Medium altitude clouds
18,000 - 8,000 ft
Clouds of the middle level 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).
Altocumulus sometimes appears in several layers. A number of different species of Altocumulus can be commonly seen; white, grey, or both white and grey, in patches, a sheet or in a layer, rolls, etc.
Altocumulus clouds are usually composed of mainly water droplets. When the temperature is low, ice crystals form and in winter and near polar regions altocumulus may be entirely composed of ice crystals. Because of its many different shapes Altocumulus is often confused with other clouds.
This is the type of cloud most of us don't want outside since they are greyish or bluish and most people have noticed that grey clouds is sometimes a sign that rain isn't far away. It can be partly or totally covering the sky, sometimes having parts thin enough to reveal the sun.
Altostratus form in stable air from a forced process. When precipitation reaches the ground, it is generally of the "continuous" type and in the form of rain, snow or ice pellets.
When Altostratus becomes so thick that the sun or moon is completely obscured and rain is falling, it is renamed Nimbostratus.
Nimbostratus is a grey cloud layer, often dark, the appearance of which is rendered diffuse by more or less continuously falling rain or snow, which in most cases reaches the ground. It is thick enough throughout to blot out the sun.
Low clouds frequently occur below the layer, with which they may or may not merge. Nimbostratus generally cover a wide area and are composed of raindrops, snow crystals, snowflakes, or of a mixture of these liquid and solid particles. The high concentration of particles and the great vertical extent of the clouds prevent direct sunlight from being seen.