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During the conference on 'passive remote sensing of clouds and the atmosphere III' in 1995, a comparison of cloud amounts derived from two retrieval schemes, APOLLO and CHAPS, was presented. It showed good results over ocean but not so good ones over land surfaces. Reasons for the discrepancy have been suggested. Meanwhile, a modified version of the APOLLO scheme has been developed and compared to a one year data set of synop data. They agree quite well. This modified version of APOLLO is used for a new comparison with CHAPS. Using the same data set, the agreement is very god. The remaining difference is probably due to insufficient cloud detection with CHAPS.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The present understanding of moist atmospheric processes and the role of clouds in the hydrologic cycle shows severe gaps of knowledge. Water vapor plays an essential part in atmospheric dynamics. For example, the release of large amounts of latent heat, due to the condensation in convective clouds, plays an important role in the general circulation. Knowledge of the distribution of clouds and its transport is essential to understand atmospheric dynamics. Clouds can have a positive as well as a negative contribution to the greenhouse effect. A cloud cover climatology in a 15 km grid resolution has been retrieved by means of the APOLLO algorithm using the 5 calibrated AVHRR channels. The monthly means of total cloud cover are about 15 percent too high compared to conventional data, the standard deviation is +/- 12 percent. The high resolution cloud cover maps show topometeorological features like 'Fohn' on single days but not in monthly means, because these events are too rare. But increased cloud cover in the luff regions are detected in monthly means as well as some cloud sparse regions like Lake Garda, Ticino or the Swiss Rhone valley. The different annual cycles of cloud cover show the different climatic regions, which are temperate, Alpine, and Mediterranean climate. This is indicated, for example, by the remarkably smaller cloud cover in the Alpine region in winter as compared to the northern and southern forelands.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.