Page 261 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 8 Weather 225
Clouds overshoot top of thunderstorm
Mesocyclone (3 to 10 km diametre)
Tornado
Anvil
Air inflows
(a) Strong wind aloft establishes spinning, and updraft from thunder- (b) A tornado descends from the base of a supercell cloud. storm development tilts the rotating air, causing a mesocyclone to
form as a rotating updraft within the thunderstorm. If one forms, a
tornado will descend from the lower portion of the mesocyclone. Animation ▲Figure 8.15 Mesocyclone and tornado formation.
Tornado Wind Patterns [(b) Photo by A. T. Willett/getty images.]
flying objects, and causing broken trees and limbs. Their highest frequency (about 70%) is from May to August.
Derecho, a term used widely outside of Canada, is a powerful, damaging straight-line wind event driven by severe thunderstorms, but specifically linked to large, or- ganized, fast-moving areas of thunderstorms. The name, coined by physicist G. Hinrichs in 1888, derives from a Spanish word meaning “direct” or “straight ahead.” These strong, linear winds, in excess of 26 m · s−1, tend to blast in straight paths fanning out along curved wind fronts over a wide swath of land. Derechos are capable of producing widespread and long-lived—over a num- ber of hours—wind damage on the order of hundreds of kilometres. Reported derecho wind events are on the increase since 2000 and may continue to increase with climate change. For more information, see www.spc .noaa.gov/ misc/AbtDerechos/derechofacts.htm.
tornadoes
A tornado is a violently rotating column of air in contact with the ground surface, usually visible as a spinning vortex of clouds and debris. A tornado can range from a few metres to more than a kilometre in diametre and can last anywhere from a few moments to tens of minutes (Figure 8.15).
The updrafts associated with thunderstorm squall lines and supercells are the beginning stages of tornado
development (however, fewer than one-half of all super- cells produce tornadoes).
tornado Measurement Pressures inside a tornado usually are about 10% less than those in the surround- ing air. The inrushing convergence created by such a horizontal pressure gradient causes high wind speeds. The late Theodore Fujita, a noted meteorologist from the University of Chicago, designed the Fujita Scale, which classifies tornadoes according to wind speed as indi- cated by related property damage. A refinement of this 1971 scale, adopted in April 2013 in Canada and February 2007 in the United States, is the Enhanced Fujita Scale, or EF Scale (Table 8.1). The revision met the need to better assess damage, correlate wind speed to damage caused, and account for structural construction qual- ity. To assist with wind estimates, the EF Scale includes Damage Indicators representing types of structures and vegetation affected, along with Degree of Damage rat- ings, both of which are listed at the URL cited in the table note. A summary of the Canadian conversion to the EF scale can be found at ec.gc.ca/meteo-weather/default .asp?lang=En&n=41E875DA-1.
tornado Frequency North America experiences more tornadoes than anywhere on Earth because its latitu- dinal position and topography are conducive to the meeting of contrasting air masses and the formation
Georeport 8.4 Storm Causes Hawai‘i Hailstorm and tornado
Although the conditions necessary to form supercell thunderstorms and large hail are rare in Hawai‘i, a March 2012 storm produced a grapefruit-sized hailstone on the windward side of Oahu—measured at 10.8 cm in diametre. The same storm
spawned a waterspout offshore that turned into a small tornado after it hit land, both rare occurrences in Hawai‘i. The NWS confirmed this event as an eF-0 tornado, with wind speeds reaching 97–113 km·h−1.