Abstract

A hand-held spectrograph has been constructed for use by an astronaut to test the feasibility of measuring cloud-top altitude from a satellite using the reflected solar radiation from the cloud in the oxygen “A” band at 7600 Å. This instrument has been used from an aircraft for preliminary measurements. The transmission of oxygen at 7630 Å with 5-Å bandpass as function of zenith distance of the sun and altitude (pressure), calculated by Wark and Mercer, were checked experimentally by us using the solar beam reflected from a white diffusing card. The instrument was used to photograph cloud spectra from an aircraft flying above the clouds. The cloud altitudes were computed using the transmission curves and found to be about 140 mb (1.4 km) lower than the actual cloud-top altitudes observed during descent of the aircraft. This apparent error is too large to ascribe to probable experimental errors. It is apparent that the absorption by oxygen inside the cloud contributes significantly to this type of measurement. A method based on the two streams theory of Schuster and the appropriate absorption coefficients of oxygen were used to compute the fractional absorption inside the cloud at the wavelength of interest for cloud models representing low, medium, and high cloud situations. The computed fractional absorption for low cloud accounts for the apparent error in the observations taken on stratocumulus clouds. Before an instrument can be used on a satellite for routine measurements of cloud altitudes, several effects must be investigated further, such as the fractional absorption inside the cloud and its variation with cloud thickness and type, and the effect of presence of lower deck of cloud and the effect of various ground albedos, especially in the case of thin clouds.

© 1965 Optical Society of America

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