Archives For wind chill

Here is a summary of the causes of the March 11 2013 “Channel Blizzard” which brought extra-ordinary “Spring” weather to SE England and the Channel Islands and N France. At RGS we had record low wind chill temperatures of -10°C at 9.30am, 36mph gusts and sub-zero temperatures all day.  Follow the numbers on the map to get a quick view of why it all happened!

1. Cold source region for Polar Air: the NE winds originated from the Polar regions with temperatures below -15°C and traveled across an extremely cold continent to reach the UK.

2. On their journey, the North Easterlies warmed a little over the North Sea (by now +4-5°C) which caused showers to form in unstable air (warming causes air to rise, clouds form and it snows). These showers formed lines called “snow streamers” which fed snow to the SE for most of the day and into Tuesday morning.

3. Much warmer SW winds at +10°C met the frigid cold Polar air mass somewhere over the Bay of Biscay but they didn’t mix well and they certainly didn’t get on!  In fact, the cold, dense polar air pushed the lighter, less dense tropical air right up off the ground, to over 4000m, where all the moisture condensed, formed cloud and snowed!  The high winds experienced across SE England were “squeezed” like toothpaste between the HIGH over Iceland the LOW over France (a high “pressure gradient”: look how close together the isobars are over SE England and the Channel!).

4. Next? Things will stay cold during mid-week as the UK remains firmly in Polar air and enjoys some dry weather courtesy of a HIGH over Iceland (unusual).  Clearer skies and frosts.  However, a Polar Low forming SE of Greenland at the moment is due to break through the Icelandic HIGH later in the week and bring unsettled conditions to the end of the week / weekend: as Polar air is still involved this may bring further snow and sleet.

Greenwich Lightship is a UK Met Office weather station in the middle of the English Channel. 3 metre waves, 50mph winds, sow and 10 foot waves were recorded.  Unbelievably horrendous conditions which the Channel Islands experienced as a blizzard.

Watch the satellite animation below and spot the storm winding up in the Bay of Biscay before it hits the Channel and says “Hello” to France and the UK!


Sunday final analysis update: crossing the Channel Monday?…be warned of v cold blizzard conditions!  LOW centre set to track thru N France most likely bringing only light snow to Reigate Monday morning but increasing risk of significant accumulation later in the day and overnight into Tuesday.  Any shift further north in the fronts could bring heavy snow to S counties pm.  Latest hi-resolution NMM model (below) shows light snow showers all day but risk increasing to heavy snow pm extending into southern counties of England, reaching Reigate late pm or evening….; UKMet Office predicts the least snow of all the models.V V tricky forecast.

So: Reigate forecast for Monday-Tuesday: emphasis on bitterly COLD, snowy day with extreme wind chill for (any) time of year with a low risk of heavier snow later and overnight to Tuesday, especially for places further South. so…accumulations 0 – 10cm by Tuesday am.

sunday 06Z NMM model

Cold weather alert! Reigate should prepare for an impressive weather change this weekend into next week.  The temperature will drop like a stone over Sunday night: shown by these Reigate maximum temperatures at midday on Saturday max +11ºC, Sunday max +5ºC, Monday max -1ºC with wind chill even as low as -18ºC (yes) possible in forecast 40 mph gusts in exposed places.  Winds will average 20 mph but even this will cause wind chill of -8ºC.  It will feel cold on Monday!  Light snowfall could start Sunday pm and continue overnight through to Monday. It may seem odd, but this snow will be caused by warm air from the South over-riding the frigid COLD Polar air blasting in from the NE: meeting at a front.  Check the temperature height diagrams to see how this works: warm, moist air circulating around the LOW in the Channel will be undercut by the dense freezing cold polar air from NE. The warm air forced to rise causes cooling, condensation, cloud formation and, as upper air temperatures are so cold… SNOW is likely to fall. Possible accumulations Sunday – Monday around Reigate could be 0-3cm (higher figures on hills) and more possible to the south of the region where precipitation could be more intense nearer the fronts.  So…positioning of the LOW is critical.  The snow maps give an idea of potential snow areas in the south on Monday.  There could be light snow showers on and off in Reigate and another moderate fall is possible Monday pm as the LOW tracks nearer to the south east of our region.  It will feel cold in strong gusts from the East: wind chills down to an outrageous -18ºC are theoretically possible if you are caught in strong easterly gusts with air temps below 0ºC all day. Temperatures will actually fall through the day on Monday as the Polar air takes a grip.  Lowest temperatures all winter are possible overnight Monday-Tuesday. Hopefully, this weather change will not take anyone by surprise: it has been on the cards for over a week and flagged up here on 2 March. The rest of the week will remain cold, but feel less cold in light winds, and drier with pressure building from the north, but more snow might precede rain later in the week as things warm a little next weekend.  More updates later as details emerge for Monday which is still v changeable!

Here is a table showing the effects of wind chill, familiar to mountaineers.  The blue zones are what Reigate and the SE has experienced in recent days with gusts of 15mph in freezing air taking “feels like” temperatures down to -13ºC.  Note that this is the zone of “maximum” danger for hypothermia because people tend to underestimate the effects of any breeze in temperatures hovering around freezing and under-dress.  Note that a slight breeze is enough to cause the greatest chilling effect.  These cool and breezy conditions are also more common in mid-latitude populated areas like the SE of England where winter temperatures frequently hover around 0ºC.  Dry air feels colder than humid / moist air: there’s more evaporative cooling into dry air and dry air also conducts heat away from your warm body more readily than does moist air.

Met Office article on wind chill


Wrap up!  Spring is cancelled for at least another week!  Mainly dry but some light snow showers and a really cold wind will be around through the weekend for Reigate, albeit mostly very light snow flurries and with little or no accumulation. Air temperatures will climb slowly above freezing to 3ºC during the day but a stronger breeze of 15mph, gusting 20mph, will make it feel even colder, especially Sunday with wind chill as low as -6ºC during the day. Night air temps will fall to -3ºC.  A tad warmer and damper by mid-week with daytime temps rising to +5C; the HIGH shifts directly over the north of the UK so winds, still easterly or northerly, will be lighter and track more across the North Sea rather than central Europe: this will feel balmy in comparison! 

A medley of pictures to show more interesting winter weather afoot!  Much talk of heavy snow on Friday as “warmer air” from the Atlantic is predicted to push in from the west. As warmer air from the west meets the dense cold continental air we expect precipitation to initially fall as snow on the leading edge of fronts and then, as warm air takes over, turn to rain … well, that’s the usual scenario. But hang-on!! Things might not be quite so text-book this time.
Charts are predicting cold windchill and moderate but persistent snowfall for Reigate from Friday pm through much of Saturday as a SLIDER LOW slips south east round the HIGH pressure and makes a wobbly attempt to bring milder air in. However, if the LOW moves South it will keep Reigate on the cold side of the fronts with a very chilly SE wind pulling in on Sunday. Probably no mild spell yet then. Charts for early next week also look distinctly wintry! Details will change but for the time being the jet stream is keeping firmly to the south of the UK, so Reigate will stay cool!

The NOAA satellite photo from Tuesday 15-Jan afternoon.