HIRS observations of clouds since 1978

Abstract

Seasonal changes in global cloud cover have been monitored with multi-spectral observations from the eleven polar orbiting HIRS (High resolution Infrared Radiation Sounder) since December 1978. The HIRS longwave infrared data have a higher sensitivity to semi-transparent cirrus clouds than visible and infrared window techniques. Clouds are found in 71% of all HIRS observations from 65 S to 65 N; high clouds are observed in 33% of the observations. Closer investigation of the tropics indicates that there has been little overall change in the global total cloud cover. There is the possibility of a small increase in high cloud cover from the first decade to the second (about 1%) however orbit drift of some sensors and instrument differences may be part of this. Significant weather events such as El Nino Southern Oscillation or volcanic eruptions may also be influencing the trends. Since 2000, the Moderate resolution Imaging Spectro-radiometer (MODIS) is starting to generate another cloud data set that must be understood and connected with the HIRS cloud data; early indications are promising that MODIS will be a more than worthy successor to the venerable HIRS.