Two Years of Cloud Cover Statistics Using VAS

Statistics of cloud characteristics over North America have been accumulated for the past 2 yr. The frequency of cloud cover with the associated heights and infrared attenuation were charted using the C02 channel radiometric data from the 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 The VAS C02 derived cloud top height and emissivity assignments have been found to be reliable in most cloud type, including thin cirrus clouds where other techniques have been inconsistent. Observations since 1985 reveal that 20%–30% of the United States was covered with thin semitransparent clouds (radiative attenuation was less than 95%), 45% was covered with thick opaque clouds, and 25%–35% had clear sky conditions. It is likely that 5% of the opaque cloud should have been identified as semitransparent cirrus. The geographical distribution of cloud cover shows a latitudinal dependence mainly over the Pacific Ocean. Moderate seasonal and diurnal changes were also found which agree with other published cloud studies.