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Fog Predictions

Discussion in 'Alpine & Southern' started by champion_michael, Feb 21, 2007.

  1. champion_michael

    champion_michael First Runs

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    Hi All.

    What sort of systems tend to bring fog / low mists etc? I've seen some great photos taken in various locations from high peaks looking down into cloudy valleys, and i'm wondering how to tell if that's likely to happen.

    Also, as a general question, is there any way of telling/predicting cloud height from the good old MSLP charts?

    I'm off to tassie for some walking shortly and i've heard april has a reputation for fog/low mist...
     
  2. Falls expat

    Falls expat One of Us

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    Firstly you need to know that fog is just cloud at ground level.

    Secondly you will not be able to accurately forecast the conditions you seek from a MSL chart.

    If you are looking for fog on the ground then you need a combination of moist air near the surface with no cloud above. Heat is free to radiate out of the ground which cools the nearest layer of air. If the this air cools down to its dew point and stays there for some time then fog is likely to form. However, not many people realise that if you have totally still conditions only shallow fog will form unless the weather pattern remains unchanged for several days. What you need is a light wind which overturns the cooling air from the ground air interface which deepens the layer of air which reaches dew point. This results in thicker layer of fog which can obscure the sky. Once this occurs the heat radiation interface moves to the top of the fog layer where continual cooling deepens the fog layer further and it becomes almost self sustaining unless strong solar heating, a higher up cloud layer or stronger winds intervene.

    Now because of ground cooling this causes a layer of air near the surface which is cooler more dense air than that above it. This is an inversion which a layer or air that is trapped from moving upwards due to negative buoyancy forces caused by temperature rising with height rather than the norm of falling with height in the atmosphere.

    These inversions don't only occur near the ground they can happen higher up in the atmosphere as well. These most commonly occur due to high pressure systems which are areas of sinking air. When the air sinks it warms and dries out as the pressure increases towards the surface. This results in a warm layer of air overiding a colder layer nearer the surface again resulting in a stable atomspheric layer. However, the air withing the layer below the inversion can and is often unstable which means cloud can form from convection. This cloud rises within that layer until it reaches the inversion level and then spreads out and forms a flat layer of cloud most commonly strato-cumulus. In most high pressure situations this inversion layer is strong enough to cap any convection at low levels, however, in some situations where there is strong heating and lots of moisture below the inversion then the inversion can be cracked resulting in strong thunderstorms. This occurs in the USA and tornados are often a result.

    Getting back to your quest of taking photos from mountain top looking down on a layer of cloud. you would need the inversion level to be below the mountain top level where you would be standing in warm dry sunny weather, with damp cloudy weather below you.

    There is no guarantee that there will be cloud under the inversion as well as it may be too dry for cloud to form.

    However, you mentioned Tassie. I believe the best condtions for this would be after a front has passed with strong high pressure building it might be possible for a layer of air below the high pressure inversion to have cloud below mountain top but you would need to know the inversion height. The best place to do that is to look at upper air ascents which can be found in various places such as
    http://weather.uwyo.edu/upperair/sounding.html

    On these you look at what height the temperature rises with height and this is where the inversion lies.

    One last thing is the best time for fogs in the valleys or at ground level in in Autumn and early winter when you get calmer weather but lots of cooling as the sun moves away northwards.

    Sorry for the long winded reply, but once you get me started.....
     
    #2 Falls expat, Feb 21, 2007
    Last edited by a moderator: Nov 17, 2013
  3. Spiceman

    Spiceman Part of the Furniture

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    Very good explanation Falls... The Waikato is famous for it's fogs. I lived in Hamilton NZ for a year and it would often be foggy until 1pm in the afternoon, to the point where you couldn't see more than 20 feet away... All the conditions you mentioned spot on..