Highlevel Clouds

High-level clouds form above 6,000 meters. The temperatures at these high elevations are cold, so high-level clouds are mostly made up of ice crystals. Highlevel clouds are generally thin and white in appearance, but can appear in a terrific variety of colors when the sun is setting on the horizon.

The most common forms of high-level clouds are thin and wispy cirrus clouds. Typically found at heights greater than 6,000 meters, cirrus clouds are formed out of ice crystals that come from frozen water droplets. Cirrus clouds usually form in fair weather and point in the direction of the air flow at their altitude.

Cirrostratus are sheet-like, high-level clouds made of ice crystals. Although cirrostratus can blanket the sky and be many thousands of meters thick, they are fairly transparent. The sun or the moon is usually seen through cirrostratus. These high-level clouds form when a wide air layer is lifted by large-scale fronts.

Table 3-4 Latin root clouds.

Latin root word

Atmospheric meaning

cirrus

curl

cumulus

pile

nimbus

rain

stratus

layer

Sometimes cirrostratus are so filmy, the only sign of their existence is a halo around the sun or the moon. Halos are formed when light refracts off a cloud's ice crystals. Since cirrostratus clouds get thicker when warm fronts advance, a halo gradually fades or disappears from around the sun or moon as the weather changes. Fig. 3-4 illustrates high-level, cirrus-type clouds (top) and midlevel altocumulus clouds.

Table 3-5 Cloud types.

(meters)

Shape

Composition

Cumulus

12,000

Vertical, fluffy, defined edges and flat bases

Condensed water vapor

Cumulonimbus

12,000+

Massive, dark, vertical towers

Water droplets and ice crystals

Cirrus

6,000+

Thin and wispy

Ice crystals

Cirrostratus

6,000

Sheet-like, almost transparent

Ice crystals

Altocumulus

2,000 - 6,000

Parallel bands or rounded masses

High humidity and water droplets

Nimbostratus

< 2,000

Dark, low

Water or snow

Stratocumulus

< 2,000

Light to dark gray, low, lumpy masses, and rolls

Weak rainfall with clear sky breaks in between

Contrail

(condensation trail)

6,000 - 12,000+

Long thin lines following a jet's exhaust path

Water droplets freeze to ice crystals

Orographic

2,000 - 6,000+

Fluffy, circling mountain peaks

Condensed water vapor

Mammatus

2,000 - 6,000

Light to dark gray

Water droplets

Billow

2,000 - 6,000

Horizontal eddies

Condensed water vapor

Fig. 3-4 High altitude cirrus clouds are wispy, while mid-level altostratus are flat bottomed.

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