Height of base: 7,000 to 17,000 ft Colour: Grey or white, generally with some shading. 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 3 fingers at arm's length. Even if the elements appear smaller than this the cloud is still classified altocumulus if it shows shading. Other 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. This is so called because in outline, the cloud tops look like a series of turrets and towers along a castle wall.