ATMOSPHERIC RADIATION AND ITS REFLECTION FROM THE GROUND

Abstract

The complex physical meanings of multiple spectral-line absorption and reradiation by nearer air molecules can be represented satisfactorily by block-spectral bands within the Planckian curves of spectral intensity distribution. Experimental determination of whole-spectrum, long-wave atmospheric transmissivity by parallel-beam radiometers permits the construction of an empirical, graphical integration chart. By this means, the half-space atmospheric radiation can be readily determined for ordinary air from soundings of temperature and moisture, if the magnitudes of these two factors are known in detail close to the flux plane. For a characteristic polar-continental air mass there is no need to separate the CO₂ from the H₂O radiation, as called for by earlier charts. The complex problem of the reflection of spectral-band radiation by a “gray” surface is evaluated by an inverted integration chart, including the estimation of absorption of reflected radiation by overlying air. The supplementation of sub-black emissive power of the ground by reflected atmospheric radiation obscures the meaning of upward radiation measured facing the ground. The Gier and Dunkle aspirated black-plate radiometer can be used on clear nights to measure the effective thermal transmissivity of the atmosphere and should prove helpful in making frost forecasts, particularly in polluted atmospheres and under cirrus clouds. A severe radiation frost is found to result from an increase of about 25 per cent in atmospheric transmissivity with the arrival of dry, polar-continental air.