Page 262 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

226 part II The Water, Weather, and Climate Systems
TABLE 8.1 The Enhanced Fujita Scale
most recent 20-year period, scientists agree that these data are unreliable indicators of actual changes in tor- nado occurrence because they correspond with more people being in the right place to see and photograph tor- nadoes and improved communication regarding tornado activity. Any actual increases in tornado occurrence may in part relate to rising sea-surface temperatures. Warmer oceans increase evaporation rates, which increase the availability of moisture in the mT air masses, thus pro- ducing more intense thunderstorm activity. Other factors in tornado development, such as wind shear, are not as well understood, are difficult to model, and cannot yet be definitively linked to climate change.
In the United Kingdom, observers report about 50 per year, all classified less than EF-3. In Europe over- all, 330 tornadoes are reported each year, although some experts estimate that as many as 700 per year occur. In Australia, observers report about 16 tornadoes every year. Other continents experience a small number of torna- does annually. Although tornadoes occur less frequently in other places such as China, Russia, and Bangladesh, population densities are much greater in some of these locations. Therefore, these violent storm events result in much greater loss of life.
tropical Cyclones
Originating entirely within tropical air masses, tropical cyclones are powerful manifestations of the Earth– atmosphere energy budget. (Remember that the tropics extend from the Tropic of Cancer at 23.5° N latitude to the Tropic of Capricorn at 23.5° S latitude.)
Tropical cyclones are classified according to wind speed; the most powerful are hurricanes, typhoons, or cyclones, which are different regional names for the same type of tropical storm. The three names are based on location: Hurricanes occur around North America, typhoons in the western Pacific (mainly in Japan and the Philippines), and cyclones in Indonesia, Bangla- desh, and India. A full-fledged hurricane, typhoon, or cyclone has wind speeds greater than 119 km ∙ h−1 (64 knots); the wind-speed criteria for tropical storms, depressions, and disturbances are listed in Table 8.2. For coverage and reporting, see the National Hurricane Centre at www.nhc.noaa.gov/ or the Joint Typhoon Warn- ing Centre at www.usno.navy.mil/JTWC. Information from the Canadian Hurricane Centre can be found at www.ec.gc.ca/ ouragans-hurricanes/default.asp?lang=En.
In the western Pacific, a strong tropical cyclone is designated a super typhoon when winds speeds reach 241 km ∙ h−1 (130 knots). In November 2013, Super Typhoon Haiyan hit the Philippines with sustained winds at 306–314 km ∙ h−1, the strongest ever recorded for a tropical cyclone at landfall (see the satellite image of Haiyan on the title-facing page of this book).
Storm Development Cyclonic systems forming in the tropics are very different from midlatitude cyclones
EF-Number
3-Second-Gust Wind Speed; Damage
eF-0 gale
eF-2 Strong
eF-4 Devastating
105 –137 km ∙ h−1; light damage: branches broken, chimneys damaged.
178 –217 km ∙ h−1; considerable damage: roofs torn off frame houses, large trees uprooted or snapped, boxcars pushed over, small missiles generated.
267–322 km ∙ h−1; devastating damage: well- built houses leveled, cars thrown, large missiles generated.
eF-1 Weak
138 –177 km ∙ h−1; moderate damage: beginning of hurricane wind-speed designation, roof coverings peeled off, mobile homes pushed off foundations.
eF-3 Severe
218 –266 km ∙ h−1; severe damage: roofs torn off well-constructed houses, trains overturned, trees up- rooted, cars thrown.
eF-5 incredible
More than 322 km ∙ h−1; incredible damage: houses lifted and carried distance to disintegration, car- sized missiles fly farther than 100 m, bark removed from trees.
Note: See www.depts.ttu.edu/weweb/Pubs/fscale/EFScale.pdf for details.
of frontal precipitation and thunderstorms. Tornadoes have struck all 50 states and all the Canadian provinces and territories. In the United States, 54030 tornadoes were recorded in the years from 1950 through 2010.
Canada places second to the United States in the world for tornado occurrences. In Canada, on average, there are 80 tornadoes causing two deaths, 20 injuries, and tens of millions of dollars in property damage each year. The actual number of tornadoes may be higher, as tornadoes that strike unpopulated areas remain unde- tected. Canada’s “tornado alleys” are southern Ontario, Alberta, southeastern Québec, and a band stretching from southern Saskatchewan and Manitoba through to Thun- der Bay, Ontario (Figure 8.16a). Other tornado zones occur in the interior of British Columbia and in New Brunswick. In Canada, annual tornado numbers have been increasing since 1990.
The long-term annual average number of torna- does before 1990 in the United States was 787. Interest- ingly, after 1990 this long-term average per year rose to over 1000. Since 1990, the years with the highest num- ber of tornadoes were 2004 (1820 tornadoes), 2011 (1691 tornadoes), and 1998 (1270 tornadoes). On April 27, 2011, 319 tornadoes were sighted, the third highest number ever recorded on a single day. Figure 8.16b shows areas of tor- nado occurrence in the United States, which is highest in Texas and Oklahoma (the southern part of the region known as “tornado alley”), Indiana, and Florida.
According to records since 1950, May and June are the peak months for tornadoes in the United States. The graph in Figure 8.16c shows average monthly tornadoes per month for 1950 to 2000, and for 1991 to 2010. Al- though the trend is toward increasing tornadoes for the