation from the sun, the relative change of equilibrium temperature (namely T/T) of the planet is of about the same order as that of the sun which is its cause. This assumes that dynamical processes in a blanketing atmosphere overlying the planet are not in control. All such effects, whether upward or downward, are superficial: the annual variation of temperature is no longer sensible thirty feet underground: and an ice age lasting 10,000 years could not be felt at 10,000 times this depth, which is about half a mile. Astronomers see down to the surface features of the planet Mars, so that there cannot be much of an atmosphere, though I observe that Dr. Simpson¹ discovers a different reason in the low temperatures (ranging from 10° C. to – 70° C). On the other hand Venus, which is subject to radiation nearly twice as intense as the earth, is entirely covered with cloud, so that inhabitants below exist in a leaden atmosphere scarcely conscious of the sun: and if the cloudy shield presents a bright surface to the incident radiation, sending most of it back and absorbing little, increased intensity of it might even conceivably diminish the temperature below by increasing the density of the shielding layer and so preventing more of it from penetrating. The Smithsonian pioneers have announced fluctuations up to one or two per cent in the solar radiation, while meteorologists seem to be disinclined to recognise any proportionate change in terrestrial temperatures. Their problem is thus to explore what are the special circumstances in the terrestrial atmosphere which lead to this result.