Breaking down the Arctic Block begins early April?

March 23, 2013 — 3 Comments

Next week is a real chiller for the whole of Europe. Average temperatures are 6°C below normal for this time of year in the UK.  One reason for the continued cold spring weather is exceptionally high pressure over the North Pole compared to that in the mid-latitudes which is measured by an index called the Arctic Oscillation.  The HIGH pressure BLOCKS out milder air and causes Polar air to flood further south than usual, reaching Reigate on several occasions this winter (albeit modified and warmed up along its journey).

Polar air escapes in negative AO

Polar air escapes in negative AO

The Arctic Oscillation index is currently “strongly negative”.  This indicates much higher than normal pressure over the Arctic and a weak jetstream in low pressure further south than usual.  Air flows from HIGH to LOW pressure.  This means that cold Polar air can easily push out from the north unimpeded by weak SW winds: hence the cold weather brought by Polar easterlies reaching the UK.  The jetstream also “holds in” Polar air like a belt, but the belt has slipped down well to the south of the UK causing “pants” weather for the UK.  Usually the jet migrates north of the UK at this time of year bringing in milder SW air at the surface.
The question, of course, is not “if” but “when” exactly Spring weather will arrive. The maps and ensemble graph seem to suggest that next weekend could see initial improvements with milder SW air reaching the UK and a break down of the Polar block… let’s hope this proves to be accurate!

3 responses to Breaking down the Arctic Block begins early April?

  1. 

    As a new subscriber, sucked in by your twitter feed I have found the best fortnight ever to become one of your mailing list and am fascinated by your predictive and explanatory skills. Info is valid as Far East as Godstone!!

    Two comments: As a parent at the junior school this stuff is the best advert for RGS I’ve seen in a very long while. Shall I send Mr Fenton some fan mail? and

    I keep asking myself “why” when reading your reports. I’m glad I now know the this weather is all due to > >> >> exceptionally high pressure over the North Pole compared to that in the mid-latitudes >>

    But why do we have exceptionally high pressure over the North Pole?

    Is that being too knowledge greedy??

    Yours gratefully

    Piers Vacher.

    R P Vacher Iron Pear Tree House, Tilburstow Hill Road, Godstone, RH9 8NA +44 7598 927 705 +44 1342 893 443

  2. 

    Piers, thank you SO much for your great feedback, really appreciated! I have a small enthusiastic junior weather team at RGS and we are keen to improve understanding and passion for weather across the school & local area in these times of dramatic climate change when we can expect more extreme weather and ought to record it – hence this site which seems to have spread beyond the school gates which is great! High pressure at the Poles: cold air sinks over the Polar regions as part of the general planetary circulation, while warm air rises over the Tropics. Where cold air meets warm that’s where the trouble starts. Unfortunately, for every reason given in meteorology there is another “why?” … which is the fascinating thing about it. Fan mail would be fantastic to HM! Enjoy the snow, though technically it really shouldn’t be snwoing at all at the moment: upper air temps are far too warm, temps barely freezing at surface and dew points too high! weird goings on… Happy Easter. best regards, Simon

  3. 

    excellent work! clear, concise and up to date. Thankyou.

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