- Middle altitude clouds are found between approximately 8,000 and 20,000 feet.clouds image by saied shahinkiya from Fotolia.com
Clouds are named based on their shape and the altitude at which they are found when measured from the cloud's base. They are made of water vapor or ice crystals that condense (turn back into liquid) on microscopic dust particles in the atmosphere. The resulting water droplets collect into clouds. Middle altitude clouds are found between approximately 8,000 and 20,000 feet (2,400-6,100 meters). The prefix "alto" denotes middle altitude clouds. - Altostratus clouds are layered clouds that appear gray or pale blue. They are primarily made of liquid water and form when the air in the middle atmosphere is moist and slowly rises. The sun can often be seen through altostratus clouds. The height of the cloud base is difficult to measure because of its unstructured and smooth appearance. Altostratus clouds are commonly seen in advance of a warm front (warmer and moister air replaces colder air) and result in precipitation as the cloud formation changes to nimbostratus. Altostratus clouds often form over the central United States from air cooled by a severe thunderstorm (outflow).
- Altostratus translucidus clouds often indicate precipitation within 10 to 20 hours if winds continue steadily from northeast to south. The sun shining from behind these clouds gives them a frosted glass appearance.
- Altostratus undulatus clouds are formed when a large air mass rises, causing condensation, combined with a change in the direction and/or speed of the wind over a short height change (vertical wind shear). (See References 6) (See References 7) Layers of air slide over one another creating waves between each layer. The clouds appear when the wave rises and dissipate as the wave falls, creating a wave pattern across the sky. Altostratus undulatus clouds are indicators of wind turbulence, but usually not severe enough to be a concern for air travel.
- Altocumulus clouds form when air is moist, producing clouds with considerable variations in appearance. They range from organized lines to random positions, with thicknesses from thin to thick, and colors from white to gray. They are commonly seen in advance of a warm front that produces altostratus clouds. When altocumulus clouds occur simultaneously with other types of clouds, a storm usually follows.
- Altocumulus translucidus clouds are composed of water and ice. Like altostratus clouds, they often indicate precipitation within 10 to 20 hours if winds continue steadily from northeast to south.
- Altocumulus undulatus clouds form when a large, moist air mass rises, condenses, and combines with wind shear and wind instability, producing parallel bands of altocumulus clouds that resemble ripples on a lake. Increasing altocumulus undulatus clouds indicate an approaching weather front that can produce rain or snow. They also indicate wind turbulence, but are not usually dangerous for air travelers.
- Altocumulus perlucidus clouds are large areas of layered altocumulus clouds formed by water and ice. They gradually rise as an approaching low pressure system causes warmer air to move into an area of cooler air. Precipitation is possible within 10 to 20 hours if winds continue steadily from northeast to south.
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