What is a Hurricane? Fig H-2: Image courtesy of Photodisc   As discussed in Physical Geography, a hurricane is an intensely developed tropical cyclone, with wind speeds exceeding 65 kilometres per hour. These severe storms go by different names in different regions of the world. In the eastern Atlantic and western Pacific Oceans they are called hurricanes. Elsewhere they are known as typhoons (western North Pacific) or (tropical) cyclones (Indian Ocean).     Origin Hurricanes start as tropical disturbances associated with easterly waves in the tropical oceans, including the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern North Pacific of the west coast of Mexico. Typhoons originate in the western Pacific and most of the South Pacific. Conditions favourable for hurricane development include deep surface layer water temperatures above 27°C (81°F) in the latitudes between 5°-20°, unstable atmosphere throughout the troposphere, and weak tropospheric winds. These conditions provide enough latent heat to be transported upward from the surface, and enough Coriolis force for strong cyclonic rotation.   FIG H-3: Image from Physical Geography     Properties A hurricane has an eye, or patch of clear sky, in the middle of strong counter-clockwise rotating and uprising air, known as eye walls. The eye size ranges from 20-50 km (15 to 30 mi). See the diagram below to get an image of the structure of a hurricane.   FIG H-4: Image from Physical Geography     Decay At the surface, hurricanes can diminish rather quickly if the conditions are right. These conditions include: (1) the storm moving over cooler water that can't supply warm, moist tropical air; (2) the storm moving over land, again cutting off the source of warm, moist air; and finally (3) moving into an area where the large-scale flow aloft is not favourable for continued development or sustainability.     Magnitude The magnitude of a hurricane can be categorized using the Saffir-Simpson Hurricane scale. This scale can be used to predict the amount of damage and flooding that will occur along the coast where a hurricane makes landfall.   Scale 1 2 3 4 5 Central Pressure (mb) >980 965-970 945-964 920-944 <920 Wind Speed (km/hr) 119-153 154-177 178-209 210-250 >250 Storm Surge (metres) 1.2-1.5 1.6-2.4 2.5-3.6 3.7-5.5 >5.5 Damage Minor Moderate Extensive Extreme Catastrophic A recent Category 5 hurricane was Hurricane Mitch, which hit Honduras in October 1998. Hurricane Mitch left an estimated 9,200 people dead and destroyed over 150,000 homes. Total agricultural losses from the hurricane were estimated to be close to $1 billion.   FIG H-5: Tracking map of Hurricane Gustav, which passed through Nova Scotia and Newfoundland on 12 September 2002. Photo from National Oceanic and Atmospheric Administration (NOAA)   To learn more about the Saffir-Simpson Hurricane Scale, click here.     Geography a. Global Geography. Hurricanes form over all warm, tropical oceans, except in the South Atlantic. Hurricanes require water temperatures to be at least 28°C. Areas which meet this requirement are outlined in the color-coded map as pink. Note that all other tropical oceans have tropical storms and hurricanes. The western North Pacific, the largest ocean area with the warmest temperatures, also has the most hurricanes, called typhoons. This region has about 30% of the world's total tropical storms and hurricanes. The western North Atlantic, including the Caribbean, only has 12% of the total (Gray, 1975).   FIG H-6: NASA   b. North American Geography. In the United States, hurricanes threaten the east and south coasts along the Atlantic and the Gulf of Mexico, while in Canada, hurricanes may impact the Maritime provinces along the east coast. The figure below shows typical hurricane tracks in the North Atlantic Ocean. We can see here that areas most affected by tropical storms or hurricanes are Gulf states and East Coast states.   FIG H-7: Image from Physical Geography   The path of every storm in this part of the world has been mapped in the American National Hurricane Center's "track book" which is updated annually. To look at archives of past years' hurricane data, click here.   Source for material in this section: Gray, W.M. (1975). Tropical Cyclone Genesis. Fort Collins: Colorado State Univ., Atmospheric Science Paper No. 234 (Available from CSU, Atmos. Sci. Dept., Fort Collins, CO. 80521).     When do Hurricanes Occur? The Northwest Pacific basin has tropical cyclones occurring all year round, though there is a distinct minimum in February and the first half of March. The main season last from July to November with a peak in late August/early September, while Canada's peak season occurs in October.   The North Indian basin has a double peak of activity in May and November though tropical cyclones are seen from April to December. The severe cyclonic storms (>33 m/s winds [76 mph]) occur almost exclusively from April to June and late September to early December.   The Southwest Indian and Australian/Southeast Indian basins have very similar annual cycles with tropical cyclones beginning in late October/early November, reaching a double peak in activity - one in mid-January and one in mid-February to early March, and then ending in May. The Australian/Southeast Indian basin February lull in activity is a bit more pronounced than the Southwest Indian basin's lull.   The Australian/Southwest Pacific basin begins with tropical cyclone activity in late October/early November, reaches a single peak in late February/early March, and then fades out in early May.   Globally, September is the most active month and May is the least active month.   Source: Neumann, C.J. (1993): "Global Overview" - Chapter 1 in Global Guide to Tropical Cyclone Forecasting, WMO/TC-No. 560, Report No. TCP-31, World Meteorological Organization; Geneva, Switzerland.     Why do some Hurricanes turn North to Atlantic Canada? There are a number of reasons that hurricanes come north to Atlantic Canada. Storms that form further out in the Atlantic (rather than close to the Gulf of Mexico) have a better chance of heading for Atlantic Canada because there is less land mass in between. Seas surface temperatures are very important for the life of a hurricane. Normally the sea surface temperatures in the waters off Canada are too cool to feed the hurricane, so it dies out before reaching Canadian soil.   Sea surface temperatures in Canadian waters have been on the rise, however, thus feeding hurricanes in higher latitudes than in the past. The Coriolis Force (caused by the rotation of the earth) and other weather systems also help to direct the hurricanes north. Weather systems play a large role in the advancement of hurricanes by blocking a path or by forcing a path on a hurricane. These three elements combine to help hurricanes to advance toward Atlantic Canada.