The Importance of High Pressure On the UK In Winter

19 Nov

Well autumn is nearly behind us and attention is most certainly being directed towards the start of December and the first of the winter months.  Clearly there has been a lot of talk about a cold start to the month and that seems increasingly likely.  One of the primary reasons for this is the potential development of high pressure (anticyclone) in and around the UK.  I have produced this blog to bring attention to the importance of high pressure across the UK through the winter and whilst this isn’t a forecast as such, may highlight some of my thoughts for the coming weeks.

As we all know high pressure brings more settled conditions than compared with low pressure.  Now from a UK’s perspective high pressure during the winter is important because it aids to bring the risk of colder conditions from a variety of sources.  Due to the location of the UK geographically and the way air masses move around high pressure, more often than not, higher pressure to the north of the UK in some shape or form, is the main ‘key’ to unlock the door to much colder conditions from the arctic.

Whilst this particular chart is at the far end of the GFS model’s time frame, it is still a good chart to use as an example to illustrate the above comments;

 

Of primary importance and significance here is the area of high pressure situated to the north-west of the UK.  The high pressure is extensive and extends from eastern Canada (1035mb) to Greenland (1030mb) and then all the way up towards Svalbard and into the arctic (1040mb).  Air masses around high pressure systems flow clockwise which is particularly important because if you get high pressure to the north-west of the UK for example, then in theory the air mass to the right of the high pressure would be from a north or north-easterly source, clearly of which is a cold source through winter.  The other important factor is the low pressure over Scandinavia here.  This is sort of acting to combine with the high pressure to allow a particularly cold region of air to move down across the UK from the north-east.

So this is one way that high pressure is important to the UK in winter, get the high pressure at a more northerly latitude to the north of the UK then the chances of colder weather certainly increase, but isn’t a certainty, then again what is in meteorology!?

The other important feature of high pressure and the UK is if the high pressure isn’t able to get a northerly location but becomes situated across the UK.  This type of pattern in winter brings little risk of wintry precipitation simply because the anticyclone is dominant across the UK and brings settled conditions, as would be expected.  However, what is important is something which is known as an inversion, which develops beneath areas of high pressure and this can still lead to particularly cold conditions at the surface, especially over a period of days.  I have explained this below in a graphic;

Air within a region of high pressure is sinking and warms as it does so until a point is often reached in the lower atmosphere where an inversion develops, where the temperature then suddenly cools (or warms if you’re rising up from the surface). What is important here is that beneath this inversion cold air can be come trapped, particularly after a period of days because as each overnight period comes and goes, as long as the skies are clear, then cooling takes place and over a period of days that results in a gradual drop in temperature. As an example, when the high pressure builds into the UK the initial maximum temperature may be 8C with temperatures falling to near or slightly below freezing at night (as long as skies are clear).  However, the net affect is for the temperature to steadily drop seeing the nights are longer than the days and if the high pressure persists for several days then the maximum temperatures within 4 or 5 days may be nothing higher than 3C or 4C and minimum temperatures fall more significantly below freezing.

So, there we have it.  High pressure is particularly important during the winter period and it would seem that given latest forecast data the end of the month and into early December will see high pressure become an increasingly important factor.

Regards to all,

M.

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2 Responses to “The Importance of High Pressure On the UK In Winter”

  1. Noodweer België November 20, 2012 at 8:37 am #

    Good and clean explanation!
    Thanks matt!

  2. Col November 20, 2012 at 9:36 am #

    Thanks Matt! Great post explaining how my favorite weather comes about!

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