On the Infrared Transmission Through Cirrus Clouds and the Estimation of Relative Humidity from Satellites

Abstract

On the basis of satellite and other types of information, it is shown, both on observational and theoretical grounds, that cirrus clouds have a higher transmission for radiation at 10 μ than for radiation at 6 μ. Thus, in the case studied, at 10 μ the cirrus clouds had a fractional transmission of about 50%, while at 6 μ the clouds were essentially opaque. This fact has an important bearing on attempts to use a “humidity diagram” to estimate relative humidity above clouds. The satellite data show that measurements at 6 or 10 μ can be used to locate regions of substantial cloudiness, which are therefore regions of high relative humidity in the troposphere. To use these satellite measurements to estimate the relative humidity above clouds would be misleading. However, the use of radiation measurements in both channels can perhaps be helpful for specifying the transmissivity of cirrus clouds, and in the absence of clouds, for locating regions of low relative humidity.