Near polar regions and in winter months, the height of their bases are lower and near the tropics and during summer months the height of their bases are higher. These clouds may be composed of water droplets, ice crystals or both. Altocumulus clouds are white, grey, or both white and grey, in patches, a sheet or in a layer, generally with shading in rounded masses, rolls, etc. which are sometimes partly fibrous or diffuse and which may or may not be merged. Most of the regularly arranged small elements usually have an apparent width between one and five degrees.
Altocumulus clouds are, at least in the main, almost invariably composed of some water droplets.
This is evident from the fairly low transparency of the elments and from the fact that the rounded masses show sharp outlines when separate. When the temperature is low, ice crystals often form and in winter and near polar regions altocumulus may be entirely composed of ice crystals. Altocumulus may sometimes be confused with cirrocumulus. In case of doubt, if the clouds have shading, they are by definition altocumulus, even if their elements have an apparent width of less than one degree. Clouds without shading are altocumulus if most of the regularly arranged elements, when observed at an angle of more than 30 degrees above the horizon, have an apparent width of between one and five degrees .