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Clouds have always been an obstacle to remote sensing using visible and infrared wavelengths. Visible and infrared measurements are altered or obscured by their presence. Clouds also are one of the controls of the heat balance of the earth. Of the different forms of clouds, cirrus have been the most elusive to detect. Ground observers under report them because they are often obscured from view by lower clouds and not considered important to aviation. Satellite measurements also have trouble detecting cirrus because their transparency makes them difficult to distinguish from ground or lower cloud backgrounds. Satellite measurements are altered by the presence of these semitransparent clouds and any remote sensing system will have to account for them or use extensive methods to detect and avoid them. The high frequency of cirrus clouds over the earth was not realized until recent sateffite cloud data sets were analyzed. There have been only four clouds studies that have covered the entire earth and have included a long enough period to be used for population statistics. They are the summary of surface weather observers by Warren et al. (1988), the limb scanning Stratospheric Aerosol and Gas Experiment (SAGE see Woodbury and McCormick, 1993)) of NASA, the International Satellite Cloud Climatology Project (ISCCP see Rossow and Lacis, 1990) and the analysis of HIRS data by Wylie et al. (1994). The frequency of semi-transparent cirrus in these studies varies from 10-45%. This reported amount of drrus varies among the studies because of differences in their ability to depict semi-transparent clouds and distinguish them from other land and ocean backgrounds or other clouds below them. Very thin cirrus are often missed by these cloud detection schemes and thus the frequency of thin cirrus may be even higher than reported by any of the studies.
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