Altocumulus clouds are part of the Middle Cloud group (2000-7000m up). They are grayish-white with one part of the cloud darker than the other. Altocumulus clouds usually form in groups and are about 1 km thick.

Altocumulus clouds are about as wide as your thumb when you hold up your hand at arm's length to look at the cloud.

If you see altocumulus clouds on a warm humid morning, then expect thunderstorms by late afternoon.