Clouds can be classified according to their usual altitudes. The altitudes listed here apply to the temperate zone. In polar regions, these clouds may be found in lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.
You find middle clouds between 6,000 and 18,000 feet (1,829-5,486 meters).
Middle clouds may look like high clouds, but they are composed mainly of liquid water droplets. Altocumulus and altostratus are the main types of middle clouds.
ALTOCUMULUS (Ac)
The presence of altocumulus clouds is often referred to as a "mackerel sky." Made up of water droplets, these clouds are typically found between 6,000 and 20,000 feet in the troposphere.
Altocumulus is Latin for "high heaps or piles."
White to gray in color, altocumulus clouds can be transparent enough for the sun to shine through them.
ALTOSTRATUS (As)
Altostratus clouds usually precede a storm system. In Latin, Alto is the term for high, so altostratus means high, spread out clouds. Found at levels of 15,000 to 20,000 feet, these clouds are sometimes made up of ice crystals, but, more often than not, they consist of water droplets.
Altostratus are gray, thick clouds. Precipitation can fall from these clouds, but it usually evaporates before it hits the ground. Precipitation may occur within six hours after altostratus clouds first appear in the sky.
