Altocumulus (Ac)

Type: Medium-level cloud

Height of cloud base: 7,000-17,000 ft

Cloud colour: Grey or white, generally with some shading.

Cloud shape: Several different types, the most common being either patches or a sheet of rounded elements but may also appear as a layer without much form. When viewed from the ground, the size of individual elements will have an apparent width of 1 to 5° when at an elevation greater than 30° (the width of one to three fingers at arm's length). Even if the elements appear smaller than this the cloud is still classified altocumulus if it shows shading.

Other cloud features: Occasionally some slight rain or snow, perhaps in the form of a shower may reach the ground. On rare occasions, a thunderstorm may occur from one type of Ac known as altocumulus castellanus - so called because in outline, the cloud tops look like a series of turrets and towers along a castle wall.