Mid-level clouds have cloud bases typically between 6500 to 23000 feet (2000 to 6000 meters). Because of their lower altitudes, they are composed primarily of water droplets. However, when the temperatures are cold enough, they can be composed of ice crystals as well.

[Image: puffy altocumulus clouds (87K)]

Altocumulus clouds generally appear as puffy masses and are located roughly 3-4 km above the ground. One part of the cloud darker than the rest, a characteristic which makes them distinguishable from the higher cirrocumulus. Altocumulus typically form from gradual lifting of air ahead of an advancing cold front.

[Image: wavelike altocumulus clouds (91K)]

In other instances, altocumulus clouds are aligned in parallel waves or bands. In the presence of rising air at cloud level, altocumulus take on the appearance of little castles and these clouds are often seen on warm, humid summer mornings, occasionally followed by thunderstorms later in the afternoon.