Clouds cover about 70 % of the Earth's surface and play a dominant role in the energy and water cycle of our planet. Only satellite observations provide a continuous survey of the state of the atmosphere over the globe, at the space-time scales at which cloud processes occur. Their record length exceeds now more than 25 years. To resolve the diurnal cycle of clouds, the GEWEX cloud project ISCCP (International Satellite Cloud Climatology Project) is using data from a combination of polar orbiting and geostationary radiometers. ISCCP cloud products are available for the period from 1983 up to 2007, and processing is being continued.

During the past decade, other global cloud climatologies have been established from various instruments, mostly onboard polar orbiting satellites. To be useful for climate studies and for evaluation of general circulation models, the accuracy and error sources of these cloud products must be determined.

The GEWEX cloud assessment was initiated by the GEWEX Radiation Panel (GRP) in 2005 to evaluate the reliability of available, global, long-term cloud data products, with a special emphasis on ISCCP. Two workshops have been held in 2005 and 2006 in Madison, Wisconsin (USA), hosted by B. Baum (co-chair until 2007, http://cimss.ssec.wisc.edu/cloud_climatology/2006).

Since 2007, ten global cloud climatologies participated, including: ISCCP (W. B. Rossow, CUNY), PATMOS-x (A. Heidinger, NOAA) using AVHRR data, HIRS-NOAA (D. Wylie, U. Wisconsin), TOVS Path-B (C. Stubenrauch, CNRS/IPSL LMD) and SAGE (P.-H. Wang, Science and Technology Corporation), as well as global cloud climatologies from the new generation of instruments aboard the NASA Earth Observing System (EOS) platforms and the A-Train, from the MODIS Science Team (S. Ackerman, CIMSS, and S. Platnick, NASA GFSC) and the MODIS CERES Science Team (P. Minnis, NASA LaRC), AIRS-LMD (C. Stubenrauch, CNRS/IPSL LMD) and CALIPSO (D. Winker, NASA LaRC). Surface observations (S. Warren, U. Washington) were also made available. Comparisons have been presented at the last GEWEX cloud assessment meeting, held in July , hosted by W. B. Rossow. First key results have been published in (GEWEX news vol. 19, no. 4, 2009 ).

A common data basis is currently prepared which will be available at this website.

The GEWEX-CA data products should not be used in publications while the assessment is underway. After the assessment has been completed, when data from the GEWEX Cloud Assessment are used in a publication, we request the following acknowledgment be included: "The GEWEX Cloud Assessment data were obtained from the ClimServ Data Center of IPSL/CNRS." In addition, references of the data sets should be given and the provider of each data set used should be specifically acknowledged. See 'Data sets' for details.