<P> Tropospheric clouds form in any of three levels (formerly called étages) based on altitude range above the Earth's surface . The grouping of clouds into levels is commonly done for the purposes of cloud atlases, surface weather observations and weather maps . The base - height range for each level varies depending on the latitudinal geographical zone . Each altitude level comprises two or three genus types differentiated mainly by physical form . </P> <P> The standard levels and genus - types are summarised below in approximate descending order of the altitude at which each is normally based . Multi-level clouds with significant vertical extent are separately listed and summarized in approximate ascending order of instability or convective activity . </P> <P> High clouds form at altitudes of 3,000 to 7,600 m (10,000 to 25,000 ft) in the polar regions, 5,000 to 12,200 m (16,500 to 40,000 ft) in the temperate regions and 6,100 to 18,300 m (20,000 to 60,000 ft) in the tropics . All cirriform clouds are classified as high and thus constitute a single genus cirrus (Ci). Stratocumuliform and stratiform clouds in the high altitude range carry the prefix cirro -, yielding the respective genus names cirrocumulus (Cc) and cirrostratus (Cs). When limited - resolution satellite images of high clouds are analysed without supporting data from direct human observations, it becomes impossible to distinguish between individual forms or genus types, which are then collectively identified as high - type (or informally as cirrus - type even though not all high clouds are of the cirrus form or genus). </P> <Ul> <Li> Genus cirrus (Ci): </Li> </Ul>

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