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Cold Undercuts Warm


crimsone

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Posted
  • Location: New Zealand
  • Location: New Zealand

    There is something I would like to know though in terms of understanding a very basic element of the current snow situation...

    To the best of my knowledge, cold air sinks while warm air rises. As a result, a cold airmass pushing against a warm one will undercut it (warm/cold - where the forwardslash is the boundry between masses). As the warm air (the bit carrying the ppn) rises, how doe this gets turned to snow? Somehow I doubt it's frozen as it falls to the ground, so I would assume it happens way up in those clouds. Also, if warm air is rising above cold air, isn't this principle conducive to convection of a sort?

    I may well have it all wrong anyway, hence the question. Following a model is one thing, but understanding the mechanics of it in real life is something else - a monkey can be trained to follow a model, but only someone who understands the mechanics too can forecast with it B)

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    [qute name=crimsone' date='11 Mar 2006, 03:00 PM' post='643209] There is something I would like to know though in terms of understanding a very basic element of the current snow situation...

    To the best of my knowledge, cold air sinks while warm air rises. As a result, a cold airmass pushing against a warm one will undercut it (warm/cold - where the forwardslash is the boundry between masses). As the warm air (the bit carrying the ppn) rises, how doe this gets turned to snow? Somehow I doubt it's frozen as it falls to the ground, so I would assume it happens way up in those clouds. Also, if warm air is rising above cold air, isn't this principle conducive to convection of a sort?

    I may well have it all wrong anyway, hence the question. Following a model is one thing, but understanding the mechanics of it in real life is something else - a monkey can be trained to follow a model, but only someone who understands the mechanics too can forecast with it B)

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    Posted
  • Location: New Zealand
  • Location: New Zealand

    Thanks John. Please note, That really should have been that a monkey could be trained to read a chart, not a model, and forgot to mention the bit about "parrot fashion" as to the method of reading and reporting on it. lol

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    Posted
  • Location: Coventry,Warwickshire
  • Location: Coventry,Warwickshire

    As warm air rises it begins to cool. When it cools it can not hold as much moisture so water droplets are condensed out. If the temperature is below freezing then it will form snow in the clouds. Once the amount of snow in the clouds gets too much it will fall to the ground.

    Cold air is denser than warm, but which is on top and which is below depend on which is moving and how long it has been moving and how quickly. Warm air moving in tends to form warm air moving over cold (warm front).Cold air moving in tends to try to just push the air ahead of it away,often undercutting the warmer air but sometimes low level drag slows the movement of air at low level. See ana and kata cold fronts.

    Convection is warm air rising, but the reason for it is different and the magnitude is different. Convection is not about warm air meeting cold but is air becoming unstable and buoyant. This causes air to rise at a very rapid rate, far quicker than frontal (air boundary) lifting, building clouds quickly.

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    Posted
  • Location: Worcestershire
  • Location: Worcestershire
    As warm air rises it begins to cool. When it cools it can not hold as much moisture so water droplets are condensed out. If the temperature is below freezing then it will form snow in the clouds. Once the amount of snow in the clouds gets too much it will fall to the ground.

    Cold air is denser than warm, but which is on top and which is below depend on which is moving and how long it has been moving and how quickly. Warm air moving in tends to form warm air moving over cold (warm front).Cold air moving in tends to try to just push the air ahead of it away,often undercutting the warmer air but sometimes low level drag slows the movement of air at low level. See ana and kata cold fronts.

    Convection is warm air rising, but the reason for it is different and the magnitude is different. Convection is not about warm air meeting cold but is air becoming unstable and buoyant. This causes air to rise at a very rapid rate, far quicker than frontal (air boundary) lifting, building clouds quickly.

    Just to add to a great explaination, CAPE refers to the energy available to a parcel of buoyent air (this is usually down to thermals in summer), so CAPE is basically the potential for convection. The greater the amount (measured in K/joules) the greater risk of convection. In summer when theres a high risk of thunderstorms the CAPE value in certian areas that have just the right conditions, is usually about 400+ in the U.K but can exceed 1000+ in the USA and other places.

    LI or (Lift Index) is also related to CAPE becuase LI is the trigger for convection which enables the Parcle of air access to the CAPE. Usually a good indiction of thunderstorms would be a CAPE of at least 300K/J and a LI of -4 (the negative meaning that there is a very high risk of the Cape being triggered).

    So CAPE on its own isnt a very good indicator of t-storms or convection, but you need together with it an LI value of at least -3.

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    • 2 months later...
    Posted
  • Location: New Zealand
  • Location: New Zealand

    Thankyou both VERY much for that. I seem to have missed it at the time, thinking that nobody had replied. I've seen it now though, and your explanations are absolutely fantastic! :D

    Thankyou :D

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    Posted
  • Location: Heswall, Wirral
  • Weather Preferences: Summer: warm, humid, thundery. Winter: mild, stormy, some snow.
  • Location: Heswall, Wirral

    It is also about relativity, the warm air above may be colder than the air below, but in terms of height, the 0C at 850hPa may be cold enough and have the right density to hold ice, whilst the warmer ground level temperature of 1C may help sustain wet snow with low dewpoints.

    the air at 850mb woulf be seen of a warmer airmass as usually perhaps you would need below -5C and 850mb to achieve 1C at ground level. The result is wet snow, and an example is the 12th March this year.

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