Clouds Help

High-level Clouds

High-level clouds form above 6000 m. The temperatures at these high elevations are cold, so high-level clouds are mostly made up of ice crystals. High-level 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 6000 m, cirrus clouds are formed out of ice crystals that come from frozen water droplets. Cirrus 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.

Mid-level Clouds

The bases of mid-level clouds usually form around 2000–6000 m. Their lower altitudes usually keep them warm enough to prevent their water droplets from freezing, but they can contain ice crystals when temperatures are cold enough.

Altocumulus may be found as parallel bands or rounded masses. Commonly a portion of an altocumulus cloud is shadowed, which helps you tell them apart from high-level cirrocumulus. Altocumulus clouds often form by convection in an unstable upper air layer. This can be caused by the gradual lifting of air before a cold front. When you see altocumulus clouds on a warm, humid summer morning, there will often be thunderstorms later that day.

Low-level Clouds

Low clouds are mostly made up of water droplets, since their bases sit below 2000 m. However, when temperatures are cold enough, they can pick up ice particles and snow.

Nimbostratus are dark, low-level clouds accompanied by light to moderately falling precipitation. However, when temperatures are cold enough, these clouds may also contain ice particles and snow.

Stratocumulus clouds generally appear as low, lumpy layered clouds that can come with weak rainfall. Stratocumulus vary in color from dark gray to light gray and may appear as rounded masses, rolls, etc., with breaks of clear sky in between.

Vertical Clouds

Probably the most familiar of the basic cloud shapes is the cumulus cloud . Formed by either thermal convection or frontal lifting, cumulus clouds can reach up to altitudes over 15,000 m. Additionally, they free huge amounts of energy through the condensation of water vapor within the cloud itself.

Fair weather cumulus look like floating cotton balls to most people. Identified by their flat bases and fluffy outlines, fair weather cumulus show little vertical growth, with cloud tops at the limit of the rising air. Give them some frontal action, though, and fair weather cumulus become tigers, turning into gigantic cumulonimbus clouds; the citadel of violent thunderstorms.

Supplied by rising pockets of air or thermals and lifting vertically from the Earth’s surface, cumulus water vapor cools and condenses to create cloud droplets. Newly formed fair weather cumulus clouds have sharply defined margins and bases, while older cumulus edges are rougher, showing cloud erosion. Evaporation around a cloud’s edges cools the surrounding air, making it heavier and causing it to drop outside the cloud.

Cumulonimbus clouds are much bigger and taller than fair weather cumulus. They either build as separate soaring towers, or form a line of structures known as a squall line . Fed by intense updrafts, cumulonimbus clouds are the giants of the cloud forms, reaching over 15,000 m. These large, nasty clouds make up huge, towering thunderstorms called supercells .

Specialty Clouds

Some clouds are very specialized. They form from specific events. For example, some of these clouds are formed by aircraft, earlier storms, and the presence of mountain peaks.

A contrail , short for condensation trail , is a cirrus-like trail of condensed water vapor that looks like the tail of a kite or a wide piece of yarn. Contrails are created at high altitudes when the heat from a high performance jet engine hits the extremely cold atmosphere. It condenses water vapor into a miniature cloud that forms a line behind the engine. Contrail formation depends on the atmosphere’s water content and the heat characteristics of the jet engines.

Mammatus clouds, formed in sinking air, are shaped like hanging fruit. Fairly scary looking, mammatus clouds are safe and not the signal for approaching tornadoes. Actually, mammatus clouds usually appear after a thunderstorm has passed.

Air is also lifted by the land’s topography. Mountains create a barrier to air currents and weather fronts. Orographic clouds are formed by air uplifted by the earth’s shape. They are formed when rising air cools and water vapor condenses. In the United States, prevailing winds often blow from west to east, so most orographic clouds form on the western side of a mountain. Orographic clouds are also seen around mountain peaks.

Billows clouds are created from the instability connected with vertical shear air flows and weak thermal layering. These clouds, formed by winds blowing at different speeds in different air layers, are often seen at air masses margins of different densities (warm air layered over cold air). They look a lot like breaking waves.

Lenticular clouds are shaped like lenses or flying saucers. These flat clouds are found in windy areas where rising air is being cooled. After the water vapor condenses into cloud droplets, it is pulled along by the blowing wind. As the droplets warm and begin to drop, they gradually turn back into vapor and disappear. These clouds are often seen downwind of mountain ranges.

Practice problems of this concept can be found at: Atmosphere Practice Test