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Need help understanding pressure vs weather systems

  1. May 4, 2016 #1
    This is a subject that confuses me for a long time. Everywhere I go to get an understanding, I seem to run into the same explanations over and over which don't really do that much explaining. I find that only the basic elements of weather are explained, and overly basic examples that utilize them are used.

    So, I would love for people to read my introductory post, answer some questions, and from there I can probably ask some follow up questions as needed to get to the bottom of some things.

    I know that in the end, for rain to happen, the water in the air must condense into liquid from its vapor form..and fall down...There seem to be different ways that this can happen though.

    One common explanation here is, to imagine hot sunny days (and presumably clear...) where the ground ends up heating up a lot. The surrounding air warms up, rises, and starts to cool at one point to form clouds and rain. Presumably the surface air has more water in it than upper air, so the new air can last only so long at higher altitude before its water condenses out.

    (1) So how can you have a condition where it is really hot for days during the summer while the air is getting pumped into the sky, and yet, the skies remain practically clear?
    (2-a) On a related note, lets say that location did end up getting rain. It is said that rain is associated with low pressure systems. Is it because as an area warms up, the air expands, pressure drops (the air is not closed off), rises, condenses, rains..etc, meanwhile the area still somehow remains relatively warm therefore the pressure is low? (2-b) But when it rains it is generally cooler than it was before. How to reconcile this? (2-c) Why in that case do we have those high pressure systems which commonly bring in heat waves or warm weather in the summers?

    That's where I would like to begin. Thank you all :)
  2. jcsd
  3. May 4, 2016 #2


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    have a read of this from Wiki

    particularly the section on Humidity and Dew Point. This explains how and why invisible water vapour in the air condenses and forms clouds

    most of that will also be answered in my link ... but briefly On those hot summer days, the air pressure and temperature will be higher and therefore the air can hold much more water vapour before condensation occurs, hence high humidity conditions arise


    EDIT: I did a post not so long ago with all this info in it .... still trying to find it for you
    Last edited: May 4, 2016
  4. May 4, 2016 #3


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  5. May 5, 2016 #4
    Thanks. I will check these out today!
  6. May 14, 2016 #5

    jim hardy

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    Might it be so simple as....... ?

    What is density of moist air compared to dry air ? Answer lies in molecular weights and parftial pressures.
    Moist air is a mix of mostly nitrogen(mw=28), oxygen(mw=32), and water vapor(mw=18),
    When dewpoint is 80F (S Florida moderate but a Phoenix monsoon),
    saturation pressure is ½ psia , so air is about 0.5/14.7 = 3% water vapor.
    When dewpoint is 40F (S Florid unheard of but Phoenix moderate)
    saturation pressure is 1/8 psia so air is about 0.8% water vapor.
    A column of dry air two miles high weighs more than a moist one (at the same temperature).
    So the pressure at its bottom will be higher.
    To solidify the concept you should calculate by how much.
    Next -
    The moist column holds a lot of "heat of vaporization" that the dry one just doesn't possess. So it's more likely to be unstable and make rain clouds.

    Armed with that simple observation plus your gas laws from high school chemistry class,
    see if this writeup makes sense of terms "Lapse Rate " and "Lifting Condensation Level" and "Adiabatic Expansions" .
    http://homepage.smc.edu/morris_pete/physical/main/notes/pg-humidity.html [Broken]

    i hope this helps "the light switch on" for you.
    Last edited by a moderator: May 7, 2017
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