Clouds of the middle level etage are usually of the altocumulus or altostratus type with bases normally in the range from 6,500 feet to 23,000 feet (2,000 to 6,000 meters). 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. Altocumulus may sometimes be confused with stratocumulus. If most of the regularly arranged elements, when observed at an angle of more than 30 degrees above the horizon, have an apparent width greater than five degrees, the cloud is stratocumulus. Altocumuus form in unstable air layers.

Altostratus clouds are greyish or bluish cloud sheets or layers of striated, fibrous or of uniform appearance, totally or partly covering the sky, and having parts thin enough to reveal the sun at least vaguely, as through ground glass. Altostratus does not show the halo phenomena.

Altostratus nearly always appears as a layer of great horizontal extent and of fairly considerable vertical extent (several hundreds or thousands of feet). Altostratus form in stable air from a forced (mechanical lifting) process. Raindrops or snowflakes are often present in altostratus and below its base. When precipitation reaches the ground, it is generally of the "continuous" type and in the form of rain, snow or ice pellets.