of embedded cumuliform buildups. The stratocumuliform group is divided into cirrocumulus (high-level, strato- prefix dropped), altocumulus (mid-level, strato- prefix dropped), and stratocumulus (low-level). Cumuliform clouds generally appear in isolated heaps or tufts. They are the product of localized but generally free-convective lift where no inversion layers are in the troposphere to limit vertical growth. In general, small cumuliform clouds tend to indicate comparatively weak instability. Larger cumuliform types are a sign of greater atmospheric instability and convective activity. Depending on their vertical size, clouds of the cumulus genus type may be low-level or multi-level with moderate to towering vertical extent. Cumulonimbus clouds are largest free-convective clouds, which has a towering vertical extent. They occur in highly unstable air and often have fuzzy outlines at the upper parts of the clouds that sometimes include anvil tops. These clouds are the product of very strong convection that can penetrate the lower stratosphere. Levels and genera 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. 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. High-level High clouds form at altitudes of in the polar regions, in the temperate regions, and in the tropics. All cirriform clouds are classified as high, 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). If limited-resolution satellite images of high clouds are analysed without supporting data from direct human observations, distinguishing between individual forms or genus types becomes impossible, and they are collectively identified as high-type (or informally as cirrus-type, though not all high clouds are of the cirrus form or genus). Genus cirrus (Ci) – these are mostly fibrous wisps of delicate, white, cirriform, ice crystal clouds that show up clearly against the blue sky. Cirrus are generally non-convective except castellanus and floccus subtypes which show limited