Archives For december

2016-01-01_12-04-44

Reigate December 2015 summary statistics:

  • Tmax 15.5C
  • Tmin 1.6C
  • Average temperature 10.1C warmest on record (2014 5.3C)
  • Total rainfall 79mm
  • max gust 37mph
  • sunshine 79.8 hours

Like the rest of England and Wales, Reigate had an exceptionally warm December and the warmest on record by a huge margin.

Central England Temperature December records smashed!

2015 warmest December CET ever recorded: 9.7C
December 1934, 1974: 8.1C

This is warmer than any March CET.
Warmer than any November barring 1994
There have been 171 colder Octobers
There have been 26 warmer Aprils
There have been 34 cooler Mays

The CET mean minimum was higher than any April mean minimum ever recorded.  It was higher than the May’s CET minimum

The gap between September and December’s CET was less than 3.0C, the gap between their respective minima was just 1.1C!

The CET mean maximum was 1.8C higher than the previous record of 1974.

(source CET data from UKWW)

In Reigate the average temperature was 10.1C compared to 5.3C, 5.8C and 6C in 2014, 2013 and 2012 respectively.  Whilst our own Reigate records only go back to 2012 (neighbour stations extend this locally back further to 2009), the Central England Temperature record extends back to 1772 and, at 9.8C monthly average, December 2015 smashed all previous mean monthly temperatures by a long way. The official Central England Temperature anomaly (temperature departure from normal long term average) came out at a whopping  5.2C above average, way above previous warm Decembers of 1934 and 1974.

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The chart above shows the monthly average data in Reigate this year as an anomaly departure from the long term average for SE England – (source MetOffice Hadobs 1910-present).  It starkly shows how much warmer December was from the long term average extending back to 1910, Reigate was a full 5.6C above normal!  How much is this down to El Nino? As discussed in previous posts the El Nino ENSO Pacific warming natural cycle cannot explain all of this extraordinary warmth.  In fact, there is little established link between El Nino and UK winter weather.  This is shown in the charts below for previous mega-El Ninos in 1982 and 1997 which had markedly different impacts on our December winter weather.  So there appears to be no real solid link between El Ninos and any crazy warm winter / December weather here.

Remember that the recent Paris climate change conference has sought to set a target of limiting global temperature rise to no more than 2C temperature increase over pre-industrial levels.  Locally we can expect spikes in temperature of course but this 5C anomaly for December, and the fact that 2015 will certainly be the warmest on record globally, shows the huge challenge that lies ahead in keeping temperatures down to less than 2C globally.

It was so warm for so long that wild flowers and plants around Reigate such as daffodils, camelias, forsythia and rhododendrons came into flower as if it was Spring.  This was a lot to do with minimum temperatures being so high with no frosts recorded at all.

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Many records were broken across the UK for highest minimums overnight, which frequently stayed in double figures. Indeed, this December has had a similar mean temperature to that expected in May. Our own station recorded the highest December Tmax at 15.5C on 19 December and the highest overnight Tmin at 12.4C on 26 Dec.2016-01-01_15-28-59

Despite the record breaking deluge further North which made this the UK’s wettest December of record overall), rainfall in Reigate came out around or even a tad below average at 79mm.

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Whilst 2014 was drier still at only 49.4mm, 2013 saw 110mm.  Regionally, South East England got around the average rainfall normally expected in December.  Like November 2015, December continued the sun-less theme with only 79.8 hours of sunshine.

2016-01-01_12-03-46

 

Apart from the incredible and perturbing warmth (which extended to the North Pole at the end of the month courtesy of Storm Frank) December 2015 was not remarkable for much else weatherwise locally.   Unfortunately, in Cumbria, Yorkshire and North Wales, there was extensive flooding which caused much misery and disruption especially over the Christmas period.  For more on the flooding and other weather stories please check this Flipboard magazine account.

Have a Happy New Year!  Data for Reigate and 2015 has been updated in the data page. 

http://www.metoffice.gov.uk/news/releases/archive/2015/early-dec-stats?WT.mc_id=Twitter_News_Pressrelease

http://www.getreading.co.uk/news/reading-berkshire-news/december-breaks-warm-weather-records-10673765

http://www.independent.co.uk/environment/uk-weather-why-the-recent-devastating-floods-will-become-the-new-normal-a6793291.html

https://xmetman.wordpress.com/2016/01/01/phenomenal-december-of-2015/

https://wansteadmeteo.wordpress.com/2016/01/01/december-2015-exceptionally-mild/

http://www.carbonbrief.org/analysis-how-december-2015-topped-chart-as-uks-wettest-month-on-record?utm_content=bufferf8440&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer

http://www.metoffice.gov.uk/news/releases/archive/2016/december-records

http://blog.metoffice.gov.uk/2015/12/31/whats-been-happening-to-our-weather/

Very warm conditions have dominated this December, especially in SE England due to a persistent warm SW sub-tropical airmass. Here in Reigate the mean monthly December temperature so far is over 10C, nearly 5C above the longer term average for the month (5.3C). Records for daily maximum and highest minimum temperatures have been tumbling as 850hPa airmass temperatures have risen 5-10C warmer than normal, making it feel more like May or even June than December, especially overnight!  In addition, the air arriving tonight is loaded with Saharan dust.

2015-12-16_16-45-01

Reigate temps December 9-16, long term av = 5.5C

Reigate daily temperatures have been consistently hitting double figures and recent nights have barely dropped below 10C.

It is the warmest start to December for years but will it break the all-time absolute Tmax record of 18.3C?

Oddly enough probably not, because the December absolute Tmax record was set in the lee of mountains in Scotland where, in the right conditions, a special foehn effect can lift temperatures beyond the reach of even the warmest airmass that we are likely to get in the next few days.  This airmass is cloudy too, so the sun is less likely to break through.

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Nevertheless, daily and local records here in the SE are tumbling and the duration of this warm spell is unusual. The cause is a stubborn high pressure over Europe and a trough in the Atlantic feeding a persistent SW airflow from the warm sub-tropics, places like the Azores.

2015-12-16_16-00-31

Azores: nice!

At this time of year the Saharan HIGH pressure is pretty strong and the southern edge of this creates winds that pour across the Sahara Desert from the east, heading across the continent to the Atlantic.

The Bodélé Depression in Chad is very likely to be the source of any dust arriving in the UK over the next few days.  Weather stations nearby have recorded windy conditions almost entirely from the east.

The Bodélé Depression produces more than half of all Saharan dust, partly due to the way super-geostrophic wind circulate around the Saharan high pressure and are funneled through a mountain barrier into the depression, accelerating wind which then lofts prodigious amounts of dust into the air.

Some 700,000 tonnes of dust can be lofted into the air every day in this location.  The result is some extraordinary rock formations due to the eroding effect of the sand blasting the rocks.

Desert sand tends to be lofted into the lower atmosphere, up to the boundary layer at most some 1km-2km or so above the surface but it can reach higher altitudes in some conditions and be transported further afield.  From Chad it is blown by trade winds into the Atlantic.  A lot of this dust continues across to America but, depending on conditions, some of it can be gathered into the mid-latitude circulation and make its way to Europe. Spot the journey of the dust on these sat pics. taken since late November.

Bodélé Depression

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21 November 2015 largely dust free Sahara

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dust storm early Dec

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Desert dust arrives Atlantic Ocean

By Thursday the desert dust is forecast to join more local particulate pollution in a warm sector to bump up pollution levels in the UK. It also coincides with exceptionally mild airmass. A breeze tomorrow should reduce the threat of a “toxic cloud” developing as stated in some media. Nevertheless, it is worth considering that desert dust is an entirely natural and vital part of the atmospheric circulation.

The outlook is for continued warm/mild weather to continue into Christmas, potentially more unsettled at times as the jetstream perks up and takes on a more direct zonal attack across the Atlantic.  The bigger reason for the mild mid-latitude weather is an exceptionally powerful polar vortex that is keeping pressure and temperature very low in the polar stratosphere.

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The lower tropospheric jetstream is subsequently strong and “locks in” the cold to the polar regions.  This situation is summarized by a strongly positive North Atlantic Oscillation: indicated by high pressure over the Azores and low over Iceland.  Until this situation changes the chances of sustained cold for us are slim.  The only hope for sustained cold this winter in a mega-El Nino year is said by experts in long range forecasting, to be a sudden stratospheric warming that will break down the polar vortex.  That can occurs most commonly Jan-Feb. Here’s hoping!

More on the importance of desert dust in the atmosphere can be found in the post here https://rgsweather.com/2015/04/14/dust/

http://blog.metoffice.gov.uk/2015/12/16/saharan-dust/

2015-01-01_14-19-40

Reigate December 2014 (2013)

  • Tmax 12.7C (11.4)
  • Tmin -2.9C (-1.9)
  • Average temp 5.3C (5.8)
  • Total rainfall 49.4mm (CoCoRaHs) (110mm)
  • Max gust 38 mph (47)
  • Total sunshine 100 hours  (92.8 hours)
  • No snow

For Reigate and the whole of SE England, December 2014 was drier, warmer and sunnier than the long-term average (1961-90), but was it less windy than December 2013?

2015-01-05_17-52-33

In Reigate and for much of Southern England a HIGH pressure moved over to create a cool calm frosty end to the year with more sunshine than usual with a total exceeding 100 hours (92 hours 2013).  Rainfall was also less than usual for December at only 49.4mm (SE Dec average 96mm).  This was despite two low pressure systems this month labelled as “bombs” in the media.  Notably there was no snowfall recorded for Reigate in December and it maintained the warmer-than average theme making 2014 the warmest year on record for England, in the longest running continuous temperature record stretching all the way back to 1772 (more on this later).

2013 was remembered for being extremely stormy for good reason as gales and rain swept the UK through the winter period causing floods and travel chaos.  However, was December 2014 really so much less windy? One storm worthy of note struck this month but the weather “bomb” on the 12 December hardly caused a stir in the SE, with most of the impact being restricted to the NW of the country. The diminutive December max wind gust in Reigate of 38 mph was caused by the non-bomb LOW that sank SE across the UK that introduced the cold final days to 2014, the so-called “arrival of the queen of freezer”!

2015-01-01_15-07-49 december 2013-14 highest av wind speed

The chart above shows the daily highest average (10 minute) wind speed for each day through December 2013 (red line) and 2014 (blue line) as measured in Reigate. The chart shows that December 2013 seems to have started less windy than 2014 but caught up and finished on a more consistently breezy note.  Nevertheless, the difference is perhaps less than might be imagined considering the stormy label given to December 2013, though of course Reigate never experienced the highest gusts nationally which were reserved for coastal areas.  Nevertheless, equally surprising is that there were 14 days when average winds exceeded 10 mph in 2014 while only 11 days exceeded 10mph in the supposedly windy 2013.  The data for 20 mph starts to show the difference between 2013 and 2014: only 1 day exceeded 20 mph in Dec 2014 while 2 days exceeded 20 mph in 2013.

The data with respect to max wind gusts also confounds the idea that Dec 2013 was windier than Dec 2014.  The average maximum wind gust in Dec 2014 was 23 mph whereas the average maximum wind gust in Dec 2013 was 22 mph, no significant difference.  Of course, this all hides the crucial MAX GUST data (peak wind gust in 24 hours): but this doesn’t help either much because there were 18 days with max gusts exceeding 20 mph in 2014 while only 15 days exceeded 20 mph in 2013.  It is left to the absolute value for maximum wind gust to distinguish the two years because it is only in this category that December 2013 markedly exceed values in 2014 with 2013 max gust being 47 mph and 2014 just 38 mph. So it is these maximum gusts that people remember and associate with “storminess” because they do the damage, even if they are only brief moments in more average wind events.  On all the other data December 2014 was windier than 2013!  This also shows, of course, how different weather data can be used to illustrate different angles on a story.

  • Max Gust 2013 47 mph
  • Max Gust 2014 38 mph

It could be concluded that the collective memory of “severe weather” is often down to a few key events that raise public awareness, more than the weather itself.  There might also be thresholds which hit the news and chime with our collective severe weather memory: flooding and snow being the obvious triggers.  December 2013 was as much to do with heavy rain as it was to do with gales, especially for the sheltered inland areas of SE England.  Rainfall this December has certainly been lower than the long term average. The chart below shows the December precipitation average as 75mm for England and Wales and just 56mm for South East England, December 2014 in Reigate is lower still at 50mm, half as much as 2013.

 

So December 2014  has been an unremarkable month for Reigate and SE England although it was nearly as windy as 2013,  but it didn’t quite hit the extremes that make headlines!

More widely December caught the imagination as freezing weather sank south across Europe and the USA where huge snowfalls were recorded in Buffalo NY and in parts of the Alps.  It snowed in Algeria while Iceland experienced balmy December days as warm as 15C.  In the SE we were stuck under a very HIGH pressure (record breaking 1044mb) that gave us the sparkling final days in 2014 with pleasant winter sunshine and some cold temperatures as low as -7C in Redhill airport.  In Reigate our lowest temperature for December of -3C shows how a town location can ameliorate extreme temperatures.  Happy New Year!

So far December weather for Reigate and across much of S England has been much the same for a while: breezy westerly winds and exceptionally mild on occasions with December temperatures breaking records in Reigate: the highest December temperature of 12.7C was recorded on Dec 18, the highest average and highest minimum were also records broken this month in the town. The December CFS chart for Bournemouth temps show how much time they have been above average this month so far.  Whilst it has been mostly relatively mild this month so far in the UK, large parts of the rest of the N Hemisphere has been even warmer with much of N America and parts of Russia being anomalously warm too.  On the other hand E Siberia has been very cold with Verkhoyansk falling to -50C at times.

Whilst a colder spell is expected to arrive soon after Christmas this will not be sufficiently extreme or long enough to move 2014 away from a record breaking warm year.  The Central England Temperature records go back before 1780 and 2014 looks almost certain to be the warmest ever recorded CET with an anomaly of +1.49C above the long term average.  This matches 2014 as a warm year globally, if not quite the very warmest.

So, apart from the rather underwhelming “bomb” depression earlier in the month, why has the weather been so mild, uneventful and unchanging for much of December, so far?  An immediate cause of our quieter weather this December is the jetstream being less active than last year.  On the charts below spot how much more active the 2013 jet was at the same time last year across the Atlantic.  A succession of Atlantic storms were driven headlong at the UK by a powerful jetstream which sat further south.  This year the jetstream has mostly been further north than the south of the UK and has been more meandering (meridional) and less powerful: hence fewer and less powerful storms for us in the south, most of the storms have tracked to the north of Scotland which has had an “ordinary” December of high winds and heavy rain!

2014-12-22_21-01-41

The reason for our milder than usual December is that much of the time our winds have arrived from a warm SW direction round a persistent HIGH over Europe.  We have had few incursions of Arctic northerlies, as yet.  Nevertheless, for parts of the month especially for the NW of the UK, there has been a sustained NW wind associated with high pressure over the Mid -Atlantic and LOW pressure over Iceland and this has pegged down our overall temperature anomaly to 1C above long term average.  This pattern of broadly westerly winds is called zonal and is indicated by a positive North Atlantic Oscillation (pressure low over Iceland and high over the Azores).

While the jetstream controls our synoptic scale pressure patterns, a more remote driver of our overall weather in the Atlantic mid-latitudes is the rather more geographically far-flung “Madden-Julian Oscillation”.  The MJO is a cycle or wave of weather disturbances (clusters of convective storms) that start in the Indian Ocean and move east through the tropical oceans into the Pacific and beyond.  It usually peters out across the Atlantic but can be observed to continue as an unbroken oscillation into the Indian Ocean where it starts another Phase 1 again. The MJO connects up with observed known weather patterns in the Atlantic and has therefore got an impact on UK weather, especially apparently, in years of weak or neutral ENSO (el-nino southern oscillation).  This year is currently a weak El Nino.  With the MJO in Phase 3-4 then a positive NAO is usually favoured but is not fixed of course, yet more drivers are likely to influence proceedings in the new year, not least “sudden stratospheric warming”.
MJO Phase 3-4 = +NAO

MJO Phase 3-4 = +NAO

DIY forecast for the possible storm 27-28 December

When upper winds (including the jetstream at 300-250hPa) match the direction of those at the surface, the weather tends not to change very much over quite long periods, like recently.

winds the same at height: no change

winds the same at height: no change

Wind direction and air masses can tell us a lot about how the weather will change and sometimes this is helpful when forecast changes are uncertain or simply fail to arrive when expected.  To an observer on the ground clouds at different heights can sometimes appear to be moving in different directions.  If on such occasions, the observer sees the clouds moving perpendicular to each other then a rapid change in weather is likely to be on the way, either good or bad!  The so-called “crossed-winds rule” is handy if you want to know how quickly expected weather events will arrive, especially if you are out and about on a day when expected changes are afoot but the timing is uncertain from forecasts.  This could prove handy later this week when a large storm might upset our run of benign conditions but the exact timing of fronts and events is uncertain, especially at this range!

2014-12-22_21-38-47

The storm, expected to intensify rapidly in the Mid-Atlantic during Boxing Day (yes, a possible rapid cyclogenesis bomb depression) could take a track modelled by the GFS that runs from the N of Scotland down through the N Sea. This SE track could pull in strong Arctic winds and, combined with high spring tides and high waves, this could cause a storm surge event for E coasts of the UK.  Note this is too far off to be certain at this stage.

Other models, such as the ECM and UKMO have different tracks that take the LOW on a less potentially damaging course.  Nevertheless, the models are worth watching because on some tracks the synoptic situation is similar to the 1953 storm which caused a lot of coastal flooding.  Of course, sea defences are much improved since 1953 so, even if the track was similar, it would be unlikely to cause anything like the same impacts.

compare 1953 storm surge with models 2014

compare 1953 storm surge with models 2014

It is also noticeable how similar the zonal pattern preceding the 1953 storm is to the current synoptic pattern: a strong positive NAO was present with both (see below).

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Back to the present… a weak cold front moving south during Christmas Eve will introduce a cool N/NW polar air flow across the country by Christmas Day. No snow is forecast for the SE with this cold front and it will be noticeably colder but brighter on Christmas Day.  A high pressure ridge on Boxing Day could start the day bright and frosty but, depending on the final track of the RaCy depression, things could go downhill rather quickly thereafter.  If you are out for a walk on Boxing Day then this is where the Crossed-Winds rule will come in handy to do some of your own forecasting!  Here’s how:

Bozing Day crossed-winds rule

Bozing Day crossed-winds rule

It will be necessary to see clouds at different heights – especially those wispy cirrus ones to check high altitude jetstream winds.  If it is completely overcast then forget this unless you happen to see breaks in the low cloud.

Find the surface wind direction: look at lower cumulus clouds or just local wind vanes or weather stations. It’s important to get the correct surface wind direction though so don’t rush this bit.

Stand with your back to the surface wind : the simple rule means that any low pressure will be on your left.  You can also tell how quickly weather will change for you locality thus…

Watch the direction of any cirrus clouds: check this with an aerial or static object.  If they can be seen to move at all then they will be shifting fast, because cirrus can be 10km up! The best cirrus for this is hooked mares tales or jetstream cirrus which appears in long impressive streaks along the core of the jetstream or similar upper winds. The more fuzzy edge is on the tropical warm side and the firm sharp edge to cirrus, if this can be spotted, is on the polar air side.

 

Now, the rule: stand with your back to the surface wind and if the cirrus (upper winds) come from the left then the weather will get worse.  The more perpendicular the winds (right angles) the more rapidly the weather will deteriorate.

If the surface and upper winds are going the same way, then there will be be no immediate change, or just very slow change.

If the upper winds come from the right then the weather will start to improve.

On the back edge of the storm, chilly Arctic air could bring down an active cold front with snow showers on the back edge into the SE, but more likely these will be restricted to NE/East coast.  It’s ages away so no details are possible but this storm looks one to watch.

Finally, the longer range outlook is rather uncertain!  Signals such as stratospheric warming, forecast to take place into early January, favour a cool January with the break-down of westerly flows into occasional cold Northerlies / easterlies.  In contrast, the MJO seems to favour more of the same unsettled westerlies for a while at least.

Quite possibly it will probably be a mix of the two!  This is suggested by lastest output by the CFS showing some zonal westerly weeks and some potential cold northerlies with the subtle position of HIGH and LOW pressure being critical to the outcome.  Lots of interest over the Christmas period and beyond, keep watching the weather!

 

December in Reigate 2013 summary stats:

A quick summary of an historically stormy and wet December in the SE.

Average temp 6c

Tmax 11.4c 16-17Dec

Tmin -2c 5 Dec

Max wind gust 39mph 27 Dec

Total rainfall 110mm of which 47mm fell 23-24 Dec (or 70mm from manual rain gauge)*

21 days with rain

Windy! Especially high up

Windy! Especially high up

For the RGS weather station there were some interesting developments regarding rainfall measurements which are worth sharing.  During the early storms in December, when wind speeds gust 35mph in Reigate, the VP2 rain gauge and ISS (integrated sensor suite), which houses all the thermometer instruments and the tipping bucket rain gauge, was blown over!  The standard tripod had been pegged down but the wind had pulled out the pegs on one side and rocked it over.  This was noticed remotely almost straight away by the lack of rainfall rate turning up whilst it was clearly raining outside! The unit was rescued within an hour, checked thoroughly and secured with guys with no adverse effects.

This prompted a comparison of rainfall between our VP2 and our standard rain gauge and some comparison checks with other neighbouring weather stations (including Kenley and Charlwood, two Metoffice “official” stations). Through studying the rainfall totals of several subsequent storms, It appears that the VP2 rain gauge is sometimes recording significantly less in comparison to our standard rain gauge, which is located a few feet away at ground level.  The most extreme example of this disparity was the 23-24 Dec storm when 70mm was recorded on the standard rain gauge at ground level, while the VP2 recorded 47mm.  This initially looks like the VP2 must be incorrectly calibrated because the standard rain gauge (collecting an “absolute” total rainfall amount in a graduated collecting jar rather than a 0.2 mm resolution of counted tipped buckets on the VP2) must be the more accurate… but hang-on! Our VP2 measurements match other local weather station and the rainfall rate correlates well with other local stations such as Kenley, Smallfield and Charlwood on a daily record resolution.  Such comparisons would never be exactly the same of course because rainfall varies a lot even across a small local area (see pics below), so relying on comparisons with other stations would certainly be incorrect in establishing the accuracy of our own figures. Nevertheless, it does at least appear we are all in agreement to a close degree regarding rain-rate trends as known storms progress across our region.

So rainfall is notoriously tricky to measure and all sorts of differences can arise between neighbouring gauges due to tiny differences including height above ground.  Height of rainfall instrumentation above the ground is especially important in turbulent gusty weather when rain can appear from all angles and get swept in all directions.  The VP2 rain collector is set at the standard thermometer height of 1.2m as it is attached as a single unit.  This means it might have been more susceptible to collecting less rainfall during the very stormy December weather due to being more exposed to the wind at over 1m.  In less gusty conditions the two gauges might agree far more, so a re-calibration at this stage would make our measurements of our more commonly occurring vertically descending rainfall less accurate!

Rainfall comparisons are on-going but evidence remains insufficient as yet to attempt a rushed re-calibration of the VP2 rain gauge immediately as it appears to be working well or, indeed, to go through any awkward resetting of the rain gauge to a location at ground level.  The conclusion is that …

  • Our rainfall measurements and trends in rainfall rate match our sister station in Reigate and rainfall rate follows the nearest official weather station at Kenley precisely enough to show broadly accurate rainfall measurements.
  • The VP2 rain gauge was calibrated last year and currently measures rainfall at 1.2m above the ground.
  • As more automatic rain gauges become the norm the height issue will be standardized.
  • Our standard manual rain gauge measures rainfall at ground level.
  • The two measure rainfall correctly but illustrate the vagaries of data recording and the caution we must use when quoting and comparing figures about the weather from different weather stations, even ones that are close together.
  • We will continue to use un-adjusted VP2 tipping bucket rain gauge readings for the time being but quote our standard rain gauge readings to use as comparison when appropriate and available.
  • A full calibration will be conducted in due course 🙂

Please let me know if you have had similar experience of rainfall data collection.

December will, of course be remembered for some extraordinary stormy weather, but with some spectacularly crystal sunny days in between and few frosts and no snow at all.  Previous posts cover flooding and some individuals storm events but here is a round-up of December pics from around Reigate as they happened.  Comparisons with last December will be analysed with the 2013 summary in due course.