Clouds have been classified by various systems according to form, altitude, and the physical processes generating them. The World Meteorological Organization (International Cloud Atlas, 1956) classifies ten genera in three major groups (cumulus or heap clouds, stratus or sheet clouds, and cirrus or fibrous clouds) by criteria essentially based on cloud form. Some of the genera are subdivided according to variations in internal shape and structure, to give fourteen species. Additional or supplementary features, e.g. transparency, arrangement, and characteristics of growth, are defined by Latin names as variants and accessory types of cloud. The cloud genera, with their abbreviations, are: cirrus (Ci), cirrocumulus (Cc), cirrostratus (Cs), altocumulus (Ac), altostratus (As), nimbostratus (Ns), stratocumulus (Sc), stratus (St), cumulus (Cu), and cumulonimbus (Cb). Clouds can also be referred to, according to their composition, as water or ice clouds; combinations of these are called mixed cloud. Clouds are also classified by the average height of their bases as: high (3–8 km, including Ci, Cc, and Cs); middle (2–4 km, including Ac, As, and Ns); and low (0–2 km, including St, Sc, Cu, and Cb).