Cloud statistics using VAS

Statistics of cloud characteristics over North America during winter of 1985–86 have been calculated. The frequency of cloud cover with associated heights and infrared attenuation were charted using the CO₂ channel radiometric data from the geostationary VISSR Atmospheric Sounder (VAS). Cloud top pressures were determined from the ratio of VAS CO₂ channel radiances in a radiative transfer equation formulation. Cloud emissivities were then calculated from infrared window channel observations. CO₂ technique derived height and emissivity assignments have been found to be reliable in all cloud types, including thin cirrus clouds where other techniques have been inconsistent. Observations during November 1985 revealed that 25% of the United States was covered with thin clouds (radiative attenuation was less than 80%), 50% was covered with thick opaque clouds, and 25% had clear sky conditions. Geographical distribution of cloud cover over the United States for the winter of 1985–86 shows a strong latitudinal dependence. Cirrus reports in frequencies of 15% in the south increase to 30% in the northwest.