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Stuve diagrams, dry and wet adiabats

  1. Aug 21, 2012 #1
    I understand the idea that an unsaturated parcel of air will rise along the dry adiabat until the condensation level is reached, at which point the air is saturated with moisture. If it rises further, it will do so along the moist adiabat - but doesn't that mean at the same time that water should condense out to form clouds?
    If you apply the Stuve diagram to any typical summer weather, you get condensation levels typically of a few thousand metres. For example, a temperature of 32c and a dewpoint of 16c gives you a saturated mixing ratio of 30.5 and actual mixing ratio of 11.3, which puts the condensation level on the Stuve diagram at around 2000 m - but the reality is that the sky is cloudless.
    So, how can you have a cloudless sky in this situation?
    Thanks for any guidance.
     
  2. jcsd
  3. Aug 21, 2012 #2
    Yes the moist adiabat is determined by the latent heat released due to condensation at about 2500 joules per gram on top of the dry adiabat.


    That depends on the actual lapse rate at that time, if it's less than the dry adiobat, then there will be no convection. Also, typical high pressure stable weather has an subsidence inversion at a few thousand feet, your parcel of air may stop rising before condensation level is reached.
     
    Last edited: Aug 21, 2012
  4. Aug 21, 2012 #3

    Dotini

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    Gold Member

    The water molecules need to coalesce and clump together for the cloud to form, and not disperse randomly throughout the air. This process is facilitated by an hydrophilic surface or nucleating point which begins to align the dipoles. Since like-charged particles or molecules attract one another because of an intermediate of opposite charges, you will swiftly build your cloud.
    http://pubs.rsc.org/en/Content/ArticleLanding/2010/CP/c000729c
    http://faculty.washington.edu/ghp/research-themes/origin-of-life/

    Respectfully submitted,
    Steve
     
  5. Aug 22, 2012 #4
    Steve, Andre,
    thanks for your replies, but maybe I'm not explaining my question clearly enough. I am trying to understand how the explanation of the Stuve diagram tallies with real weather.
    I am supposed to take the temperature and RH and plot a dry adiabat up to the condensation level where the RH is 100%, then take the moist adiabat line that goes up at a constant 100% humidity. This is fine on paper, but surely the moist adiabat implicitly means that the water is condensing? If so, how can that be the case on a sunny day with no clouds? If there are no clouds then clearly the water is not condensing, so how can the lapse rate be governed by the moist adiabat?
     
  6. Aug 22, 2012 #5
    I think I can see your confusion. Stuve diagrams, along with (skew-T, Tephigrams and the rest) are normally plotted with Radiosonde balloon data. This is done every 12 hours all over the world and the real time data can be found here: http://weather.uwyo.edu/upperair/sounding.html . A radiosonde makes a whole set of temperature and humidity measurements as it rises up through the atmosphere.

    You can't figure out the weather from extrapolating one measurement of temperature and humidity at ground level, without knowing the actual temperature lapse rate of the atmosphere (which is what you get from a radiosonde). What you do is ask 'if air at ground level, were to rise, what would its temperature be relative to the real atmospheric temperature at each increment of altitude?'.

    If a dry air parcel rises, it will follow the dry adiabat initially. However if a radiosonde measures that the atmospheric temperature decreases more slowly than the dry adiabatic lapse rate, then any rising parcel of air will find itself more dense than its surroundings and will want to sink again. In this case the atmosphere is stable, and as Andre pointed out, convection is suppressed.

    So the crux of the matter is: the reason you see clear skies is because at some point any rising air finds itself cooler than the surrounding atmosphere and stops rising because it is more dense than its surroundings. This happens before it reaches saturation. Your confusion arises from your assumption that air must continue to rise. It doesn't have to.
     
  7. Aug 23, 2012 #6
    That's exactly what I tried to say but apparantly my wrap up was too brief. Sorry about that.

    Picture a Stuve diagram where the data of a radio sonde is plotted upon for the actual atmospheric conditions. So for a rising parcel of air at the surface with a certain temperature, with the dry adiabat, you would have to stay right of the radiosonde curve until dewpoint is reached, to have clouds. If you hit that curve before dewpoint, no clouds.
     
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