Cirrus clouds appear as delicate feathery, wispy formations in the sky that twist and spread due to wind currents. These types of high altitude clouds often signal the approach of a warmer air mass. Cirrus clouds also signify nuances in the jet stream, which is a narrow band of strong wind occurring in the upper atmosphere. Cirrus clouds accompany a phenomenon known as an upper-level jet streak contained within the jet stream. Jet streaks are areas of the jet stream where air flows at a stronger rate than the majority of the jet stream.
This high altitude cloud type is similar to cirrus formations; however, cirrostratus clouds appear as thin sheets and form a wide, veil-like layer. Cirrostratus cloud formations are common above clouds known as thunderheads. Sunlight and moonlight is dispersed by ice crystals shaped as hexagons inside cirrostratus clouds, creating a visual halo or ring in the sky. As warm air approaches, cirrostratus clouds are formed from cirrus types. Cirrostratus clouds are thicker versions of cirrus clouds and often become other types of mid-level clouds as warm air moves into the atmosphere.
Cirrocumulus clouds appear as small, puffy patches at high altitudes in the sky. These layered cloud formations often have a lumpy look and commonly form in rows or waves across the sky, creating alternating areas of cloudiness and sunshine. Cirrocumulus clouds contain both ice crystals and liquid water droplets.
Cumulonimbus clouds begin as low-level cumulus clouds. Cumulus cloud formations, considered fair weather clouds, have round tops and flat bottoms and appear to be piled on top of one another. A cumulus cloud will begin to rise into the sky when instability is present in the atmosphere. As warm, moist air rises; updrafts are created, adding additional height to the cumulus cloud. These clouds may continue to rise to altitudes of up to 75,000 feet. Cumulus clouds become cumulonimbus formations as they reach into middle and high altitudes. The result of a cumulonimbus cloud formation is often a thunderstorm with heavy rain, thunder and lightning, and occasionally hail. Cumulonimbus clouds often contain severe thunderstorms.
Contrails, also referred to as vapor trails, are long, narrow clouds formed from the exhaust of aircraft flying at high altitudes. These types of clouds form as water vapor contained in the aircraft's exhaust condenses in the cold, moist air found at high altitudes.