Explain the Wind to Me

Yeah, you read it right.

I have always assumed, without really thinking about it, that wind happens when a body of air is moving from one place to another. If pressed, I might have clarified this by saying that when you have a wind, you have a bunch of air molecules which in aggregate are moving together in some particular direction. (Of course not every air molecule is moving in that direction, but in aggregate, I figured, the average of all of their motions is a motion in the direction of the wind.)

It occurs to me this morning (as a result of reading the thread in the “Cecil’s Columns” board on sucking spaghetti) that I may have the wrong idea as to how wind works. An alternative that occurs to me is as follows: Wind happens when there is a mass of air which contains a pressure differential. No “sub-body” of air within this mass of air needs to be “moving” in order for there to be a wind–rather, the pressure differential causes it to be the case that non-air objects within the wind tend to be pushed in a particular direction depending on their position within the mass of air, and this creates the illusion of a mass of wind “moving” and thereby “pushing” the object around.

I don’t think these are exclusive explanations. (In fact, it’s not clear to me that the second explanation doesn’t end up implying that some masses of air within the major mass are in fact moving in aggregate, despite what I stipulate in the exposition of the explanation itself.) But I’m not sure whether just one of them is correct, or closer to correct, than the other. Or perhaps neither of these are correct.

Another thing I am confused about is the role of air density in all this. You can increase pressure without increasing density, by increasing temperature. (Right?) Are pressure differentials within bodies of air under normal circumstances due to temperature differences, density differences, or some of both?


That’s the way I always understood it. A cold front is the area of pressure differential between a high and low (or less high) pressure systems. Wind is the result of air masses rushing around as the pressure is being equalized between the two systems. Yes, bodies of air move around as you say, just as the body of air moves from the inside of a baloon to the outside when a hole is created. Nature abhoring a vacuum, and all that.

It’s either that or air elementals.

I agree that both explanations are about equally right. Wind is air in motion, normally masses of air much bigger than a breadbox. :wink: Pressure differential is a big explanation for why air would suddenly decide to move somewhere, since gases will tend to move to equalize differences in pressure. The pressure differentials themselves are often due to temperature conditions and other factors.

Complicating all of this on a worldwide scale, I believe, is the coriolis force of the spinning earth, which means that when a large high pressure zone tries to move into a low pressure space, it sometimes ‘misses’ slightly and pushes into the medium-pressure air at the edge of it.

Hmmph. Thought there was finally a thread I was an expert on, but, no, you guys want to talk about that kind of wind.

:slinks away:

I’d just like to add that they call the wind Mariah. :smiley:

You’re right, in that it’s both. The air only moves (on average, anyway, you’re right about it moving constantly in all directions) because of the pressure differential (how else could a gas be moved?)

It’s not one or the other. If you have air in a sealed, rigid container, you can indeed heat it to raise the pressure with net movement of air.

But a little chunk of the atmosphere isn’t in a sealed rigid container. It’s unconfined, and the only pressure it is subject to is from the weight of the air above it. If you heat up a mass of unconfined air, the pressure starts goes up, which immediately makes it expand into space formerly occupied by air at the original temp and pressure.

Now the hot air is less dense (same mass, more volume). The lower part of the hot air mass has the same volume of air pushing down on it as before, but less mass (the hot air is less dense), so it’s at a lower pressure. That means the cooler air on all sides will start moving in to this area, which is why the hot air rises (as demonstrated by hot-air balloons, etc.)

In the end, I think the major factor in wind can be pretty much boiled down to “hot air rises, cold air moves in to replace it”

>non-air objects within the wind tend to be pushed in a particular direction depending on their position within the mass of air, and this creates the illusion of a mass of wind “moving” and thereby “pushing” the object around

If I understand this statement right, no, this isn’t the case. Your first version is correct, and this is different.

It’s really the air moving around. If it moves vigorously enough it can catch visible objects and move them around some, too, but this is a minor issue as far as the wind is concerned.

If you build a campfire, it warms the air above it, which goes up (and which smoke particles and sparks illustrate for you). That air goes up because the same mass of air takes up more room when it’s hotter and at the same pressure - it’s lighter, or more accurately less dense - it goes up for the same reason that less dense bubbles go up in more dense water.

You can figure that the density of air is proportional to its absolute pressure and inversely proportional to its absolute temperature. Both of these numbers can change by 10% or more in the outdoors.

They speak of high pressure regions and low pressure regions. You can think in terms of these regions causing winds, accelerating air and keeping it moving dispite energy losses as it rushes over hills and through trees. But you can also picture the campfire, and think of lakes and oceans that are warmer than the land, or of mountains and valleys that have different temperature because they have different exposures to the sun. The pressure version and the other versions are different statements explaining the same underlying physical situation.

So, when the wind blows, the air molecules move. They also move around at the speed of sound on short little frictionless journeys, averaging about 70 nm, between random collisions with each other. But that’s a random motion that doesn’t transport anything (well, except heat in a thermal gradient). The motion of the wind is a systematic or average nonrandom motion that does transport things. For example, it transports goddam smog from Baltimore to my house, at just around the time it has aged to be most irritating. But I digress.

I agree with Napier that this is incorrect. When there’s wind, a mass of air is definitely moving - it’s no illusion.
An oversimplified but useful way to understand wind is to think of air as a mess of molecules, analagous to ping pong balls. These surround the earth in a reasonably thin layer (effectively about 20 miles thick, but more than half the balls are within 4 miles of the surface of the earth). If the top of that layer of balls were everywhere constant, there would be no wind.

But various things (the most important of which is uneven heating from the sun) cause the balls to slosh around - the layer gets thicker in some areas and thinner in others. Gravity then goes to work, causing the balls to tend to flow away from a thick area and toward a thin one (think downhill). That flow is called wind.