Pressure waves in air, such as sound, are longitudinal waves, which means that the individual air molecules don’t move in the direction the wave is travelling - only the compression is propagating. Is the same true for wind? Or are, in the case of wind, the actual air molecules moving horizontally? If so, are the wind speeds that are commonly reported by meteorologists the speed by which these molecules are moving, or are they a different metric?
First off, in a longitudinal wave, the individual molecules are moving in the same direction as the wave as a whole (or in the opposite direction). They’re just not moving very far, before they collide with another molecule and start moving back the other way.
But to answer your question about wind, we have to take a step back. Even without wind, when we would say that the air is still, the individual molecules are still moving, and very, very quickly: The average speed (that is, without considering direction) of any individual molecule is greater than the speed of sound in the gas. All of these individual molecules moving quickly are what gives a gas its bulk properties like temperature and pressure. But these individual molecular motions are all in random directions, and so the average velocity (which does consider direction) is zero.
If there’s any bulk movement of the air, what we call wind, then you still have all of these rapid random motions of individual molecules, with the bulk movement of the air on top of that. So in the prevailing wind around here, there are still some molecules moving east, and some moving west (and some moving north and south and up and down, and everything in between), but on average, the ones going east are going a little bit faster than the ones going west, and there are a little bit more of the east-moving molecules than the west-moving ones. And so, on net, the air moves.
then, this is different from wave-propagation in water, where - AFAIU - the water molekules do NOT move with the wave, but pretty much stay put.
my guess: given that water cannot easily be compressed, the movement is passed on like the momentum of balls on a pool table …
but air-molecules have a lot more space to move
It’s not a wave, it’s a mass flow. A stream of water’s water molecules also move, even when the stream is in the same medium.
Depends if you are talking about the movement of a wave on the surface of water, or a wave inside a body of water.
Inside a body of water the waves are much the same as a sound wave in air - they are compression waves - and the mean location of each molecule is constant - but subject to a displacement due to the passage of the wave, and all imposed over the general random motion.
A surface wave is a different phenomenon. There is an interface and the wave propagates in a constrained form: it cannot propagate above the plane of the fluid, and tends to move as a surface effect, so long as the water is deep enough. There the motion of the molecules is in a roughly circular motion. The mean location is fixed, but as the wave propagates the molecules rise and fall, but also move forward and backwards. We see the rise and fall easily, but the forward and back is there as well.
A breaking wave displays the forward part of the wave. When the propagating wave reaches shallow water the circulating wave motion interacts with the bottom and the propagation slows, but the peak of the wave doesn’t slow, and it overtakes the body of the wave, eventually cresting and then breaking.
So, still an average fixed location for each particle, but a different propagation.
The water molecules don’t travel with the wave, but their movement (back and forth) is the the thing that causes the wave to propagate.
No. Wind is not a wave. It’s a current.
Currents happen in fluids, including in air, where they are often called ‘wind’, and in water, where they are often just called ‘current’ or ‘flow’
That’s a very comprehensive explanation, thank you. Just for clarification: Suppose a meteorological chart (say, an animated one like they have on TV) shows a high-pressure area in one place, a low-pressure one a thousand miles away, and wind blowing from the former to the latter. Then there is no flow of molecules that actually travel all the way along these thousand miles? I mean, possibly some are, simply because there’s so many of them and they move erratically anyway; but the bulk of the air molecules do not travel, but rather it’s more that lots of air molecules along the way shift, individually, a little closer towards the low-pressure area?
I think this is incorrect. The air will move that 1,000 miles, it may take 100 hours for the air to get there if the wind is at 10mph, but the air, the moisture, particulates, they all move. Granted, a lot of things will get caught up on the surface, or picked up from the surface along the way.
Let’s say an air current picks up moisture from the ocean, turns into a storm and hits Florida. The water molecule dropping as rain was picked up 1,000 miles away, and transported to Florida by the air. That water molecule didn’t travel 1,000 miles while the air sat mostly still, all the molecules travelled together.
Air movement such as wind is a bunch of molecules flowing, in bulk or as-one and aperiodically. The net movements (direction, velocity, volumes) are dependent on geometry, starting conditions and the energy source causing the movement.
By analogy, passengers move about inside a rolling train. Everyone is moving with the train along the tracks but also shuffling around within the train cars, maybe sitting in groups for a while, climbing up to the observation deck or laying down for a nap. These are little turbulances, eddies, microclimates in a weather system. On an individual basis, vibrating devices carried by passengers like phones or electric shavers also wiggle a little but do so within someone’s pocket which is traveling with the train.
The nose knows. When there’s a forest fire on the other side of the planet and you smell smoke, that’s because smoke molecules have, in fact, traveled all that distance. On average, molecules will have the same velocity as the air, though some will be faster, some slower, and some even in the wrong direction (those molecules that don’t exactly match the average are why smells also spread out in still air, though except at very close ranges, that’s very slow on average compared to wind speeds).
I think perhaps Schnitte is correct to think no flow of molecules travel all this way, for the specific reason that they propose local high and low pressure areas as the driver. A little air movement will relieve this difference, won’t it? The atmosphere sloshes around for various reasons, and in this specific example it might actually be fair to think of it as wave motion, albeit with a time frequency on the order of days. There are also long distance transport phenomena, such as air rising in the tropics and falling over the poles, or such as jet streams, or such as coriolis forces, but Schnitte’s example is specific here and may not actually transport molecules all the way from H to L.
Can anybody shed light on this particular point?