Science Questions

What is wind?

Thu, 16th May 2013

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Question

Rohit Patel asked:

 

I want to know, what is wind? Not the type you get from eating lots of baked beans(Ha! Ha!) but the one that we feel on our face and in the trees. I want to know whether it is all gases that make up our atmosphere or something else? Where does it come from or go and what is giving its energy/driving it? what is solar wind?

 

Await your response.

 

Many thanks,

 

Pradip Patel

Answer

Hannah - So, what is wind and why is it with us? For the answer, we turn to meteorologist and weather forecaster, Jim Bacon from Weather Quest at the University of East Anglia.

Jim - The wind is what you feel when the molecules of air start moving. The question is, what makes them move? And this is the point where you can get a clue by listening to Queen’s famous song, Under Pressure. Because it’s the pressure difference that makes all of these happen. The atmosphere generally doesn’t like things to be at a balance. It likes things all even. Just about any feature of the atmosphere would rather be in balance, well, if you go to a place where there's a lot of air above you, the air is denser, the pressure at the surface is higher. And where there's less air above you, the pressure is lower. This is an imbalance and it gives us areas of high and low pressure on the weather maps. And what the wind does is try to smooth things out by taking the air from where there's too much and moving it to where there's not so much. And the result of this is usually to reduce areas of high pressure and fill in areas of low pressure, if it quite achieves it. But the bigger the pressure difference that you start off with between high and low pressure, the quicker the wind wants to flow between them and therefore, you have a stronger wind. Well, it starts off as you might reasonably think, blowing straight from high to low pressure. But because the earth is spinning, other forces come into play, it gets a bit complicated, and in fact, it almost ends up flowing around the highs and the lows, but slightly towards the low. So, it’s always trying to fill in the lows. And you could see how fast the wind might be flowing by looking at a weather map because on there, you'll find lines of equal pressure - isobars - and when they're close together, this means there's a bigger pressure difference between the highs and the lows, and therefore, the winds are stronger. In other words, the molecules that make up the air are under pressure to move from areas of high to low pressure.  

Hannah - Thanks, Jim. So, it’s the Earth’s rotations and heat from sunlight that causes air density and pressure differences across the globe and this results in wind.

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A picture is worth a thousand words (and the thickness of the Earth's atmosphere shown below is greatly exaggerated in order to show the circulating air).



As you will read, wind doesn't merely go from here to there but basically goes around and around in various ways, and it is powered by the Sun!

Compared to the rest of the Earth, the equatorial regions (that is, near 0° latitude) receive the greatest amounts of the Sun's rays — specifically its thermal rays, which is why I did not call it sunlight. The equatorial regions then heat the air above them more than elsewhere, and this difference causes the air there to rise more than elsewhere. This causes a chain reaction that results in what scientists call Hadley Cells (see diagram above). The physics of everything involved here limits the length of Hadley Cells to about ±30° latitude.

The same difference in heating also occurs between about ±60° and the nearby poles, causing what scientists call Polar Cells. Between these two kinds of cells (that is, between ±30° and ±60°) are what scientists call Ferrel Cells, but they rotate in the opposite direction because the Hadley and Polar Cells have more effect on the air there than does the Sun's rays. If you think of all these cells as gears, then you can see why the Ferrel Cells turn opposite to what the Sun's energy would drive them to do.

As the air in these cells gain altitude, the physics involved causes the water in them to fall out, which causes more rain in the tropics than elsewhere. This also tends to make the air over the ±30° latitudes rather dry, and so little rain falls there, which in turn causes the major deserts to form in those regions (see diagram below). (Everything happens for a reason!)



Another physical phenomena, called the Coriolis Effect, causes the movement of air at the surface to deviate from their simple north-south directions and adds an east-west component to the direction of the winds in the various regions (see diagram below) — that is, adding an easterly component to the tropical and polar winds, which travel toward the direction of the equator, and a westerly component to the temperate winds, which travel in the direction away from the equator. Keep in mind that the names of winds indicate where they seem to be coming from, not going toward. So, for example, the northeast winds in the northern tropics come from the northeast and move toward the southwest.


So there you have the general global wind patterns. Our ability to predict the wind and weather is limited due to the complexity of all the physics involved, although computers — especially supercomputers — which can "crunch" more data more quickly than is humanly possible, now give us the ability to predict wind and weather more accurately several days into the future and beyond. Lmnre, Sun, 12th May 2013

Very nice description Lmnre. yor_on, Sat, 1st Jun 2013

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