Aice marine Science as level nasa earth Observatory by Steve Graham and Holli Riebeek · November 1, 2006
Download 448.17 Kb.
|
- Navigate this page:
- Thunderstorm Triggers
Hurricane Formation and Decay Hurricanes form over tropical waters (between 8 and 20 degrees latitude) in areas of high humidity, light winds, and warm sea surface temperatures [typically 26.5 degrees Celsius (80 Fahrenheit) or greater]. These conditions usually prevail in the summer and early fall months of the tropical North Atlantic and North Pacific Oceans, and for this reason, hurricane “season” in the northern hemisphere runs from June through November. 
Tropical cyclones ususally form in each hemisphere’s summer and early fall, when ocean waters are warmest. (Graphic by Robert Simmon, NASA GSFC.) The first sign of hurricane genesis (development) is the appearance of a cluster of thunderstorms over the tropical oceans, called a tropical disturbance. Tropical disturbances generally form in one of three ways, all of which involve the convergence of surface winds. When winds come together (converge), the force of the collision forces air to rise, initiating thunderstorms. Thunderstorm Triggers One trigger for convergence is the meeting of the Northern and Southern Hemisphere easterly trade winds near the equator. The meeting of these wind belts triggers numerous, daily thunderstorms in a region called the Intertropical Convergence Zone (ITCZ). Occasionally, a cluster of thunderstorms will break away from the ITCZ and organize into a more unified storm system. Another mechanism that can lead to the formation of a hurricane is the convergence of air along the boundary between masses of warm and cold air. Along the boundary, denser cold air can help lift warm and moist air to form thunderstorms. Occasionally such boundaries, called mid-latitude frontal boundaries, drift over the Gulf of Mexico or the Atlantic Ocean off the East Coast of the United States, where developing storms can organize into hurricanes in one of two ways. Either thunderstorms organize into a large system that forms a new area of low pressure, or a pre-existing, weak, non-tropical cyclone will form along the front and will develop into a hurricane. Cyclones that form along mid-latitude frontal boundaries are often called mid-latitude orextratropical cyclones, and they typically have cold air at upper levels over the cyclone center. In contrast, hurricanes (tropical cyclones) have warm air over their centers. To change into a tropical cyclone, the cold air over an extratropical cyclone must change to warm air. This change can happen if thunderstorms occur near the cyclone center. The thunderstorms form along the frontal boundary as warm air rises over the colder air mass. As the air rises, it cools, and water vapor condenses into clouds. The heating released by condensation then helps to warm the air, and eventually the extratropical cyclone transitions into a tropical cyclone. Waves that occur within the dominant easterly winds over the tropical Atlantic cause areas of converging and diverging winds. The convergence forces air to rise, triggering numerous thunderstorms that can go on to become hurricanes. (Graphic by Robert Simmon, NASA GSFC.) 
The last and most common mechanism that triggers the development of a cyclone is the African easterly wave,an area of disturbed weather that travels from east to west across the tropical Atlantic. Essentially, an easterly wave forms because of a “kink” in the jet of air that flows west out of Africa. The jet is created by the strong temperature difference between the Sahara Desert and the Gulf of Guinea. The warm air over the Sahara rises and, several kilometers above the surface, turns southward toward the cooler air over the Gulf. The rotation of the Earth turns this air current westward to form the African Easterly Jet, which then continues out over the Atlantic Ocean. Occasionally, a “kink” will develop in the jet and move from east to west, hence the name easterly wave. Converging winds on the east side of the easterly wave trigger the development of thunderstorms, and some of these large thunderstorm systems go on to become hurricanes. Most Atlantic hurricanes can be traced to easterly waves that form over Western Africa. Hurricanes are commonly formed by easterly waves.The waves are “kinks” in the African Easterly Jet, a strong wind that blows over the Atlantic from the West African coast. The easterly waves trigger strong thunderstorms that move eastward. Over the warm waters of the Atlantic, the thunderstorms embedded in the easterly wave can grow into a hurricane under the right conditions. 
These images and animations show an easterly wave crossing the African coast during late summer 2005. The system later developed into Hurricane Irene. Cold cloud tops appear white in these thermal-infrared images, acquired by the Meteosat-8 satellite. As the easterly wave, visible as a circular area of clouds, approached the African coast it began to rotate. On August 4, the system became a tropical depression over the Atlantic. (Images copyright 2005 EUMETSAT, provided by the British Atmospheric Data Center.) Directory: ourpages -> auto -> 2016 2016 -> Typhoon (the Northwest Pacific Ocean west of the dateline) 2016 -> A kq2a: What is the pattern of air movement in the troposphere? Variations in global insolation 2016 -> District 26 family night at citi field september 20, 2016 New York Mets vs. Atlanta Braves 2016 -> C/Linux Programming Syllabus – Fall 2016 Instructor: Carol Yarbrough, asfa computer Science Email 2016 -> Mid-term exam schedule january 2017 Mon., Jan. 23 – 8: 30am 2016 -> Science Skills Center High School Dr. Dahlia McGregor, Principal ap computer Science A 2016 -> Short Essay Response to Emily Dickinson's Poetry (1 page minimum, 1 ½ page maximum) Due Date: Tuesday, March 8th 2016 -> Program: University of Illinois Chicago Chance & stem academy University Name 2016 -> Fourth Grade ela/ss winter Packet Thursday, December 22 2016 -> Chapter Questions Download 448.17 Kb. Share with your friends:
|