Why Wind Blows

The push: from high to low

In Lesson 3 we met highs and lows. Wind is the atmosphere trying to even them out. Wherever air is piled up (high pressure), it pushes toward where air is thin (low pressure). Meteorologists call this push the pressure gradient force, but you already know it well.

Let air out of a bicycle tire and it rushes from the high pressure inside to the lower pressure outside. Open a door between a warm room and a cold one and you feel a draft. The bigger the difference, and the shorter the distance between them, the stronger the rush.

That is why forecasters look so closely at isobars on a map. When isobars are spaced far apart, pressure changes gently over a long distance, and the wind is light. When they are packed tightly together, the pressure changes sharply over a short distance, and the wind howls. It is just like contour lines on a hiking map: tightly packed lines mean a steep slope.

The curve: the Coriolis effect and friction

If the pressure push were the only thing going on, wind would blow straight from highs to lows and fill them in within hours. Instead, air swirls around them, and the systems can last for days. The reason is that our planet is spinning.

Coriolis in plain words

Picture two people playing catch on a spinning playground merry-go-round. Throw the ball straight at your friend and, by the time it gets there, they have rotated away. From where you sit, the ball seems to curve. Nothing pushed it sideways; the ground under it turned.

The Earth is a giant, slow merry-go-round. Air moving over long distances is deflected to the right in the Northern Hemisphere (and to the left in the Southern). This apparent sideways tug is the Coriolis effect. It is zero at the equator, strongest near the poles, and grows with the wind’s speed. It only matters for big, long-lasting motions such as weather systems. It is far too weak to decide which way your sink drains.

Friction near the ground

A kilometre or more above the ground, the pressure push and the Coriolis effect settle into a balance, and the wind ends up blowing along the isobars rather than across them. Near the surface, though, the air rubs over grass, crops, trees, buildings and hills. That friction slows it down. Slower wind means a weaker Coriolis tug, so the pressure push wins a little, and the wind cuts across the isobars at an angle, toward the low.

Put all three together and you get the familiar pattern from Lesson 3: air spiralling clockwise out of highs and counterclockwise into lows.

Gusts, turbulence and wind direction

Why wind is never steady

Wind near the ground is not a smooth river. It tumbles. As air flows over buildings, trees, fences and the edges of coulees, it breaks into swirling eddies of every size, like water churning around rocks in a stream. On top of that, faster wind from higher up keeps getting mixed down to the ground in bursts. The result is gusts: brief peaks, lasting only seconds, well above the average speed.

Sunshine makes this mixing stronger. On a sunny day the heated ground sets the lower air churning, which drags faster winds from aloft down to the surface. That is why windy days on the prairie often get gustier through the afternoon and ease off after sunset.

That is also why weather stations measure wind high above the ground. The international standard is 10 metres, clear of nearby obstacles, and the West Lethbridge Weather wind sensor sits on a 10-metre tower for exactly that reason. Down at shoulder height in a sheltered backyard, the wind can be much weaker and more erratic.

Wind direction is where it comes from

This trips up a lot of people. A west wind blows from the west, toward the east. A north wind comes from the north. Weather reports always name the direction the wind is coming from, because that tells you what kind of air is arriving: a north wind in January brings Arctic air, while a southwest wind in winter can mean a chinook.

Wind chill: what the wind does to you

Your body warms a thin layer of air right next to your skin. On a calm day that layer acts like a light blanket. Wind keeps stripping it away and replacing it with cold air, so your skin loses heat much faster. Wind chill puts a number on that: it tells you roughly what temperature on a calm day would make your skin lose heat as fast as it is losing it now.

Canada’s wind chill index is written like a temperature, but without the degree sign. A wind chill of minus 30 means your face will lose heat about as fast as it would on a calm day at minus 30°C.

Why Lethbridge is so windy

Lethbridge is often described as one of the windiest cities in Canada, and anyone who has wrestled a car door here would agree. Several ingredients line up:

• We sit in the westerlies. At our latitude, the main flow of air high above us usually runs from west to east, often at high speed.
• The Rockies are right next door. When strong westerly winds cross the mountains, the air can plunge down the eastern slopes in waves, bringing fast-moving air from aloft right down to the prairie.
• Pressure patterns pack tight here. Lows often form or deepen on the lee (east) side of the Rockies, squeezing the isobars together over southern Alberta.
• The land is open. Few trees and wide, flat prairie mean little friction to slow the wind once it gets going.