A few years ago National Geographic, in an article on Saskatchewan, mentioned that we sometimes have a little wind. (I trust I’m not revealing any secrets.) But, one man was quoted as saying, “In Saskatchewan, we don’t consider it really windy until we have whitecaps in our bath water.”
Wind and Saskatchewan seem to go together, like sunburn and summer or Captain Kirk and Mr. Spock. But just what is wind, and where does it come from?
I thought you’d never ask.
Wind is basically the movement of air that arises from differences in air pressure either horizontally or vertically. It occurs at three different scales.
On a planetary scale, there is a vast and constant movement of air within the troposphere, the bottom 11 kilometres of the atmosphere. At the equator, air is heated and rises to the top of the troposphere. It can’t go higher because the next layer, the stratosphere, is even hotter because of the extra ultraviolet energy absorbed at that altitude.
However, more air is constantly being heated and pushed up under it, so the air at the top of the troposphere has to go somewhere. That somewhere is sideways, resulting in a constant flow of air poleward from the equator.
This flow would be directly north or south if not for the rotation of the Earth, which produces the Coriolis effect. Something at the equator moves eastward faster than something closer to the poles, so the air pushed poleward carries with it a strong eastward momentum. It is this eastward momentum, along with friction between the rotating Earth and its atmosphere at low altitudes, that makes weather systems in the northern hemisphere and southern hemispheres rotate in opposite directions.
(Contrary to popular belief, water does not flow down the drain in opposite directions in the two hemispheres; the Coriolis effect is too weak to be noticed on such a small scale. Water can spiral down the drain in either direction in either hemisphere.)
As the air cools, at about 20 degrees latitude, it sinks and flows back into the low-pressure area created by the rising air at the equator. This flow of air back to the equator at ground level creates constant “trade winds,” so-called because they played such an important role in trans-oceanic trade in the days of sailing vessels.
These vast, rotating cells of air are called the Hadley cells. In the mid-latitudes similar cells called Ferrel cells operate in opposite fashion, carrying surface air north and upper tropospheric air toward the Hadley Cells. The trade winds blow eastward, but in these latitudes we have prevailing westerlies.
The next level of winds are “synoptic.” These are generated by those funny symbols meteorologists put all over their maps. Air pressure is not constant; it varies from place to place in the atmosphere, on many scales, from “standing waves” of high and low pressure thousands of kilometres long in the upper troposphere to relatively localized, surface high and low-pressure areas (also called anticyclones and cyclones). Winds flow around these surface system, counterclockwise around lows in the northern hemisphere and clockwise in the southern hemisphere.
Finally, there are mesoscale winds–the winds associated with thunderstorms and tornadoes, for example, or winds that arise because of local geography. A good example is the sea wind/land breeze cycle of coastal regions, where the difference in heating and cooling rates between ground and water result in a surface breeze from the cooler water to the warmer land by day, and vice versa at night.
Another good example, and one we’re more familiar with in Saskatchewan, is the chinook. This is born in the mountains, where an “adiabatic” temperature change occurs as air flows over the peaks. (An adiabatic temperature change is one that occurs without any addition or subtraction of heat.) When air is lifted it expands and cools; when it sinks, it contracts and warms. Generally the wind going up the mountains carries moisture, which condenses and drops out as the air rises and cools, thereby moderating the temperature change. However, without moisture, the air heats up more on its descent than it was cooled on its ascent, producing either warm, dry conditions–or very high winds that raise the temperature quickly.
One other special wind that should be mentioned is the jet stream. Jet streams are upper-air bands of relatively strong winds (30 metres per second all the way up to 75 metres per second to the east of major continents) nine to 18 kilometres in altitude, located above areas of strong temperature gradients. The one we hear about the most is the polar front jet stream. The polar front is the sharp temperature gradient between the polar and mid-latitude air masses. The polar front jet stream is strongest in winter because that’s when temperature differences are greatest. This powerful wind is important because it helps steer major low-level weather systems. When it takes an unusual route, it can really mess up our normal weather patterns.
Like the jet-stream, surface winds are usually strongest in the winter, for the same reason–and, in Saskatchewan, strongest of all in April and May (when we sometimes have our worst blizzards and/or dust storms) because of the extreme temperature gradients between the still icy land north of us and the rapidly warming land to our south.
Year-round, our winds change with shifting cyclonic tracks. In the winter, weather systems generally track across the middle of the province, along a trough of low pressure between a stable high in the north and one in the south. In the summer this track is displaced northward, reducing wind velocities.
Winds in the north are generally weaker than those in the south due to forest cover. The mean wind velocity in the north is between eight and 16 kilometres per hour; in the south the mean wind velocity ranges from 19 to 26 kilometres per hour, with winds occasionally ranging as high as 50 to more than kilometres per hour.
Which, by my reckoning, is just about enough to put whitecaps on your bathwater.