Solar And Terrestrial Radiation Considered

The surface of the earth absorbs the heat of the sun during the day, and radiates heat into space during the night. The envelope of gases and vapour, which we call the atmosphere, exerts highly important functions upon these processes. Thanks to the researches of Professor Tyndall, we are now enabled to understand these functions much more clearly than heretofore. His elaborate, patient, and remarkably sagacious series of experiments upon radiant heat, have satisfactorily demonstrated tha dry air is as transparent to radiant heat as the vacuum itself; while air perfectly saturated with aqueous vapour absorbs more than five per cent. of radiant heat, estimated by the thermal unit adopted for the galvanometer indications of the effect upon a thermo-electric pile.

Aqueous vapour, in the form of fog or mist, as is well known, gives to our sensation a feeling of cold, and interferes with the healthy action of the skin and the lungs; the cause being its property of absorbing heat from our person.

Air containing moisture in an invisible state likewise exerts a remarkable influence in radiating and absorbing heat. By reason of these properties, aqueous vapour acts as a kind of blanket upon the ground, and maintains upon it a higher temperature than it would otherwise have. “Regarding the earth as a source of heat, no doubt at least ten per cent. of its heat is intercepted within ten feet of the surface.” Thus vapour—whether transparent and invisible, or visible, as cloud, fog, or mist—is intimately connected with the important operations of solar and terrestrial radiation. Cloudy, or humid days, diminish the effect upon the soil of solar radiation; similar nights retard the radiation from the earth. A dry atmosphere is the most favourable for the direct transmission of the sun’s rays; and the withdrawal of the sun from any region over which the air is dry, must be followed by very rapid cooling of the soil. “The removal, for a single summer night, of the aqueous vapour from the atmosphere which covers England, would be attended by the destruction of every plant which a freezing temperature could kill. In Sahara, where ‘the soil is fire and the wind is flame,’ the refrigeration at night is often painful to bear. Ice has been formed in this region at night. In Australia, also, the diurnal range of temperature is very great, amounting, commonly, to between 40 and 50 degrees. In short, it may be safely predicted, that wherever the air is dry, the daily thermometric range will be great. This, however, is quite different from saying that when the air is clear, the thermometric range will be great. Great clearness to light is perfectly compatible with great opacity to heat; the atmosphere may be charged with aqueous vapour while a deep blue sky is overhead; and on such occasions the terrestrial radiation would, notwithstanding the ‘clearness,’ be intercepted.” The great range of the thermometer is attributable to the absence of that protection against gain or loss of heat which is afforded when aqueous vapour is present in the air; and during such weather the rapid abstraction of moisture from the surface of plants and animals is very deleterious to their healthy condition. “The nipping of tender plants by frost, even when the air of the garden is some degrees above the freezing temperature, is also to be referred to chilling by radiation.” Hence the practice of gardeners of spreading thin mats, of bad radiating material, over tender plants, is often attended with great benefit.

By means of the process of terrestrial radiation ice is artificially formed in Bengal, “where the substance is never formed naturally. Shallow pits are dug, which are partially filled with straw, and on the straw flat pans containing water which had been boiled is exposed to the clear firmament. The water is a very powerful radiant, and sends off its heat into space. The heat thus lost cannot be supplied from the earth—this source being cut off by the non-conducting straw. Before sunrise a cake of ice is formed in each vessel.... To produce the ice in abundance, the atmosphere must not only be clear, but it must be comparatively free from aqueous vapour.”

Considering, therefore, the important consequences attending both terrestrial and solar radiation, it appears to us that observations from radiation thermometers are of much more utility in judging of climate than is usually supposed. These observations are very scanty; and what few are upon record are not very reliable, principally from bad exposure of the instruments, while the want of uniformity in construction may be another cause. Herschell’s actinometer and Pouillet’s pyrheliometer, instruments for ascertaining the absolute heating effect of the sun’s rays, should, however, be more generally employed by meteorologists. In comparing observations on radiation it should be kept in mind, that “the difference between a thermometer which, properly confined [or shaded], gives the true temperature of the night air, and one which is permitted to radiate freely towards space, must be greater at high elevations than at low ones;” because the higher the place, the less the thickness of the vapour-screen to intercept the radiation.