Archives For cold temperatures

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!


Some charts for next week show the Polar LOW which is heading our way for this weekend sitting on top of the UK and a HIGH building to the NORTH. This could drag in cool easterlies for some parts of the country again and keep Spring locked out. Look carefully at the chart which shows upper air pressure and temperatures at 500hpa: around 5000m (half way up through the atmosphere: a good place to predict air flows without the disruption of annoying surface features which complicate things).  It shows Russia warming up and Greenland distinctly spring-like but the UK stuck in a pool of cold air from N Scandinavia.  Details are not certain but don’t break open the beach-wear and bbqs just yet, looks like the Kangaroo Spring is set to continue with a hop back to winter yet again next week.


Has the sun set on the first and last day of Spring?!  Spring will return but certainly not for a while.  The rest of this week will be rather wet but mild. An octopus-like LOW sitting out west in the Atlantic will sweep several tentacle-like fronts across the country (count them = 8) and this will bring frequent, mostly light rain to Reigate, heavier rain does threaten on Thursday: check back for details. The big news is the temperature drop between Saturday (+11C) and Sunday (+2C!): this temperature plunge will feel bracing after our all-too brief flirt with Spring and the mild conditions this week. Over the weekend the LOW looks like it will track south of the UK and drag in amazingly chilly air from a BLOCKING HIGH developing over Scandinavia to the NE.  The HIGH will force the LOW to the south and east of the UK and we will pick up a NE / E air flow.  This air flow could stick around for much of next week.  By Tuesday things could be really cold in the upper atmosphere (1500m= -14C!). Down here in Reigate things will return to a wintry feel with hard frosts, cold temperatures around freezing and possible snow now and again. If the coldest upper air temps reach us then heavier snow from the E and NE is a possibility.  Tricky to tell this far out.  Brace yourselves and keep watching for updates!


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! 

Snow is very fussy. It requires particular conditions throughout the depth of the atmosphere to form. Any one of these conditions not met, and it will fall as sleet or rain. For example, snow requires particularly cold temperatures (obviously!) extending right the way up through the atmosphere to allow deposition of ice crystals instead of water droplets: indicator temperatures are lower than -27°C at 5500m, lower than at least -5°C at 1500m and surface temperatures at least below 2°C, so it doesn’t melt on the way down.  Also, a dew point below freezing is a pre-requisite for snow to fall. On Sunday and overnight to Monday not one of these conditions is met over Reigate, so snow in the current model forecast is not likely: 5000m temperatures are too warm (-25°C), at 1500m the temperature is a balmy +1°C and the dew point is +5°C through most of that period.  The only period when all the snow-forming conditions are met over Reigate (as things stand currently) is on Monday afternoon / evening.  Unfortunately, at that stage the LOW will have drifted off and taken much of the precipitation with it.  So… does this discount snowfall for Reigate over the weekend? Not at all, snow is still very possible because of nocturnal cooling and evaporative cooling and any slight change in the track of the LOW, but this is another story for later. Stay tuned!