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What are Contrails?

A contrail, also known as a condensation trail, is a cirrus-like trail of condensed vapor (often resembling the tail of a kite) that is produced by jet aircraft flying at high altitudes. Contrails are produced at altitudes high enough for water droplets to freeze in a matter of seconds before they evaporate. Temperatures at such altitudes are typically below -38 degrees Celsius.

Contrail Spread Apart - Click for a Larger Image

Contrails form through the injection of water vapor into the atmosphere by exhaust fumes from a jet engine. If there is sufficient mixing between the cold upper tropospheric air and the hot exhaust gases to produce a state of saturation, ice crystals will develop. Even tiny nuclei released in the exhaust fumes may be sufficient enough to generate ice crystals.

Contrails spread apart and evaporate  with time. If the air in which the cloud develops has a low relative humidity, the cloud particles will quickly evaporate.  However, even in the presence of higher relative humidities, upper level winds can spread contrails apart, forming a horizontal sheet-like cloud. For a contrail to remain in tact for a long period of time,  the air must have high a relative humidity and light winds.

High Level cirrus clouds and Contrails

High level clouds typically form above 20000 feet (6000 meters) and since the temperatures are so cold at such high elevations, these clouds are primarily composed of ice crystals. They are typically thin and white in appearance, however during sunrise and sunset, these clouds can appear in a magnificent array of colors as unscattered components of sunlight (red, yellow, and orange) are reflected by the underside of the clouds.

The most common variation of high level clouds are cirrus clouds. Cirrus are thin, whispy clouds composed of ice crystals that originate from the freezing of supercooled water droplets and exist where temperatures are below -38 degrees Celsius. Cirrus generally occur in fair weather and move from west to east, pointing in the direction of the prevailing winds at their elevation.

Cirrus Clouds - Click for a Larger Image

Cirrus can form from almost any cloud that has undergone glaciation and can be observed in a variety of shapes and sizes. Possibilities range from the "finger-like" appearance of cirrus fall streaks, commonly seen during pleasant weather conditions, to the uniform texture of more extensive cirrus clouds associated with an approaching warm front.

Fall streaks form when snowflakes and ice crystals fall from cirrus clouds.  The change in wind with height and how quickly these ice crystals actually fall determine the shapes and sizes the fall streaks attain. Since ice crystals fall much more slowly than rain drops - about 1 meter per second compared with about 8 meters per second for large raindrops - fall streaks tend to be stretched out horizontally as well as vertically.

Fallstreaks - Click for Larger Image

Contrails and a Warm Front

A warm front is defined as the transition zone where a warm air mass is replacing a cold air mass. Warm fronts generally move from southwest to northeast and the air behind a warm front is warmer and more moist than the air ahead of it. When a warm front passes through, the air becomes noticeably warmer and more humid than it was before.

Symbolically, a warm front is represented by a solid line with semicircles pointing towards the colder air and in the direction of movement. On colored weather maps, a warm front is drawn with a solid red line.

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