Wind by Sander Schimmelpenninck
sander@idirect.com

This article in my How-to series for The Ontario Weather Page covers more-or-less horizontal winds, their causes, and how to measure or estimate them. Wind, of course, is the movement of air. You can feel it but not see it. However, you can see its immediate effects, like flags flapping or smoke or steam carried in its path.

Naturally occurring winds come in two main flavours: geostrophic and ageostrophic. Geostrophic comes from the Greek roots ge (earth) and stroph (turn). You probably know that air tends to move from areas of high pressure to those of low pressure. Anyone who has seen the billowing cheeks of the jazz trumpet giant Dizzie Gillespie knows that.

The winds we see most often are caused by big systems: high- and low-pressure areas that cover several hundred kilometres on weather maps. However, big-system winds do not travel straight from highs to lows; that would be far too simple. Instead, they deviate from their expected course almost as soon as they get going, and thus sidle up to the low as in reluctant courtship. To understand this geostrophic behavior, consider the record player still found in the dusty attics of some senior citizens. Cover the turntable with a paper disk. Imagine that the spindle is the centre of a low-pressure area and a spot on the rim a high. Start the machine. Move a pencil from your high to your low. Stop the record player and observe your trace. Right: it's a spiral.

In nature, the same thing happens. The wind obediently heads from the pressure high to the low, but meanwhile the earth's surface rotates under the path of the wind. The windstream does not know that. As a result, the wind flows clockwise around pressure highs and counterclockwise around lows, at least in the Northern Hemisphere. The opposite happens south of the equator, because they still drive on the left in Australia.

My intellectual peers will recall Bugs Bunny's immortal words: I must have taken the wrong turn at Albuquerque. Meteorologists blame the Coriolis Effect, after Gustave Gaspard de Coriolis, 1792-1843. Coriolis was assistant professor of mathematics at the Ecole polytechnique, a French Army university for geniuses. Not all work and no play, Coriolis wrote A theory on the effects in the game of billiards. He later described the wind's circuitous path from highs to lows, no mean feat, because weather maps came into use only decades later, after the invention of the telegraph.

Despite its detours, you can estimate the speed of the geostrophic wind when you know the pressure gradient. That term means the pressure difference per unit of distance. Some weather maps show isobars, lines of equal pressure. When those are packed close together, like suburban moms lining up for a Sears November special on hockey sticks, you have high pressure gradient. For instance, when you have a pressure drop of 6 millibars over 300 km (the length of Lake Ontario), you can expect a windspeed of about 60 km/h almost gale force. That applies during the day. At night the surface wind usually dies down, due to friction with the surface of the Earth.

So much for the geostrophic wind. As mentioned, (quasi-)horizontal winds are also caused by other effects than those of big systems in your weather map. Here are some.

If you live near a large body of water, you may have noticed sea- or lake-breezes, the delightful onshore winds that cool you on sunny August days. At our place, about 1 km inland from Lake Ontario between Toronto and Hamilton, they usually flow at about 8 knots or 15 km/h. The sun warms the ground, raising its temperature 10 or 20 Celsius above the lake temperature. In turn, that warms the air and makes it lighter causing it to rise. Something has to fill its place, and that something is cool air above the lake. You're right: over-land heating causes a low-pressure area. However, the distances are so small that the Coriolis effect does not get a chance. For that reason the lakebreeze flows straight from high to low, at a right angle to the coastline.

Thunderstorms cause another kind of ageostrophic wind. We have all experienced the sudden winds associated with their gust fronts. In extreme cases thunderstorms spawn tornados, the strongest naturally-occurring winds, with speeds up to 400 km/h.

The wind also does strange things on a smaller scale. When faced with a constriction, highway traffic slows which is why traffic reporters earn their outrageous salaries. Not so for the wind. When it passes through a narrows, e.g. the canyon between two sets of high-rise buildings, it speeds up, so that the air molecules at the far end can catch up with those that flowed above or around the obstruction. Generically that is called the Venturi effect, but meteorologists and sailors know it as channelling and tunnelling. Our driveway is oriented northeast/southwest and flanked by 10-metre tall cedars. When the wind is out of the southwest (its favorite direction), tunnelling magnifies its speed.

Once again, this chapter covered more-or-less horizontal winds. Others blow up and down as if there were no tomorrow. I plan to write about them in an article about the upper atmosphere.

To imagine seemingly-erratic wind flows, think of the wind as a river or perhaps a babbling brook. Have you noticed the rushing eddies and nearly-stagnant pools in the Rhine or the Mad River (near Ontario's Devil's Elbow provincial campground, ten sites, no staff and no running water, near Alliston, and a great source of watercress)? That is how air behaves, and that it why the wind in your back yard may blow from the south when its general direction is from the north.

Now how to measure wind direction and velocity. The instrument of choice is the arrowvane windset, a miniature airplane and propeller mounted swivelling on a 10-metre TV tower hence the term 10-metre wind, commonly known as the surface wind. Canada's weather office, the Atmospheric Environment Service (AES), wants to see no obstacles whose height above the arrowvane (not the base of the tower) exceeds 1 metre for every 10 metres of distance (or 6 degrees above horizontal).

Without a government-specified windset, both wind direction and speed are guesses, so let yours be educated.

For direction, know your true, cardinals: North, East, South, and West by landmark. Use a magnetic compass to determine landmarks, and apply your local compass correction, the difference between magnetic and true (or geographic) north. For that correction, known in aviation as variation, ask a pilot or sailor friend, or call your nearest NavCanada flight service station.

Find a good reference for wind direction, e.g. the flag on an isolated nearby building. Remember how local eddies can fool you.

For windspeed-estimating nothing rivals the Beaufort Scale, invented by a British admiral before you were a gleam in your father's eye. It appears elsewhere on the Ontario Weather Page. Contact me at sander@idirect.com if you want help.

AES defines windspeed as the mean (or average) over the past minute and gusts as the peak value in the past 10 minutes before the reported-observation time. Don't report a peak gust unless your mean wind is at least 10 knots, or 19 km/h.

That reminds me: pilots still navigate in nautical miles. Aviation weather reports therefore express windspeeds in knots, nautical miles per hour. One nm equals exactly 1.852 kilometres.

When estimating windspeed, remember the local vagaries described earlier in this chapter. Pick a relatively open spot. It's hard to know how accurate your windspeed estimates are. I can suggest only that you record yours and compare them with reports from your nearest airport within, say, 20 km. In doing so, ignore observations when showers or thunderstorms occurred at either or both sites.

This article is Copyrighted 1999: Sander Schimmelpenninck and used with his expressed written permission.  No unauthorized use allowed without prior written consent by him.