Wikipedia:Today's featured article/September 29, 2011



Cirrus clouds are atmospheric clouds characterized by thin, wispy strands, often bunched into tufts. They range in color from white to a faint gray and form when water vapor undergoes deposition at altitudes above 16500 ft in temperate regions and above 20000 ft in tropical regions. They also form from the outflow of tropical cyclones or the anvils of cumulonimbus clouds. Since these cirrus clouds arrive in advance of the frontal system or tropical cyclone, they indicate that the weather conditions may soon deteriorate. While they indicate the arrival of precipitation, cirrus clouds themselves produce only fall streaks (falling ice crystals that evaporate before landing on the ground). Jet stream-powered cirrus clouds can grow long enough to stretch across continents, but they remain only a few kilometers deep. When visible light interacts with the ice crystals in cirrus clouds, it produces glories, sundogs, and fire rainbows. Cirrus clouds raise the temperature of the air beneath them by an average of 10 °C (18 °F). (more...)

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