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Towering cumulus clouds are often associated with cloud suck

Cloud suck is a phenomenon commonly known in paragliding and hang gliding where pilots experience significant lift due to a thermal under the base of cumulus clouds, especially towering cumulus and cumulonimbus. The vertical extent of a cumulus cloud is a strong indicator of the level of lift beneath it, and the potential for cloud suck.[1] Cloud suck commonly occurs in low pressure weather and in humid conditions.[2]

Cloud suck is typically associated with an increase in thermal updraft velocity near cloud base. As a parcel of air lifted in a thermal rises, it also cools, and water vapor will eventually condense to form a cloud if the parcel rises above the lifted condensation level. As the water vapor condenses, it releases its latent heat of vaporization, thereby increasing the buoyancy of the parcel.[3] The updraft is amplified by this latent heat release.[1][4]

Paraglider pilots have reported being unable to descend in strong cloud suck, even after bringing their canopies into full stall, which would normally result in a rapid vertical descent.[5] Cloud suck is especially dangerous for paraglider pilots, whose maximum speed is less than 30 knots, because the updrafts can exceed their ability to get away.[6]

On 14 February 2007 while practicing for a paragliding contest in Australia, Polish-born[7] German team pilot Ewa Wiśnierska-Cieślewicz was sucked into a cumulonimbus cloud, climbing at up to 20 meters per second (4,000 feet per minute)[8] to an altitude of 9,946 meters (32,600 feet).[9] She lost consciousness due to hypoxia, but regained consciousness after 30 minutes to an hour, and landed still covered in ice after a three and a half hour flight.[10] [11] Chinese paraglider pilot He Zhongpin died after he was sucked into the same storm system and struck by lightning at 5900 meters (19,000 feet). Mr. He's body was found the next day 15 km (9 miles) from his last known position prior to entering the cloud.[12]

Compared with hang-gliders and paragliders, sailplanes have much higher top speeds (often over 250 km/h), they could easily escape powerful cumulo-nimbus clouds by flying away quickly or by using their powerful airbrakes. Furthermore, since lee waves are nowadays used by glider pilots to gain great heights instead of thunderstorms, occurrences of unplanned ascents are rare.

The wreck of the Shenandoah.

Cloud suck is also a concern for powered aircraft, but usually not a lethal hazard, except in extreme weather situations.[6] The USS Shenandoah, the first rigid airship built in the United States, and the first in the world to be inflated with helium, was lost in a cloud suck accident associated with a squall line. At about 6:00 am on the morning of 3 September 1925, near Ava in northern Noble County, Ohio, the Shenandoah was suddenly caught in a violent updraft, while at an altitude of 2,100 feet rising at the rate of a meter a second. At about 6,200 feet the ascent was checked, but the ship began to descend. When halfway to the ground it was hit by another updraft and began to rise rapidly at an even faster rate. Ultimately the keel snapped, and the ship broke up while still more than a mile above the ground. Shenandoah's commanding officer and 13 other officers and men were killed. Twenty-nine members of the crew survived the break-up, although some received serious injuries.[13][14]

References

  1. ^ a b Pagen, Dennis (2001). The Art of Paragliding. City: Black Mountain Books. pp. 105, 108. ISBN 0936310146. "Remember, the thickness of a cumulus clouds is the biggest indicator of the level of lift beneath it and the potential for cloud suck..."  
  2. ^ Pagen, Dennis (1992). Understanding the Sky. Spring Mills, PA: Sport Aviation Pubns. p. 230. ISBN 0936310103. "Cloud suck seems to occur most commonly in low pressure weather and especially in humid conditions."  
  3. ^ Macgorman, D. (1998). The Electrical Nature of Storms. Oxford Oxfordshire: Oxford University Press. p. 164. ISBN 0195073371. "As the water vapor condenses, it releases its latent heat of vaporization, thereby increasing the buoyancy of the parcel."  
  4. ^ Fox, Peter (2000). Geophysical and Astrophysical Convection. Washington: Taylor & Francis. p. 251. ISBN 9056992589. "The updraft is amplified by latent heat release..."  
  5. ^ Simpson, Joe (2003). The Beckoning Silence. Seattle: Mountaineers Books. p. 62. ISBN 0898869412. "As she instigated the stall she was alarmed to realize that far from free-falling she was still slowly being pulled upwards."  
  6. ^ a b Anderson, Fletcher (2003). Flying the Mountains. McGraw-Hill Professional. p. 219. ISBN 0071410538. "Paraglider pilots, whose maximum speed is less than 30 knots, have a particular phobia about flying under large cumulus clouds."  
  7. ^ "I have fun living like a bird", Standa Hlavinka & Ewa Wisnierska Cieslewicz, www.skyfly.cz
  8. ^ "Paraglider survives wild flight". Herald Sun. 2007-02-17. http://www.news.com.au/heraldsun/story/0,21985,21239133-661,00.html. Retrieved 2007-05-20. "Ewa Wisnierska, 35, was catapulted upwards like a leaf at speeds of up to 20 metres per second..."  
  9. ^ Besser, Linton (2007-02-16). "Ewa sucked into storm and lives to tell". The Sydney Morning Herald. http://www.smh.com.au/news/national/dead-luck-ewas-flight-of-fury/2007/02/16/1171405421626.html. Retrieved 2007-05-20. "Ewa Wisnierska, 35, passed out due to a lack of oxygen and flew unconscious for up to an hour covered in ice after reaching an altitude of 9947 metres"  
  10. ^ "Paraglider Cheats Death In Thunderstorm". CBS News. 2007-02-16. http://www.cbsnews.com/stories/2007/02/16/world/main2486010.shtml. Retrieved 2007-05-21. "'The glider kept climbing, climbing, and I couldn't see anything. Then it got dark. I was already shaking, all wet, all the instruments were wet and frozen' she recounted."  
  11. ^ Meldung des DHV mit persönlicher Stellungsnahme der Pilotin zum Vorfall (in German)
  12. ^ Braithwaite, David (2007-02-20). "Lightning killed paraglider". The Sydney Morning Herald. http://www.smh.com.au/news/national/lightning-killed-paraglider/2007/02/20/1171733745020.html. Retrieved 2007-05-20. "A lightning strike killed a Chinese man who disappeared after being sucked into the storm survived by German paraglider Ewa Wisnierska last week."  
  13. ^ U.S. National Park Service. "Shenandoah Crash Sites --Aviation: From Sand Dunes to Sonic Booms: A National Register of Historic Places Travel Itinerary". http://www.cr.nps.gov/nr/travel/aviation/she.htm. Retrieved 2007-05-20.  
  14. ^ Waller, Douglas (2004). A Question of Loyalty : Gen. Billy Mitchell and the Court-Martial That Gripped the Nation. New York: HarperCollins Publishers. pp. 15–16. ISBN 0060505478. "The squall now quickly lifted up the helpless airship to 6,300 feet...then it plunged in a matter of minutes down to 3,200 feet."  

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