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Gravity, pressure, Coriolis, friction = wind

The bigger the difference between areas of high and low pressure, the faster the air will move between them.

Most public schools get a couple of weeks off over Christmas and some private schools will give three weeks, but here at Weather School we’re so nice that we give over six weeks of holidays from school! The last time we were in school we discussed the heating difference between land and water, and were tying that into the overall energy balance of our planet.

It is this energy balance – or rather, lack of balance – that drives pretty much all of our weather. When we talk about weather it means different things in different parts of the world. Whether it is warm temperatures, rain, snow or icy cold temperatures, things are very different weather-wise depending just where you are on this great planet. While all these different weather factors change dramatically from one location to another, there is one constant aspect of weather that pretty much everyone on Earth has experienced no matter where they live, and that is wind.

So just what the heck is wind and, more importantly, what causes it to blow? When we talk about winds there are three main categories we can use to classify them. The first of these are primary winds that consist of the general circulation of the atmosphere (westerly winds, for example). The second category of wind is secondary winds. These are the winds associated with the movement of high-and low-pressure systems. The final category of wind is the tertiary winds which include such local winds as the chinooks.

OK, that’s great, there are three general categories of winds, but really, what actually causes the wind to blow? As with most things in our atmosphere there is a simple answer that explains the basics, but when you get right down to it, even the reason for wind blowing can get fairly complex. In this week’s article we will keep things basic and simply go into the four main controlling factors of the wind.

Driving forces

Before we do that, let’s first take a short pause and actually define what we mean by wind. After looking up literally a dozen different definitions of wind, the one that I thought was best was probably the simplest: wind is the horizontal movement of air across the Earth’s surface, with turbulence occasionally causing a vertical component to this movement. I knew that. Now, on to the four main driving forces behind wind.

The most important force that helps drive our winds is actually one of weakest forces in nature: gravity. If we didn’t have any gravity on Earth, then the atmosphere wouldn’t have any weight. Without weight, the atmosphere would not compress itself and there would be no increasing density as you get closer to the Earth’s surface. This would mean that without gravity, there would be no atmospheric pressure.

If you want to discount gravity and simply say it is a given and it shouldn’t be taken into account, then atmospheric pressure would be the top force driving the wind. Actually, it is the difference in atmospheric pressure that creates the wind, or what we call the pressure gradient force. In reality, this is really what drives our wind.

The best way to think about this is to picture two large pails. One pail is full of water, while the other is empty. If we were to connect the two pails with a hose, the water from the full pail would flow into the empty pail. Why? Because there is a pressure difference between the two pails. The bigger the pressure difference, the faster the water will flow. This same analogy holds true for our atmosphere: the bigger the difference between areas of high and low pressure, the faster the air will move between them. The faster the air flows, the greater the wind speed.

Well , that pretty much describes why we have winds, so why are there two other forces involved? Well, as we dig deeper into what drives winds we will discover that the Coriolis force complicates things by deflecting winds as they travel across the globe. Remember I pointed out there is a basic and a complex understanding of wind? Now we are getting into the complex area.

Finally, there is the insidious force called friction, the subtle force that slowly drains away energy from any and all systems. Even within the weather there is no getting away from this sinister force. OK, maybe I’m being a little too melodramatic; after all, without the force of friction we wouldn’t be able to shelter ourselves from the wind, and the weather as we know it would not be the same. The reason why this is will have to wait for our next lesson. Until then…

About the author

Co-operator contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.



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