# what causes High or low pressure in air?

I have been under the (perhaps shaky) understanding that pressure is exerted equally in all directions in a fluid. Which is why I’m suddenly struck by the question of why the pressure should vary from place to place at the bottom of a large container of air - the earth’s atmosphere. If it were water, the pressure at any particular depth should be the same across a horizontal plane, shouldn’t it? I think that in large areas, a mass of colder, denser water may exert more pressure on the bottom than an equivalent volume and depth of warmer, less dense water. But why isn’t the pressure the same horizontally then? And, If that’s true, does the same thing happen in air? And what about the pressure underneath waves? of air or water? Whoooo. Remind me not to wonder about stuff I’m so hazy on. thanks. xo C.

Energy from the sun. This energy heats the air directly, as well as heating the land or sea masses on the earth’s surface. These media all have different heat capacities, and conduct and convect heat to the air, which undergoes a change in density as a result. Add to that the fact that the air and sea masses experience fairly strong currents which move the heat around (wind), and you get a system which is constantly striving to reach equilibrium, but can never quite get there as the input variables are always changing.

A high pressure center is a sinking column of air. The compression of air at the bottom, usually the surface of the earth, causes the higher pressure. As the air sinks, it gets warmer, and this suppresses the formation of clouds, so you usually have fair weather.

Since the air must go somewhere, it moves outward, in a clockwise circulating fashion.

A low pressure center is the opposite - the air in the center is rising, and new air rushes in to take its place at the bottom, spiraling in with a counterclockwise spin. The rising air condenses moisture, forming clouds, and so you get rain and storms.

But why does the air rise and fall? Upper and lower level currents, lateral vortices, hemispherical mixing, turbulence, solar heating, butterflies flapping their wings… it’s a complex, chaotic system. A computational meteorologist could explain specific mechanisms…

I hope one comes along. It would be fascinating.

Thank you for your input. However, regardless of the fact that my questions are not particularly articulately phrased, I can’t derive an answer from your responses. How can the pressure at the more or less level bottom of a container of fluid vary horizontally? How do vertically oriented areas of higher or lower pressure move through a fluid? I presume the density of the fluid does change, and therefore the pressure changes. But I just don’t understand what’s really at work here. Can anyone take a shot at trying to help me understand? I’m massively confused. C.

CC: it comes down to the definition of “fluid”. Briefly, air doesn’t behave like water. Liquids are near-as-dammit incompressible, so their fluid dynamics differ from gases, which are compressible.

I’ll take a stab at trying to clarify things for you. I get the impression that you are viewing the atmosphere in the same context as you might view a body of water. In the ocean, pressure at any given depth is proportional to the height of the water column above, since it is gravity acting on the mass of that water that creates the ambient pressure.

pressure § = density (rho) * gravity (g) * height (h)

The air in earth’s atmosphere behaves similarly, but only in a more generalized context. Air pressure does decrease with altitude, which is why you need supplemental oxygen at the summit of Everest, and why aircraft have maximum attainable altitudes.

The difference, however, between the behaviour of air and that of water, is that air, being a mixture of gases, is a compressible medium. Water has the same density at any depth, barring minor variations due to salinity and temperature. Air, on the other hand, exhibits a significant change in volume as it changes temperature. Air will change mass significantly as the water vapour content changes (warm air holds more moisture). Also, the particles which make up the air are moving very quickly, and their kinetic energy is large with respect to gravity. What this means is that compressible fluids like air are not as rapidly driven to equilibrium as an incompressible fluid such as seawater.

All of these factors boil down to the fact that, given enough time and without any external disturbances, the atmosphere would eventually reach an equilibrium wherein the horizontal pressure differential is zero, as you surmised. This, of course, never happens due to the constantly varying energy input.

Water is the same. If you stick a heating element in a glass of water, you can observe a convection current in the glass. The hot water near the element is less dense, and gets displaced vertically in favour of the surrounding cooler water.

If you imagine looking at the earth from space, you can see that the most intense sunlight falls around the equator, between the two tropics. This energy heats the earth, and the warmed air in the region ascends vertically as cooler air takes its place. This air has to go somewhere, and so spreads away from the equator until it has cooled and become less dense, at which point it falls to earth, creating a high pressure system. Ever heard of “the doldrums”? This is an area (or rather, a latitude) which experiences consistent high pressure due to this effect, and hence not any wind. The winds we experience on the earth’s surface are due to air moving from high pressure areas to low pressure areas in order to establish equilibrium.

This is the same principle behind the lava lamp (I don’t know why that example occurred to me, but I digress…) The wax in the lamp is heated, becomes less dense, and rises, until it cools and sinks. Air in the earth’s atmosphere is doing the same thing, only it also moves laterally, so it sinks in a different place from where it rose - resulting in lateral pressure differentials across the earth’s surface.

Clear as mud?

Well, the pressure is trying to equalize. That’s one of the reasons that we have wind. But the speed at which wind can equalize pressure isn’t fast enough to keep up with the other forces changing pressure – temperature and humidity.

Think of yourself as an ant at the bottom of a reasonably constant-depth swimming pool. If the surface of the pool is calm, you’ll measure the same pressure everywhere. If the surface is all roiled up, some parts of it are higher than others; under the high parts, you’ll measure higher pressure.

Of course, gravity will cause the high parts to move downward, and the low parts to fill in, so pressure will tend to equalize. But if there’s continual energy input, the roiling will be a more or less regular phenomenon.

Ok, so now I have a much better understanding of the dynamics of that particular fluid. What I had ignored was the difference between a liquid fluid and a gaseous fluid. I was also a bit unclear as to the nature of the pressure underneath the crest of a wave, but if Xema is correct, then directly under a wave crest the pressure at the (level) bottom is greater than that in surrounding areas - which are NOT directly underneath the crest. This had confused me, as I had thought that the pressure would have been transferred laterally, but I see that it would only be transferred a very tiny distance. At that point, a lesser pressure would exist because of a shorter column of water above that spot - a spot directly underneath a part of the wave that is not a crest. And, I presume, since the surface of our atmosphere is not a clearly defined mesa, but probably more like a wispy thinning swirling boiling roiling fluid, that there will also be places that effectively have more air under them and therefore exert more pressure on the ground underneath them. I thank one and all. C.