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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 5,000 m (16,500 ft) in temperate regions and above 6,100 m (20,000 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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• China launches its first space station, Tiangong 1, as part of its human spaceflight program.
• Kenyan environmentalist Wangari Maathai (pictured), the first African woman to receive the Nobel Peace Prize, dies at the age of 71.
• Patrick Makau Musyoki of Kenya wins the Berlin Marathon, setting a new world record.
• King Abdullah of Saudi Arabia announces that Saudi women will be permitted to vote and stand for election.
• At the road cycling world championships in Denmark, Mark Cavendish of Great Britain wins the men's road race and Germany's Tony Martin wins the men's time trial.
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September 29: First day of Rosh Hashanah (Jewish New Year) (2011, 5772 AM); Michaelmas

• 1829 – The Metropolitan Police (Scotland Yard pictured) of Greater London, originally headquartered in Great Scotland Yard, Westminster, was founded.
• 1957 – An explosion at the Soviet nuclear reprocessing plant Mayak released 74 to 1850 PBq of radioactive material.
• 1962 – Alouette 1, Canada's first satellite, and the first satellite constructed by a country other than the Soviet Union or the United States, was launched.
• 1963 – The University of East Anglia was founded in Norwich, England, after talk of establishing such a university in the city began as early as the 19th century.
• 2006 – Gol Transportes Aéreos Flight 1907 collided in mid-air with an Embraer Legacy business jet near Peixoto de Azevedo, Mato Grosso, Brazil, killing 154 total people, and triggering a Brazilian aviation crisis.

More anniversaries: September 28 – September 29 – September 30