What is a tornado? FIG T-2: Image courtesy of NOAA   A tornado (also called a cyclone or a twister) is a violently rotating column of air that hangs from a cumulonimbus cloud. A tornado is nearly always observable as a dark-coloured "funnel cloud" or tube. It is the most destructive of all atmospheric phenomena. The vortex, having an average size of 100 to 500 m (330 to 1650 ft), whirls at speeds from 160 to more than 480 kilometres per hour.     What causes a tornado?   A tornado is produced inside a severe thunderstorm cloud called a cumulonimbus, and is usually accompanied by hail, lightning, and thunder. This thunderstorm may be associated with frontal boundaries, squall lines, mesoscale convective complexes, supercells, and tropical cyclones.   Figure T-3 shows the life cycle of a thunderstorm (developing, mature, and dissipating stages) while Figure T-5 shows how a squall-line storm can begin to generate more severe weather in the form of hailstones and tornadoes.   FIG T-3: Image from Physical Geography   FIG T-4: Image from Physical Geography   The pressure in the centre of such a spinning funnel may be 100 to 200 mb below the pressure of the surrounding air. This difference in pressure triggers highly destructive winds. Tornadoes tend to follow relatively straight paths that can leave swaths of destruction up to 160 km (100 miles) long and 900 m (300 ft) wide. Their dark colour is accentuated by the vegetation, loose soil, and other objects sucked into the centre tube of this gigantic vacuum.     How are tornadoes classified?   Tornadoes are classified according to a system known as the Fujita or Fujita-Pearson scale, named after Professor Theodore Fujita from the University of Chicago, and Dr. Alan Pearson, the former director of the National Severe Storm Forecast Center (NSSFC), who devised the system in 1971. This scale relates the wind speed of a tornado to the amount of damage done.     Fujita Tornado Damage Scale   SCALE WIND ESTIMATE (KMPH)* TYPICAL DAMAGE F0 64-116 Light Damage. Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged. F1 117-180 Moderate Damage. Peels surface of roofs; mobile homes pushed off foundations or overturned; moving autos blown off roads. F2 181-252 Considerable Damage. Roofs torn off frame houses; mobile home demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground. F3 253-330 Severe Damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown. F4 331-417 Devastating Damage. Well-constructed houses leveled; structures with weak foundations blown some distance away; cars thrown and large missiles generated. F5 418-509 Incredible Damage. Strong frame houses leveled off foundations and swept away; automobile-sized missles fly through the air in excess of 100 metres (109 yards); trees debarked; incredible phenomena will occur.   *Important Note About F-Scale Winds: Do not use F-Scale winds literally. These precise wind speed numbers are actually guesses and have never been scientifically verified. Different wind speeds may cause similar looking damage from place to place - even from building to building. Without a thorough engineering analysis of tornado damage in an event, the actual wind speeds needed to cause that damage are unknown.   Sources for F-Scale Table: NOAA and Environment Canada   For another description of the Fujita-Pearson scale, complete with photos of damage typical of each category, please visit the NOAA and Environment Canada.     Other Tornado-like phenomena   Other tornado-like phenomena include:   Waterspouts: A waterspout is a tornado over water. One of the most recent waterspouts to occur in Canada was on 9 August 2004 in the Northumberland Straight. The spinning funnel of water closed the 12.9 km Confederation Bridge for three minutes after being seen on security cameras.   Waterspouts are tornadoes by definition but they don't officially count in tornado records unless they hit land. They are smaller and weaker than the most intense Great Plains tornadoes, but still can be quite dangerous. Waterspouts can overturn small boats, damage ships, cause significant damage when reaching land, and can result in deaths. Special marine warnings are often issued when waterspouts are likely or have been sighted over coastal waters, or tornado warnings are announced when waterspouts seem to be moving towards shore.   FIG T-5: Image courtesy of NOAA   Landspouts: These non-supercell tornadoes arise from the intensification of pre-existing, shallow, vertical vortices near the surface, through simple vortex stretching when a developing convectional updraft moves over them. These narrow vortices look like waterspouts, hence the name.   Multi-vortex tornado: Multi-vortex (or multiple-vortex) tornadoes contain two or more small, intense subvortices orbiting the centre of the larger tornado circulation. When a tornado doesn't contain too much dust and debris, they can sometimes be spectacularly visible. These vortices may form and die within a few seconds, sometimes appearing to train through the same part of the tornado one after the other. Subvortices are the cause of most of the narrow, short, extreme swaths of damage that sometimes arc through tornado tracks. From the air, they can mow down crops and stack the stubble, leaving cycloidal marks in fields. Multi-vortex tornadoes are the source of most of the old stories from newspapers and other media before the late 20th century which told of several tornadoes seen together at once. However, on rare occasions, separate tornadoes can form close to one another as satellite tornadoes.   FIG T-6: Image courtesy of NOAA   Gustnadoes: These very small-scale, shallow vortices may develop near the surface along outflow boundaries and/or cold fronts with or without deep convection aloft.   Tornadocane: This unusual cluster of thunderstorms assumed a hurricane-like shape complete with an eye that produced several damaging tornadoes in North Carolina in April 1999.   More on the tornadocane.     Where do tornadoes occur?   Tornadoes can occur anywhere in the world, although they occur most frequently in North America and Australia. Tornadoes occur mainly in the mid-latitudes, where warm and cold air masses meet under strong polar jet streams.   a. Canada. Tornadoes continue northward from the central states into Canada. In Canada, during an average year, eighty tornadoes cause an average of two deaths and twenty injuries, plus tens of millions of dollars in property damage. Many more tornadoes may go undetected because they touch down in unpopulated areas.   Canada's "tornado alleys" are southern Ontario, Alberta, southeastern Quebec, and a band stretching from southern Saskatchewan and Manitoba through to Thunder Bay. The interior of British Columbia and Western New Brunswick are also tornado zones. Ontario receives approximately 15 tornadoes a year (90% of all which are measured at F0-F1) while Quebec receives roughly 6 tornadoes. On 31 July 1987 a tornado smashed into the city of Edmonton, Alberta, and its eastern suburb of Sherwood Park. Twenty-seven people lost their lives and over 350 people received debilitating injuiries.   The geographic location and features of Tornado Alley allows for the collision of northward-moving maritime tropical air from the Gulf of Mexico with southward-moving continental polar air. When the maritime tropical Gulf air collides with the drier air, the different temperature and humidity characteristics of these air masses create a volatile mixture that can unleash especially violent thunderstorms. Tornado formation occurs most frequently in the Spring due to the strong air mass contrasts during this season.   b. The United States. In the United States, tornadoes touch down most frequently along a wide strip running southwest to northest between the southern Plains and the lower Great Lakes region. This strip can be seen in the map below. However, tornadoes have been known to occur in many other states as well.   Fig T-8: Image courtesy of NOAA     When do tornadoes occur?   a. Canada. The peak season for tornadoes in Canada is May thorugh September with the most active months being June and July. Tornadoes normally occur in the afternoon or early evening.   b. United States. Peak tornado season in the United States is March through May in southern states and during the summer in northern states. In an average year, about 1,000 tornadoes are reported across the country, resulting in eighty deaths and over 1,500 injuries.   For the United States as a whole, tornadoes peak from April through July, with May normally having the largest numbers.