Toronto “Tornado”

A severe thunderstorm's shelf cloud slowly engulfs the city.  August 20th, 2009.  From Janine Massey's Flickr photostream.

A severe thunderstorm's shelf cloud slowly engulfs the city. August 20th, 2009. From Janine Massey's Flickr photostream.

On August 20th, a severe thunderstorm passed through the city, and north of us it spawned a tornado (around Highway 7 and Martin Grove Road). There were a few other tornadoes too, one in Windsor, and another in Durham which unfortunately went on to kill an 11-year-old boy at a day camp.  There are a lot of myths and half-truths that surround tornadoes, gleaned from movies or decades of folk wisdom.  It can be tempting for Torontonians—who generally think of themselves as an island of tranquillity free of severe weather—to overreact a little.

The fact is, though, that Canada is second only to the United States as the favourite touchdown spot of tornadoes (roughly 70-80 per year—NRCan‘s stats say 70, EnvCan‘s stats say 80), and of the ten worst Canadian tornadoes, Ontario can lay claim to four (see here and scroll down for “Canada’s Worst Tornadoes”).

Although our true “Tornado Alley” lies along an axis from Sarnia to London, Toronto happens to be right on the edge of an area that gets 2.5 to 4.9 tornadoes (per 10,000 km2) every year. So they are not exactly scarce to this part of Ontario.

Map of the Annual Number of Tornadoes in Canada.  Source: Natural Resources Canada

Map of the Annual Number of Tornadoes in Canada. Source: Natural Resources Canada

One thing you will see a lot of—especially on YouTube—is people shooting the shelf cloud or wall cloud and labelling it a tornado, presumably because its large, low-hanging semi-circular shape  vaguely resembles a good-sized town-wrecking tornado.  This video (with a hat tip to neighbour Joey de Villa) is a prime example.  I initially misidentified it as a wall cloud, but on further reflection realised it is merely the shelf cloud, as it is too close to the storm’s leading edge.  The shelf cloud (which can be semi-circular) trails the storm’s gust front, pushed outward by inflow of warm air and the outflow of cool air from the storm cell.  You can tell it is the shelf cloud because a) it is very close to the storm’s front edge, and b) there is no rotation.

Tornadoes spawn from something known as a mesocyclone (or more colloquially a “wall cloud“).  The wall cloud is the visible manifestation of a large, rapidly rotating updraft, and it is this spinning updraft which may eventually form a tornado.  The wall cloud of a mesocyclone does rotate slowly, but mesocyclones do not typically form at the leading edge of the storm; they tend to be located in the right rear side of supercells (see also: hook echo).  This later video (again with a hat tip to Joey) may be just such a wall cloud; although the cameraman is under the humourous misapprehension that it is a tornado itself, and that his life is in danger.

One other common misapprehension is that green skies are indicative of tornadic conditions.  As this Scientific American article states, there is no definitive correlation between sky colour and the presence of tornadoes:

Threatening green skies during a thunderstorm also proved entirely independent of the type of severe weather that came with it. Gallagher measured hailstorms where the dominant wavelength of light was green as well as hailstorms where it was the typical gray-blue color of thunderstorms. Tornado-producing storms proved similarly divorced from any particular sky color, other than dark.

Researchers remain undecided about the exact mechanisms that cause the sky to appear green in certain thunderstorms, but most point to the liquid water content in the air. The moisture particles are so small that they can bend the light and alter its appearance to the observer. These water droplets absorb red light, making the scattered light appear blue. If this blue scattered light is set against an environment heavy in red light—during sunset for instance—and a dark gray thunderstorm cloud, the net effect can make the sky appear faintly green. In fact, green thunderstorms are most commonly reported in the late afternoon and evening, according to Beasley.

— Knight, Meredith.  “Fact or Fiction?: If the Sky Is Green, Run for Cover—A Tornado Is Coming.Scientific American, 14 June 2007. [Emphasis is mine]

You will not be surprised to learn that Thursday’s thunderstorm, which featured green skies, occurred in the late afternoon.  Where green skies may have predictive powers, though, is in heralding the arrival of severe weather.  They may not be a good indicator of tornadoes, but they typically do indicate that you are going to get a serious thunderstorm.

It is also not unusual to think that thunderstorms that do inflict significant property damage had some undetected tornadic activity.  This is not so.  The gust front at a storm’s outflow boundary can contain very high winds, and these will advance several kilometres in front of the storm itself.  If your area suffers a microburst, this can also create tornado-like damage without the presence of any funnel cloud or visible phenomena.

Finally, it is important to pay attention to the nomenclature that your local forecasters give to the storm.  While many of us may think of “severe” as just another adjective, in the parlance of forecasters it actually carries a very specific meaning.  A “severe thunderstorm” is one whose wind gusts (or peak wind speed) have been clocked at 50 kts (93 kph) or greater.  These winds in and of themselves can do significant property damage, whether or not a tornado shows up.  Thursday’s storm was, no surprise, tagged as severe.  Remember that when the weather office declares a severe storm warning, they are expecting 50kt/93kph (or greater) winds, and the gusts can get an awful lot faster than that.  It is no picnic.

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