Archives For Reigate weather

A little delayed but here is the summary of weather data from our VP2 weather station in Reigate, Surrey for the month of September.

This September in Reigate was characterised by a very warm start at 31c, a wet unsettled middle month and a warmer conclusion.

Rainfall was low at only 47.2mm (2012 64.4mm)

Average temp was a 14c (2012 13c).  This is similar to the “average” Met Office Central England temperature CET recorded for September at 13.7c.

February 2013 weather in Reigate. Highest temp 11.6C; lowest temp -2.5C; rainfall total 42.6mm.

Also winter 2012-2013 summary available above too.

FULL update of RGS weather data available on the DATA page here!

HIGH pressure means air sinks, warms up as it descends and dries out: this should mean less cloud… right? Unfortunately not always. Reigate has seen very little sunshine in the last few weeks despite a persistent HIGH pressure. Stratocumulus cloud and stratus cloud has hung low over the North Downs. This low cloud has been trapped by a temperature inversion at 1000m, where temperatures rise with height putting a “lid” on the weather. Any rising air will hit this inversion and convection will stop. Clouds forming beneath the inversion will simply spread out, join up and cover the sky: creating overcast dull gloomy conditions. Once the cloud forms it is difficult to shift because, whilst the sunshines above the cloud warming the air, the cold air persists below exacerbating the inversion. A stronger breeze or drier air or a shift in the position of the HIGH will release us from this weather underworld. The good news is the weekend looks brighter with winds less likely to be blowing from a cool, damp North Sea. Sunday looks the brightest with temperatures reaching 9 or 10C by the afternoon: almost spring like. The rest of the week from Wednesday will become more unsettled, feeling warmer but wetter as the HIGH at last moves away to the south east allowing Atlantic westerlies back for a breath of wet air!

To Spring… early forecast models suggest wetter conditions with slightly below average temps for the south of England with possible frequent easterly winds as LOWS track to the south; and drier, warmer and slightly above average conditions in the north of the country.  Essentially HIGH to the north and LOW / jet stream to the south of the country which will keep the south cooler and unsettled with the north enjoying better weather.  In short, more of what we had in late winter, just a warmer version?! Updates later.

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! 

The impending LOW approaching from the NW this weekend is a real teaser!  It will track SE across the country and the centre of the LOW will pass to to the SW of Reigate sometime over Sunday night. It will then “stall” for a day or two and drag in cool winds from the East.  Fronts associated with this LOW will bring every type of precipitation known…probably!  A weakening warm front slowly drifting east across the country on Saturday is not likely to reach Reigate until the evening. Saturday will be cloudy, calm and cool but hopefully remain dry. The leading edge of the warm front could bring some light snow by Saturday evening.  Sunday will be the BIG ONE… wet, cold and miserable, all day. North of London there could be heavy snow.  Reigate is on the VERY EDGE of the forecasted snow area (see map). Temperatures will remain above freezing at up to 3-4C, too warm for snow. However, despite this, heavy rain could turn to snow any time.  From Sunday afternoon onwards a process called evaporative cooling could cause snowfall. Some raindrops always evaporate on their descent from clouds, even in really cold air. Evaporation (turning a liquid into a gas) uses energy / heat (think of how chilly you feel stepping out the shower; the cause = evaporation!). Now imagine millions of tiny raindrops evaporating, for every evaporating raindrop some energy (heat) is taken out of the atmosphere and lowers the air temperature. In marginal snow conditions this temperature drop can be sufficient to turn HEAVY RAIN (the more raindrops the more cooling) into SNOW, even if surface temperatures are above freezing.  So, the question for Reigate this weekend is not “will it snow on Sunday?” but “will it rain hard enough to snow on Sunday?”  Enjoy the precipitation anticipation!

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!

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Thursday update: situation on Sunday – Tuesday is mostly GRIM, wet, miserable and cold but details are still uncertain regarding snowy-ness here in Reigate: could be lots or just sleet and rain mix.  Snow up north definitely but SMALL changes in location and speed of LOW centre and wind direction will make a BIG difference to what weather we get in Reigate. Stay tuned for latest! (nb re-reading my post here: “mild” means “relatively mild” at Tmax 4°C Sunday, Monday “cooler” means Tmax 2°C: so cold, wet and miserable right through from Sunday am!)

The Arctic wind built threatening, angry stratocumulus clouds over Reigate early today (pictured) and some built into perky sleet showers by the end of the day.  The rest of this week should be brighter, clearer, frosty and less windy but sadly no sign of Spring yet – models show cool conditions continuing next week. Light snow is possible in Reigate on a decaying occluded front over Thursday night.  The Arctic wind will shut down in the next few days as a HIGH pressure builds from the SW.  Significant snow is a possibility in the first part of next week anytime from Sunday night through to Tuesday for Reigate and SE England as winds swing Easterly associated with a trough slipping SE. It’s tricky to forecast whether it will be snow, sleet or rain and where exactly it will fall at this stage but ,despite this vague detail, models do agree that a SLIDER LOW will move down from the NW and bring, initially warmer southerlies and rain on Sunday, but then drag in some cool Easterlies which could cause a spell of snow, possibly for the rush hour on Monday.  Early next week will be cold and snow / sleet showers threaten anytime. It’s a tricky forecast because the temperature of these Easterly winds is now creeping up as Spring approaches and surface temperatures will hover at the upper threshold of snowfall (2° or 3°C) so that it could easily fall as sleet or rain. Places further east and higher up on the Downs are more likely to get snow rather than sleet: it is that close. Follow 1-10 on the slides to check the causes of this potential snowy event: it is quite different from our Skyfall last month. 

It looks like February could see a return to winter conditions for Reigate (2 small colour charts) and maybe even cold enough for some snow; though with the sun being that bit stronger it is unlikely to be as chilly as last week. The jetstream, currently right over the UK and bringing our stormy conditions and unseasonably high temperatures (12°C today at midday in Reigate), is likely to be shifted south during the second week of February. A blocking upper ridge in the Atlantic and building HIGH pressure over the North Pole will combine to swing winds round to a cool northerly direction over the UK. A LOW over Europe will add to the chill by bringing in periods of cold Easterly winds over the UK as well and it is these which often bring significant snow to the South East.
Meanwhile, this week’s extraordinary weather is not over yet: a deep depression (see big synoptic chart) is forecast to rapidly swing up out of the SW on Friday. Depending on it’s precise track over the UK it could bring heavy rain and strong winds or EVEN some snow. The models disagree by a critical hundred miles or so: MetOffice has the LOW tracking south of Reigate= easterly winds = snow; GFS has it tracking north of Reigate = westerly winds = rain. (Update: UK Met Office model is now on it’s own with snow in the south; all other models take the track of the LOW further north leaving strong SW gales and rain on Friday but no snow for Reigate – snow will fall further north across the UK; so the synoptic chart above is probably WRONG already!!).  In both scenarios the back of this LOW will see nippy northerly winds bringing temperatures right down on Saturday, but at least it should be bright.

An unusual winter pressure pattern is building in the early part of next week: a HIGH over Spain and NW Africa and deep LOWS over the Atlantic to the NW of the UK will combine to create what some meteorologists call the “blowtorch”. The relative position of HIGH and LOW pressure will funnel Tropical air from the Azores direct to Reigate possibly raising temperatures to 12°C on Tuesday (the big map shows temps for today, Saturday). This effect is sometimes called the “BLOWTORCH” (or hairdryer) because it can bring unseasonably high temperatures and melt snow rapidly across NW Europe (this blowtorch could reach as far as Russia where snowfall has been extreme). Reigate snow will be long gone by Tuesday but the warming effect could still be marked and a big contrast to a week ago when Tuesday temperatures fell to -6°C. Don’t break out the flip-flops just yet! Unfortunately, the blowtorch is unlikely to last more than a day or two at most and, in any case, it will be accompanied by rain and wind which might make it feel fairly unremarkable! Watch the video clip below which shows temperatures rising across the continent and the cold being beaten back as BLOWTORCH blasts through Europe!