Sunday, February 12, 2006

Column 11: Weather myths (originally opsted November 1, 2005)

We all know that weather story, the one that we know isn't true but we keep hearing it be perpetuated. You know which one I mean.

Well, I think it's high time we debunked the myths of the weather variety.

I'll start off this thread with a couple of well-used myths, and then hopefully you'll come up with some (or ask about something!)

1. The Canadian prairies are dry dry dry.
Fact is, the Canadian prairies, at least in summer, can be as humid as or more humid than southern Ontario! In mid-summer, the typical hot-and-humid temperature/dewpoint combination in Toronto is 30/20. Pretty steamy, producing a humidex of 37. Well, in (especially the eastern) prairies, it gets like that every summer. It may not last as long, but visit Winnipeg from mid-June to mid-August, and don't be surprised to see 30/20 there. Further, the most humid reading I've ever seen in Canada happened in Manitoba. 34/27. Yuck.

2. "It's too cold to snow".
orry, folks, but this one is a bald-faced lie. It's nestled within a grain of truth, though, which has been milled to produce mythical flour. When the temperature is -30 or colder, the air, even if saturated, is very very dry. Cold of this type is not conducive to the production of ice crystals, which are what start off snowflake formation. However, the problem is this: air that cold is extremely dense. Almost always, air this cold is also in a layer only a couple of hundred metres thick, and above it is much warmer air. The problem is that the density of the cold air makes it hug the ground, where we live, and force us to wear ridiculous poofy clothing.

So even though it may be very cold at the surface, aloft it may be warm enough--right in that perfect temperature range to promote ice nucleation and snow production.

3. The Coriolis force makes tornadoes mainly spin counterclockwise in the northern hemisphere
Nope, nope, nope. At least not directly. The Coriolis force acts on length scales of the order of continents, not the order of metres. The amount of Coriolis deflection of air inside a tornado is so small (less than a millimetre) as to be considered zero.

So what's going on then? It's called cyclostrophic balance--the balance between the pressure gradient force (that's why a tornado is so windy) and the centrifugal force.

There is a caveat, however. The Coriolis force does act on low-pressure systems, making the wind blow counterclockwise around them (in the northern hemisphere). This large-scale wind flow is generally needed for tornadoes to form in the first place, so it can be said that the Coriolis force has an impact on tornado formation. Just not in the way that most people think.

As an aside, in a likewise manner, toilets do not flush counterclockwise nor do sinks drain counterclockwise because of the Coriolis force. The direction of swirl is entirely dependent on the motion imparted on the fluid being drained in the first place. Don't believe me? Pull the plug on a full sink, but just before you do, make a slow clockwise circle with your hand in the water. See what ensues.

4. Lightning never strikes the same place twice.
Tell that to the CN tower.
Although, in one sense, this is true. The Earth is spinning through space on an orbit on an orbit (earth spins, revolving around the sun, which is revolving on a spiral arm around a central point in the galaxy). This means that, in true space, lightning likely doesn't strike the same place twice, even if it does. But that's just a bit more philosophical than I need to get here.

5. A green cloud predicts a tornado
This one was helped along by that guy in Twister. "Going green" is what I think he said. Green hues in a cloud merely tell that the cloud has copious water involved in it, so it's absorbing most of the light given to it. It indicates the likelihood of heavy rain, and can sometimes mean an enhanced risk for hail. but a green sky in itself means nothing for tornado formation.

I could go on and on, but then that would take the fun out of it. Go nuts and let me know about some weather myths you've encountered!


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