Weather: Tornado alert! Canada, too, is vulnerable
Monday, March 31, 2008
About 100 tornadoes a year strike Canada - mostly in southern Ontario and the Prairies. Fortunately, today's improved forecasting is giving those in their path precious minutes to take shelter
by HENRY HENGEVELD
This past winter, North Americans had another reminder of how destructive and terrifying some weather events can be.
On Feb. 5, a massive, humid air mass from the Gulf of Mexico worked its way up the southeast of the United States, only to run head on into another air mass - this one cold and dry - working its way south-eastward through the central American plains.
The warm air, being lighter, began to slide over the top of the cold air mass, rising and cooling in the process. As a result, the large volume of water vapour within the air mass began to condense into water droplets, releasing vast quantities of heat into the surrounding air.
The added energy caused the air to rise even more rapidly, starting a complex pattern of updrafts and downdrafts which gradually developed into intense thunderstorms. In places, these vertical winds developed a spinning motion and triggered the birth of a tornado.
Late that day and the following morning, a total of some 30-40 such tornadoes roared across Arkansas, Alabama, Mississippi, Tennessee and Kentucky. They killed at least 59 people, demolished hundreds of homes into kindling wood, and tossed vehicles around like toys. Experts indicated that this was the deadliest such outbreak in 23 years. They also noted that such events usually occur in spring and summer, not in mid-winter.
While it is tempting to shrug our shoulders and declare that this is an American problem, the fact is that such tornadoes do also happen in Canada. Two regions - southern Ontario and the Canadian Prairies - are particularly vulnerable. On average, about 100 tornadoes are reported in Canada each year (compared to 800 in the United States). Most of these are weak events causing modest damage, and limited fatalities or injuries. None appear to occur in winter.
Environment Canada analysts, over the years, have gone to great lengths to develop a good record of past tornado events in Ontario. Those that have occurred during recent decades are generally well documented. That is partly because intense media interest ensures that each is brought to the attention of experts in a timely manner for investigation and confirmation.
Doppler radar and lightning detection networks provide the instrumental data to help these experts determine whether the events were really twisters, rather than a wind burst related to a thunderstorm, and to estimate their magnitude. Doppler radar can detect the spinning air motion caused by a twister, while the lightning detection confirms the presence of frequent electrical discharges usually associated with tornadoes.
However, for early records, developing accurate estimates for frequency and intensity is a much greater challenge. This requires hours of carefully reading eyewitness accounts of the nature of damage caused, usually from newspaper clippings. Even then, the statistics are almost certainly underestimates, since sparse populations and poor communication facilities during the earlier parts of the 20th century would have resulted in many tornadoes not being reported.
Despite these limitations, the available data shows that tornadoes are quite common throughout southern Ontario. About two-thirds of these appear to be in the F0 and F1 categories. That's the technical jargon used by experts to identify tornadoes that have maximum wind speeds of 180 km/hour or less. While intense enough to overturn cars, uproot trees and damage building exteriors, they are relatively weak and do not level well-built structures.
However, there have also been numerous occurrences of more intense events. Figure 1 (see page 52) indicates the distribution of tornadoes in southern Ontario reported between 1918 and 2003 which are estimated to be in the categories F2 and above (wind speeds greater than 180 km/hour).
About two per cent of these reach the F4 magnitude, all in the south-west. At this magnitude, winds increase to devastating strengths of 331 and 417 km/h - enough to level even well-built homes, and to throw large steel and concrete missiles great distances.
While tornadoes of the F5 category - the mega-tornadoes that can be as much as a mile wide and sustain winds in excess of 500 km/hour - have never been officially confirmed in Canada, newspaper accounts of damages suggest that one or two of these may have occurred in the past in Saskatchewan.
While no weather model is yet able to forecast a specific tornado, considerable progress has been made in identifying conditions that can result in tornadoes and in detecting the formation of twisting actions within thunderstorms which precede the full development of a tornado that touches the ground. Doppler radar, which can measure winds within clouds, have been particularly useful.
Such information allows forecasters to issue warnings to those in the area of greatest risk, and to alert them when actual cloud spinning action begins to develop. The minutes of advance warning provided though these alerts allow people in the path of a tornado to take shelter - preferably in basements or closets (but not in a car).
One of the common myths about tornadoes it that it is the low pressure at the centre of the tornado that causes buildings to "explode" as the tornado passes overhead. Some argue that opening windows in advance can thus help equalize inside and outside pressures more quickly and reduce the damage caused.
In fact, most of the structural damage from these storms is caused by the intensity of the wind and the debris that it slams against the building. Opening windows will only allow the wind from even relatively weak tornadoes to enter the structure more readily and create havoc inside. Far better to use the precious time available to seek a safe haven to shelter in!
While this all may sound worrisome, the good news is that the odds of dying from a tornado are about 12 million to one. That's less risky than driving a car! BF
Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada.