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    johnholmes
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    Polar Lows

    Before you read this article I would suggest you read my article on Air Masses:

    Air Masses

    And the guide by @Thundery wintry showers on Uk winter setups

    Winter Setups

    Having read all that lot then I hope this fairly short article will help an understanding of Polar Lows.

    How they form and an idea of how to predict their track

    Below is a satellite picture of one, courtesy of Arctic Climatology and Meteorology. I think you will agree it looks most impressive. One can almost ‘see’ the snow showers falling under those large white clumps of cloud!

    post-847-1129824997_thumb.jpg

    A NOAA-9 polar orbiter satellite image (visible band) of a polar low over the Barents Sea on 27 February 1987. The southern tip of Spitsbergen is visible at the top of the image. The polar low is centered just north of the Norwegian coast. Image contributed by S. Businger, Department of Meteorology, University of Hawaii.

    A Polar Low is what is termed a mesoscale feature. By that it is a relatively small feature within the size of normal depression types. Covering hundreds of square kilometers not thousands as the main Polar Front Depressions do. It only occurs in winter time but otherwise has many characteristics of a ‘comma cloud’. This in a simple explanation is a mass of deep convective cloud which tends to organize itself into a comma shape. Its formation is as a ‘heat’ low! This may sound odd in mid winter but its the heat of the sea and the very cold air over the top that causes the Polar Low to form. It can even, on a satellite picture, give the appearance of a hurricane, not that it is of course. Although strong winds can occur in its vicinity producing blizzard conditions.

    Please forgive me if I keep things as simple as I possibly can. I do this in any teaching session. That way more people come to understand the basics and others can ask questions at the end of the item or read the various other links.

    Polar Lows tend to form, in the area of our interest, in a north or north west airflow. It is possible for it to occur in the Norwegian Sea in a north east flow but is less common. One area they seem to favour is just south of Iceland. Quite why is open to discussion. It may be that some kind of ‘lee’ effect is the first trigger. By that I mean air flowing over the mountains of Iceland then coming over markedly warmer sea to the south. The two combine and a polar low is formed. They will normally form in areas of relatively lightish winds, sometimes on the edge of a strong surface flow. Again some kind of lee or eddy effect may help this formation. Initially the cloud is all convective, Cumulus and Cumulonimbus, but as the feature develops middle and sometimes high level cloud will be created. As the feature moves further south its source of energy is the sea. Coming south it encounters increasingly warmer sea thus aiding more instability and more convective cloud along with all the moisture it needs from the sea. It usually, not always, tends to track in the overall direction of the isobars. Another thing to look at is the skew-T diagram and the winds at 5,000ft and 10,000 ft also give it some of its direction. Once near the British Isles it will have a tendency to try and stay over water whilst it can. So the entrance to the North Channel between Ireland and Scotland is quite a favourite. Trying to predict its track is not easy. But following the suggestions above will give good guidance. Another thing to look for are pressure falls, as with all lows, they tend to move in the direction of the largest fall of pressure. Indeed if it is possible, and this is not always so due to sparse data over the north east Atlantic. However, if it is possible then drawing the area of higher pressure rises behind it with the higher falls ahead of it, then joining this imaginary line can give quite a good idea of its steering over 3-6 hours.

    Will it snow or not? The age old question. So what, if we have a Polar Low approaching, are the things to look for.

    1) time of day

    2) link to 1) with temperature

    3) the dewpoint

    4) freezing level(0 deg isotherm)

    There are plenty more but these are readily available to anyone on Net Weather and will give a fair guide to what may happen. Look at its track, using the Met Office Fax charts as they come out. Make a rough diagram of the area between Iceland and the UK and plot its movement each 6 hours. (The map below may help). This will give a good indication of its likely track for the next

    3-6 hours or so. By which time the next Fax chart should be available. Watch out for your Net Wx forecast team issuing forecasts or Alerts!

    Below is a map, with acknowledgement which might help. It has the area needed, latitude and longitude and a scale. I hope it helps.

    post-847-1129825049_thumb.jpg

    A more technical definition by Rasmussen and Turner (2003) states:

    'A polar low is a small, but fairly intense maritime cyclone that forms poleward of the main baroclinic zone (the polar front or other major baroclinic zone). The horizontal scale of the polar low is approximately between 200 and 1000 kilometres and surface winds near or above gale force.'

    This shows how rough at sea it can be near a Polar Low.

    John Holmes

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