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High and low pressure

  1. Feb 19, 2014 #1
    I am having trouble with differentiating between what seems to be two phenomena in the atmosphere. Maybe someone could comment on it. Here is an explanation of the problem.

    1) An air mass has an inherent pressure due to its temperature. Relatively cold air masses have relatively high atmospheric pressures associated with them because the denser air has more mass in it per cubic unit, causing any point on the surface of the earth to bear more weight from the gravitational pull on that mass. Relatively warm air masses have a relatively low atmospheric pressures associated with them because the air is relatively less dense and so there is less mass to be pulled by gravity against each square unit on the surface of the earth. (Correct me if this understanding is incorrect.)

    2) Low pressure areas also develop in areas of relatively warm air because the rising warm air causes a low pressure below it as it draws matter upward out of the space below it. I believe this is true at the equator among other places.

    Which of these would describe the pressure gradient that causes wind. It seems clear to me that the second one would be responsible for some kind of wind simply because air is drawn into the low pressure area that is formed by the rising warm air. This moving air would of course be wind.

    But I have read that air masses tend not to mix. (This non-mixing provides the opportunity for fronts and precipitation.) So, it does not seem that when speaking of air masses in terms of their inherent pressure as described in #1, that wind could flow from a cooler (higher pressure) air mass to a warmer (lower pressure) air mass without mixing going on.

    In both of these I know that I am ignoring the Coriolis effect and cyclonic action and all. I simply would like someone to point out if I am not understanding the matters of high and low pressure areas correctly.
  2. jcsd
  3. Feb 19, 2014 #2


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    1 & 2 are related. Warm air rises because there's colder air nearby waiting to fill the space. Start with warm air next to cold air, and it will tend to reconfigure itself so that the warm air is higher than the colder air because the heaviest thing wants to be on the bottom. If the air masses just rearrange themselves in this way, then mixing hasn't necessarily happened.
    Now put together ascending/descending air with rotation and you get all kinds of nifty winds. :)
  4. Feb 20, 2014 #3
    There are two different types of pressure.

    Hydrostatic Pressure : weight of the air column. If you heat air up, then air expands, and then density drops, this lifts itself, due to archemedian forces. However, that would meen a lack of air in the place where it was, and that lack is not immediately filled up, and that generates wind gradiants.

    This expansion of air would mean that the same airmass is spread over larger area, thus the weight of a column corresponding to the initial smaller area is dropping, and that is now what is called low pressure.

    Now you lift the air, and there is a momentum discnotinuity in the sense of low air density in the lower layers of the atmosphere, and regardless of the total air colum weight, the instruments in the lower altitude see a lower pressure as the weight of the air which has risen up is not immediately convyed to the lower layes, like a pascals hyraulic instrument, due to compressibility of air.

    Then this lack of density causes other air to move. You get wind.

    Besides this we have gas pressure, which increases with increasing temperature. see Boyle.
  5. Feb 20, 2014 #4
    Thanks, I get the picture.
  6. Feb 24, 2014 #5
    To quote Prof. Robert Fovell (UCLA)
    "Temperature differences cause pressure differences and pressure differences drive winds"
    As for mixing, you're right in that 'packets' of air tend to resist mixing, however they will still seek to even out pressure differences, whereby a cold air packet will push under a warm air packet, raising it up, so that any particular column of air is still trending towards equilibrium.
  7. Mar 1, 2014 #6
    Zamedy has the right of it. Warm air does not rise automatically. Nothing moves against the pull of gravity unless pushed by a stronger force. Warm air does not "draw in" cooler air. The cooler air displaces the warmer air, forcing it up. When the cooler air stops pushing, the warmer air stops rising.

    Winds are caused by many things besides pressure differences: differences in density under isobaric conditions, differences in temperature under the same conditions, changes of phase, etc.. Gravity winds almost always blow from areas of low pressure towards areas of higher pressure, as do all sinking air masses and the downward leg of every Hadley Cell.

    The free atmosphere is very complex. What happens in the free atmosphere and what happens in a closed laboratory container are two very different things. The equation PV=RT does not apply when the wind is blowing or when changes of phase are taking place.
    Last edited: Mar 1, 2014
  8. Mar 2, 2014 #7


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    High and low pressure are simply references to a reference pressure, typically 1 atm (1 bar) at sealevel.

    In the context being used, the local pressure is determined by the mass of atmosphere above. Gravity is pulling the air/gas down. The high pressure implies the mass (density) in the volume above is greater than some other location. Density is a function of temperature (and composition), and thus we may refer to 'warm' air being of lower pressure.

    Winds (flow) because at the same elevation, there is some mass of denser (cooler) air falling somewhere to displace the less dense (warmer) air. We talk of [natural] convection. Air mixes by diffusion, and if velocities are high enough, by turbulence/convection. Otherwise, air mixes by diffusion and heat is transferred by conduction or radiation.
  9. Mar 2, 2014 #8
    Atmospheric molecular number density is a function of air temperature and pressure:
    n = P/kBT. Number density is the key, not mass density. The acceleration of gravity is the same for atmospheric iodine molecules as it is for atmospheric hydrogen molecules.

    Warm air is not necessarily of lower pressure. Over the face of the Earth, there are many, many hot places that have higher surface pressures than many much colder places. The subtropical "highs" are good examples of these. By and large, surface temperatures and surface pressures are poorly correlated. This is particularly true when "weather" is occurring.

    Once insolation is absorbed by the atmosphere and the surface, advection (winds and ocean currents) is far more important in the global redistribution of heat than are conduction or radiation.
  10. Mar 3, 2014 #9

    Thanks. That helps a lot. I have heard it termed that the heating of the air at the equator causes the air to warm and rise causing a low pressure area at the equator. This is clearly not an exact explanation. In reality, it seems, the air at the equator is heated, reducing its density, causing a lower pressure than the air at the immediately higher latitudes and so the higher density and pressure air presses into the lower density and pressure air near the equator causing the air over the equator to rise. This is one thing that contributes to the Hadley cell circulation. (?)
  11. Mar 4, 2014 #10


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    I don't think it's necessary to go around saying this explanation is "wrong." There are rarely explanations in words that are "exact." The summarized results are indeed that heating at the equator causes air to warm and rise under the fully reasonable condition that there is un-warmed air next to the air, and when gravity pulls downward, the denser, un-warmed air wins.
  12. Mar 4, 2014 #11


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    In the atmosphere, or column of gas, ΔP = ρ g Δh, where ΔP is the differential pressure, ρ is the mass density (= m n, where m = molecular mass, n = number density), g is acceleration of gravity, and Δh is the differential of height.

    The integral of the mass density over the height above a given point gives the pressure.

    Ref: http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/barfor.html#c2

    Given a column (in a container) of oxygen and a column of nitrogen with the same number density connected in a circuit at top and bottom of the columns, the heavier oxygen would settle and the nitrogen would rise. Similar, a balloon filled with hydrogen or helium, with the same number density as the surrounding air would rise (due to buoyancy), which arises from the mass differential.

    Regarding advection vs convection, it is worthwhile to understand "Historically, these terms were used interchangeably and denote the macroscopic transport (or movement) of a fluid and its properties (temperature, salinity, oxygen, etc.) by the fluid's organized velocity field. Convection involes a transport of both mass and the property. common usage now is that advection is the horizontal transport of a fluid and its properties and convection is the vertical transport of a fluid and its properties." Ref: http://oceancurrents.rsmas.miami.edu/glossary.html
  13. Mar 4, 2014 #12
    Maybe a more exact way to say it is that the heating at the equator causes the air there to have a lower pressure which allows the higher pressure higher latitude air to push toward the equator displacing the lower pressure air and forcing it upward causing what we know as convection.
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