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February 28, 2015

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Reigate Grammar School, UK. Local weather station and forecasts for education.  Reporting global weather. SkyWarnUK storm spotter. RiverSearch River Mole. RMetS education committee. Town VP2 updates website every 10mins, wind every 5secs. CoCoRaHS manual rain gauge. Data to Met Office + Weather Underground.  Status: all good :-)

eclipse shadow

eclipse shadow

Sadly in Reigate and across the SE of England a blanket of thick cloud persisted throughout the eclipse period and we had no direct view of the amazing spectacle. Elsewhere in the UK views were mostly better, so we had about the worst possible situation: thick stratus and stratocumulus that stubbornly didn’t move until midday.  Nevertheless, effects of the eclipse were recorded and experienced. The afternoon was cloudless blue sky, so the weather played with us.  On the bright side, our student weather club eclipse forecast turned out spot on here! NewEx RESULTS: scroll down

cloudy in the SE

cloudy in the SE

Darkening skies: eyes adjusted to the fading daylight but the timelapse below shows the light fading as the camera exposure and shutter speed were taken off auto and set to manual. Spot the lights automatically coming on.

Cooling down: as solar radiation faded, temperatures locally fell a little but only by less than 0.5C or so.  As the eclipse was relatively early in the morning it seems that the usual climb in temperature was somewhat held back by the eclipse cooling.  Dew point, rather surprisingly, also dipped somewhat showing a somewhat drier atmosphere for a short period during the eclipse.

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Pressure change? Although we cannot directly experience this, pressure fell markedly towards the end of the eclipse period.  The overall forecast for the day was for pressure to fall… but was the pressure “held up” somewhat by cooling subsiding air from aloft?  Well, we won’t know for sure but the pronounced “pressure cliff” seems to nicely coincide with the maximum of the eclipse period.  The wind moderated somewhat through the eclipse which can been seen by the slight lowering of max wind gust speeds below.  Nationally, NewEx found little evidence of the “eclipse wind” (see below).

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Overall, the eclipse in the South East and for us in Reigate was spoilt by cloud and we didn’t get to see this rare event.  Nevertheless, there is some good meteorology that will come out of this, not least to investigate the influence of solar radiation on weather models.

another eclipse shadow

another eclipse shadow

National Eclipse Weather Experiment: summary of findings quoted from University of Reading Meteorology Department “StarGazing Team”.

“After the data had been uploaded, we collected together the observations from the different sites and averaged them. From combining the measurements from all the participants, these show a clear drop in temperature across the country.

Temperature

Temperature change

A reduction in cloud in central England during the eclipse is also apparent. This is a very interesting result for further analysis, and one which would be hard to obtain other than through the efforts of a disciplined group of distributed human observers such as yourselves.

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cloud cover change

This finding is therefore almost certainly unique to NEWEx. As you may have noticed, winds were mostly light across the country during the eclipse, which meant the circumstances were not well suited for detecting changes in the wind. The so-called “eclipse wind” unfortunately remains elusive, so more work will be needed on this.”

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wind change

more here from NewEx http://www.met.reading.ac.uk/outreach/newex_2015/index.html

Low cloud on a slack N/NE flow has sat over the SE overnight left by the weak Channel low.  This low cloud will clear during the morning, the question is will it clear by the eclipse maximum at 9.30am?  Low cloud can be seen on the satellite image below but the cloud mask image shows clearance occuring in Wales, parts of N England and E England.  This is behind a weakening frontal feature (no rain) which is slowing as it moves south. Our student forecast still holds true from yesterday.. so check our original BBC school report eclipse forecast here

Sat pics this morning suggest the NetWx charts are doing well with cloud forecasts. If this turns out to continue then we can expect a relatively prompt clearance of cloud between 9 – 10am. As cloud is likely to remain in some places locally it will be down to a lot of luck but regionally some in the SE, especially further NORTH should get a view of the eclipse between 9-10am. **as we know it stayed cloudy across Reigate and SE England during the eclipse and didn’t clear until midday. London had a better view so the clearer weather was never far away but took its time to arrive: better luck next time!** UPDATE ON IMPACTS OF ECLIPSE ON WEATHER FOLLOWING SOON

Watch out for subtle weather impacts such as an eclipse wind, changes in cloud formation and a slight dip in temperature during the event.  More on this from our weather students here 

Here is a reminder of the times and % cover of the sun during the eclipse.  Remember not to look directly at the sun at all.  Use a pin hole camera.

Most models currently forecast HIGH pressure ridging in from the west by Friday.  Update Sunday 00Z run: shows more agreement.  ECM has been out-performing other models recently so this might prove to be most accurate: high edges in from west, cool N/NE flow, then pressure falls by next weekend as HIGH regresses back west allowing spell of cool northerlies.

Latest charts show clearest view over Midlands/Wales/N England… SE is hanging on to lingering cloud from a shallow LOW passing south into France from N Sea, this will enhance cloudy N/NE winds through Thursday.  Cloud should be clearing from the North quite quickly during Friday morning so hopefully it’ll hurry up and so by 9.30am!  It could turn out to break quite suddenly with little high and medium cloud above to hinder views thereafter.

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more model agreement on high by Friday

If this forecast comes out then the weather looks reasonably good    iffy! (update Mon: dirty high : lots of cloud, especially in east nr North Sea coasts) for chances of catching a view of the partial eclipse passing over the UK. Latest chart for Friday am from UKMetOffice shows a band of cloud / light rain moving S/SE during the early morning over SE England.  Further west and north and away from east / SE coast currently looks more promising for seeing the eclipse.

metoffice cloud Friday eclipse 9am

UKmet weather cloud chart

The partial eclipse peaks around 9.30am for us in Reigate and the SE.  Details of local cloud cover are not reliable at this range especially circulating round a high pressure such as this.  So, if a view is important to you, continue to check professional forecast providers carefully throughout the week.  On Thursday our student RGS weather team will be putting together a video weather forecast for the eclipse for a BBC School Report so please check back then for details!

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Reminders: never ever look at the sun, even in an eclipse (check here for methods of viewing).  Also, without taking away any of the excitement and value of viewing and understanding this awesome spectacle, let’s check the comment from our expert friends at Meteorwatch so we know what to expect… :-)

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OK, back to the weather… the GFS chart below shows the current synoptic situation: a Scandinavian HIGH pressure is bringing cool, dry but mostly cloudy easterlies through this weekend.  A weakening front from the continent will bring cloudier wetter conditions on Sunday and cloud could linger on Monday as pressure ebbs away slightly.   A lighter southerly flow looks to warm and brighten things up by mid week and it should stay dry.

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Saturday 14 March synoptic : Scandinavian HIGH

By Friday, eclipse day, the HIGH over Scandinavia is forecast to ebb south and melt away to be replaced rather quickly by a LOW pressure over Scandinavia.  At the same time, the GFS shows the Azores HIGH building a ridge from the SW.  The GFS and GEM has this anticyclone building somewhere to the W/NW or even over the UK by Friday.  The exact position and strength of the HIGH will determine wind direction and cloud cover.

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Partial Eclipse day chart Friday 20 March

The ECM chart below puts the HIGH further west and introduces a purposeful northerly wind with a hint of a cold front moving south down the N Sea during Friday morning.  Though the ECM is on its own with this it is worth watching as this could be a spoiler for those wanting a view of the eclipse.

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cold front on ECM

The GFS Atlantic charts below show a ribbon of cloud stretching the length of the total eclipse path, roughly between Iceland and the Faroes and Svalbard.  This ribbon of cloud is associated with a humid SW air stream and frontal cloud ahead of low pressure nudging into the HIGH from Greenland.  If this HIGH pressure comes off as shown below then the UK could well be the best location to get a relatively cloud-free view of the partial eclipse.  Unfortunately the charts show that views along the path of totality through the Faroes might be less clear, but it’s early days and cloud forecast charts are notoriously flakey!

The position of the HIGH to the west / NW of the UK will bring in North or NE flow through the North Sea and into SE England.  Such a flow will be cool and places near North Sea facing coasts might risk low cloud off the sea or even fog early on.  Fog may not ruin a view of the eclipse and could add to the mystery but lifting low cloud might spoil things of course.  Eclipses can influence cloud cover and shape, so watch for any subtle changes to cloud as the eclipse occurs. Reports to NEWEx welcomed by Reading University Meteorology Department.

Temperatures you experience if out viewing the eclipse from 8-10am will vary locally depending on sunshine(!), cloud cover and fog and exposure to the N/NE breeze. Temperatures first thing are cool 2-3C but should climb to 8-9C, though a dip of 1-2C is expected during the eclipse.  Windchill is likely to be a factor near coasts and hills.

So: exact details tricky to pin down: but the outlook seems reasonable for seeing the eclipse in Reigate on Friday morning.  The risks will be if the N/NE wind picks up and drags in low cloud from the coast, which is quite possible; or if the ECM chart comes off that cold front pushing down the North Sea could be a spoiler too.  However, the overall pattern looks to be optimistic.  Updates nearer the time and please check back to view our student video forecast on Thursday!

2015-03-14_13-48-13

eclipse in the UK

Pleasant mild sunshine of late looks to give way to some cool drab easterly winds this weekend as high pressure builds over Scandinavia.

Easterlies this time of year are less cold than in winter as the continent warms under stronger Spring sunshine.  In any case, the continent has had a relatively mild winter so there is not much deep icy cold in our continental source regions so we cannot expect much real cold in this easterly episode.  Cool, drab cloudy skies are most likely … called anticyclonic gloom.  This is built when a large scale temperature inversion occurs: a cold cloudy air mass near the surface stays cold while above the air warms in sunshine and prevents convection going higher.  Low level convection, unable to rise, creates low cloud that spreads out as a thick low blanket of stratocumulus often with occasional drizzle. Most unimpressive!

2015-03-12_22-42-21

Further ahead there are hints of a possible colder mid/late March as some models suggest northern blocking: the Greenland high joining in with increasing pressure over the Pole and lower pressure blocked to the south.  This situation can allow polar air to escape into mid-latitudes given the right pressure pattern.   A forecast Arctic Oscillation (below)  diving steeply negative is also an indication that this might be on the cards.  Similarly there is a negative tilting North Atlantic Oscillation too.  Both these show indications of going negative.  The Madden-Julian Oscillation (below) is an indicator of the activity and location of  cyclical tropical convective wave systems that travel east along the Equator.  Different phases of the MJO have been connected with influencing different global weather patterns – including tenuously with European/Atlantic weather. The MJO is moving into Phase 7 and this has been correlated with northern blocking and a negative NAO, at least 50% of the time anyway!

Some of the models are hinting at the idea of some colder weather this month, but it’s a way off so not certain, worth watching though as a possibility of winters last gasp might catch people out.

 

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February 2015 Reigate weather summary

 

Weather statistics summary for Reigate during February 2015

Average temperature 3.9C (UK 3.5)

Tmax 10.8C 25/02

Tmin -3C 22/02

Total precipitation 59mm

max wind gust 32mph 06/02

February was around the long term average at 4C in Reigate.  The CET stood February at just 0.3C above the long term average.

After an intially cold start to the month the temperature rose.

Overall February came out around average temperature but note the much warmer continent.

2015-03-12_21-58-44

A notable cirrostratus halo occured around moon on 02 Feb and this accompanied by some brief excitement over a snow band moving south down the eastern side of the country.

This band delivered an insignificant snow flurry on 03 Feb, overnight, with  a cm or so of briefly lying snow that melted rapidly during the morning.

Models threatened easterly winds on occasion but this didn’t arise.  In any case Europe and the continent experienced a warmer than average month so the deep continental cold was not available.  In fact after another brief flirt with snow at the beginning of the month, the temperature climbed through the middle of the month with westerly influences with temperatures exceeding 10C overnight on occasions.

Rainfall 58mm was just a little below average and sunshine hours, at 90 hours, was about average.

 

 

 

MetOffice February summary

http://www.metoffice.gov.uk/climate/uk/summaries/2015/february

 

How high can we expect temperatures to go and how Spring-like is this weekend set to be in Reigate? Find out below!

This weekend and possibly for much of March, the weather is set to be dominated by HIGH pressure nearby to the south and low pressure to the NW which will bring in mild southerly or SW winds for Reigate.  Cooler and cloudy easterly wind directions are also possible later next week if the high pressure slips NE to Scandinavia which models are suggesting.  Whilst this weekend is expected to be pleasantly mild and spring-like and initially sunny on Saturday, temperatures are not going to break any Spring records because cloud cover is gradually going to spill from the north on a weakening cold front.

2015-03-06_20-08-11

synoptic chart Fri 6 March

The synoptic chart above shows the HIGH over the continent to the south of the UK and a deep low to the NW between Iceland and Greenland.  This is dragging in, with the help of a lively SW jetstream, a SW moist airflow over Scotland.  In fact NW Scotland has an amber warning for huge rainfall totals above 140mm over the next few days assoicated with the stalled cold front that will sit near or over Scotland for much of the time.  Warm air flow ahead of this cold front is advecting large amounts of moisture in a plume over the mountains which is causing the high totals over the NW. The charts below show the unsettled NW compared to the calm, mild and dry SE of the UK under the influence of the HIGH pressure.

The charts below show the story for this weekend.  Initially a dry airmass sits over the SE on Saturday morning but this is gradually replaced as cloudy conditions slip SE.  Temperatures through the weekend are looking mild, with Tmax 15C possible on Saturday, but anything higher is less likely on Sunday as cloud further thickens with the arrival of a weak cold front.  Saturday is probably the most pleasant day with brighter sunnier conditions especially in the morning.  The cold front in Scotland slowly migrates SE during the weekend but weakens as it does so.  By the time it reaches the SE on Sunday evening it is probably only going to bring low cloud and some drizzle.  Throughout the weekend wind in the SE is set to be light, especially on Sunday.  Misty conditions might occur overnight into Sunday and later Sunday evening in light winds.

Next week is looking generally mild and with HIGH pressure not far away to the south mostly dry.  A couple of LOW pressure systems are forecast to pass across the NW of the country and their trailing fronts will be weak in the SE but could bring cloud and some light rain.

2015-03-06_18-50-29

LOW passes to the NW on Monday

Overall the high pressure looks set to dominate Europe during next week bringing dry and warmer than normal conditions.

The GFS and ECM both suggest that the HIGH could slip over Scandinavia by the end of the week, as the chart below suggests.  This would introduce cooler easterly winds to the UK but nothing too icy at this time of year, it would also remain mostly dry. Unfortunately, easterly winds are often cloudy as they pick up moisture from the North Sea that creates days of anticyclonic gloom under an inversion.

2015-03-06_20-30-58

Scandinavian HIGH… would bring dry, cool but gloomy March weather

The charts below summarise the weather outlook: high pressure domintating bringing mostly dry and mild conditions.  Nights next week could turn colder with possible frost returning.  The longer range models suggest March could turn out to be a very dry month especially in the south.

BBC summary

http://www.bbc.co.uk/weather/feeds/31774369?ns_mchannel=social&ns_campaign=bbc_weather&ns_source=twitter&ns_linkname=news_central

 

A system stirred up by a low pressure tracking out of Mexico into the Gulf of Mexico this week is on track to bring unsettled conditions for part of the weekend to the UK.  It’s nothing too severe for most but is an interesting feature that will bring some wind and rain everywhere.  Below is a satpic showing the development of this system as it interacted with a lively jetstreak on Saturday 28/02/2015.

development of LOW on jetstreak

development of LOW on jetstreak

 

This LOW illustrates nicely how extra-tropical systems can rattle clean across the Atlantic in a few days if they are picked up by an active jetstream.   This one does precisely that.  Spot the system leaving Florida on the chart below for today and the sat pic.  This system started as a low pressure crossing from Mexico into the Gulf of Mexico mid-week, so for weather systems it will be an aged fellow on arrival here in the UK.  Its’ longevity is partly due to the exceptional COLD over NE US which interacts with the warm tropical air and causes further deepening.

The Gulf low pressure is tracking quickly NE skirting the US east coast before being picked up and deepened further by an active jetstream.  The jetstream itself is particularly powerful at the moment due to intense cold spilling out of an exceptionally wintry NE USA meeting warm tropical air issuing from a strong subtropical Azores HIGH pressure converging with the moist Gulf airmass.  A result of the powerful jetstream is a positive North Atlantic Oscillation: the NAO is a measure that indicates the difference in pressure between Iceland and the Azores.  In positive NAO conditions the jetstream is often active, producing a strong westerly zonal flow keeping Europe mild and unsettled especially in winter, or early spring!

Our Gulf LOW is due to pass over Reigate fairly rapidly through Saturday pm and overnight into Sunday am and bring some moderately wet and windy weather, likely to go unnoticed because of the nocturnal transit.  Winds gusting in excess of 40mph are possible for Reigate into Sunday am in exposed places.  Notably, due to the TROPICAL origins of this airmass the temperature overnight Sat-Sun could climb to double figures in Reigate.  Tropical air crossing the Atlantic also picks up a tremendous amount of moisture so attendant fronts are likely to bring a lot of rain too, possibly exceeding 10mm overnight, which is a moderately wet night.

Here are the synoptic charts showing the Gulf low progress across the Atlantic, deepening and occluding into the North Sea.  Note the secondary wave which could bring additional rain later on Sunday to the South.

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The additional rain later Sunday afternoon / evening looks potentially heavy for the South and SE.  It’s a rapidly developing wave feature that needs attention as, on the northern edge, it looks to raise the possibility of snow across the Midlands.  Heavy rain is possible for the SE and #Reigate with a period of gales on the south coast.

2015-02-26_22-44-28

The outlook for next week is for the Azores HIGH pressure to extend a ridge to the north and cause a NW then northerly airflow for the UK.  This will bring cooler temperatures to the UK.  Whilst it is likely to be mostly dry for Reigate and the SE with pressure rising, wintry showers especially on east facing coasts of the North Sea could be possible depending on how the HIGH develops.  Frost is likely with temperatures dipping below freezing at night from mid-week.

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Atlantic ridge builds to the west and brings in northerlies

How long this Atlantic block persists, and the cooler weather, is uncertain.  The coldest scenario would depend on the HIGH moving north and east and building over Scandinavia to pull in easterlies from a cold continent.  This scenario is preferred by the ECM by later next week whilst the GEFS topples the high to the SE and brings back a zonal mild westerly flow from the persistent Icelandic LOW pressure that erodes the edges of the HIGH from the NW.  The charts below show the uncertainty as a wide spread of possible pressure and temperature towards the end of the first week in March.

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Gulf low for the weekend, then pressure builds

The cool start to March is shown below.  The overall outlook is for a persistent positive NAO and Arctic Oscillation to persist and this would suggest a brief cool epsiode without the formation of a persistent Scandinavian High.  Models have flirted with possible easterly winds by the end of next week but the outlook is for the positive NAO to persist and this rather suggests a quick return to milder zonal westerlies.  As the high builds in early in the week various troughs and fronts could even push some wintry precipitation as far as the SE on Tuesday (spot the pink on the rainfall chart below for Tues)

First week of March starts cool: MetOffice synoptic chart for mid-week shows Azores high briefly ridging north to block mild zonal westerlies and usher in a cold polar airmass, albeit briefly as this ridge looks to topple SE and by next weekend we could be in a pretty mild SW flow hitting mid-teens possibly.  So a cool, mostly dry middle part of the week for Reigate and much of S England but precipitation, some even wintry, pulling in on NW / N winds is not ruled out with a North Sea low possible.  As the Atlantic is likely to push westerlies back in later in the week we can expect more purposeful frontal rain pushing east across the whole country.

2015-02-28_09-19-25

 

link to accuweather take on this system

http://www.accuweather.com/en/weather-news/winds-to-whip-uk-north-sea-coa/43063930

 

2015-02-08_22-03-51

HIGH pressure dominates but is it all calm?

High pressure is known for calm, clear conditions, with little wind, cold frosty and foggy nights especially when there is little cloud. Pretty unexciting weather.  However, HIGH pressure is not as unexciting as all that.  Anticyclones can sometimes be surprisingly windy especially round the edges.  We spend a lot of time learning about LOW pressure, with associated storms and gales and torrential rain but understanding the inner workings of HIGH pressure is important to get the full picture of mid-latitude weather.

So… buckle up for the ride and let’s get super-geostrophic!  Wind blows from HIGH pressure to LOW pressure.  The wind speed and direction is the result of two forces: the pressure gradient force (PGF) is the difference between high and low pressure and sets up the strength of the wind and the overall direction which is for winds to blow directly from HIGH to LOW pressure.  Coriolis force (or Coriolis Effect) is a result of the spin of the Earth and deflects resultant winds to the right of their intended path in the northern hemisphere.  Here are some video links to review these forces before proceeding with super and sub-geostrophic winds. Skip below these videos if you already know about PGF and Coriolis.

 

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The winds do blow from high to low… but get pushed to the right by that Coriolis fellow!

The pressure difference between high and low pressure determines the speed of wind.  Winds do blow from high to low due to the pressure-gradient but are deflected to the right by another force called the Coriolis effect! Below is a chart showing upper winds at 850hPa (1500m) blowing round the same HIGH pressure shown on the synoptic chart at the top of the post.  Note the relatively high wind speeds circulating round the HIGH in the north of Scotland, the North Sea and across France and Biscay especially.  Winds obviously blow faster across the ocean but remember this is an upper wind chart so is above the boundary layer of most frictional forces upsetting the wind.  In any case, none of these locations is associated with a trough… it is all anticyclonic super-geostrophic wind.  So why is the wind blowing so strong when there is no LOW for miles?

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Given the same isobar spacing the wind speed aloft round high pressure ridges is often greater than the wind flowing around troughs and low pressure. This is surprising because we associate gales and windy weather with “storms” and low pressure systems.  The chart above illustrates super-geostrophic winds circulating around the Azores high across Europe.  These look pretty strong at 850hPa (1500m), the level above frictional effects of the surface.  The chart also shows the trough of low pressure over the Mediterranean where, given some of the locations with similarly spaced and even tighter isobars, the wind strength is not especially any greater and perhaps even less than that circulating freely around the HIGH.

hilo2015-02-08_22-26-30

Wind is a result of pressure differences across the planet surface.  Wind wants to blow from high to low pressure.  This is called the pressure gradient force.  Due to the spin of the Earth winds in the northern hemisphere are deflected to the right of their intended path.  The two forces, pressure gradient and coriolis force, actually balance out to produce a theoretical wind that flows parallel to the isobars called the geostrophic wind, shown above. Unfortunately, isobars are almost always curved so the geostrophic wind hardly ever actually blows.

subg2015-02-08_22-18-44

Assuming a constant isobar spacing.  Around troughs of LOW pressure the wind is sub-geostrophic. This means it blows less than the expected geostrophic wind.  In the chart above the wind is shown as a black arrow.  In addition to the coriolis force, the centrifugal force acts to “push” the wind away from the low centre and is acting in the same direction as the coriolis force.  Note that the resultant wind is pointing slightly away from the LOW towards the HIGH, which is of course not possible because the wind would be moving into and against increasing pressure.  As the pressure gradient force cannot change, the coriolis force must weaken to allow the wind to return parallel to the isobars.  This means that the wind flowing around troughs of LOW pressure has reduced force acting on them given the same isobar spacing of a similar HIGH. These winds therefore blow slower than geostrophic wind and are called SUB-GEOSTROPHIC.

superg2015-02-08_22-18-44

Here is the HIGH pressure situation.  This time the centrifugal force is acting with the pressure gradient force to push the wind into low pressure.  As the pressure gradient cannot change the coriolis force must INCREASE to pull the wind back parallel to the isobars.  This means that the wind flowing around ridges of HIGH pressure has GREATER forces acting upon them than winds flowing round lows with equivalent isobar spacing.  These winds therefore blow faster than geostrophic wind and are called SUPER-GEOSTROPHIC.

Usually, of course, low pressure cyclones and depressions exhibit tighter isobar spacing than HIGH pressure and so resulting wind speeds round LOWS are most frequently higher than the HIGH pressure feeding them.  Nevertheless, assuming the same pressure-gradient force, winds exiting anticyclones can produce higher wind speeds than those entering depressions.

 

useful reference

http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/fw/grad.rxml

http://www.metoffice.gov.uk/learning/wind/what-causes-wind

http://education-portal.com/academy/lesson/factors-that-affect-wind-pressure-gradient-forces-coriolis-effect-friction.html

 

2015-02-04_18-44-20

January 2015 summary for Reigate, Surrey

Average temp 4.5C (UK av 3.7C, CET 1C above long term average)

Tmax 14.7C (12.2C 2014)

Tmin -4.9C (-1.5C 2014)

Total precipitation 69mm  (183mm 2014)

Max wind gust 42mph (52mph Jan 2014)

snowfall was recorded on 3 days, snow lying on 2 mornings

Total sunshine 96 hours (98hrs 2015)

2015-02-04_18-50-532015-02-04_18-50-11

January was mostly under the influence of an occasionally strong westerly and NW airstream with some stormy weather for the NW of the UK but, predictably, Reigate and the SE was sheltered from most of the weather action and our rainfall total was about on the long term average for the South East at 69mm. This was, of course, considerably less than the 183mm rainfall last January 2014!

2015-02-08_20-12-44

Overall, January was just about average temperature in the UK. This average hides the variation though… the start of January was extremely mild with some very warm Tmax temperatures early in the month reaching nearly 15C in Reigate.  The end of January was considerably colder than average although nothing extreme.

The end of January cold snap ran into the first week of February.  During this period Reigate saw temperatures drop modestly below average and we experienced 3 spells of modest wet snowfall, albeit lying snow from night time falls rapidly melted by early morning and the lowest temperatures we a mere -4C.  Reigate experienced one notable but brief “thundersnow” event on the afternoon of 29 Jan. This caused some local traffic disruption.

The cause of our swings in weather a neatly summarised in the mean monthly sea level pressure and 500hPa anomaly chart from the JMA.  Here you can see the building high pressure over the Azores towards Iceland and the low pressure to the North and NE nudging towards Scandinavia.  It is this configuration of building ridge in the Atlantic and LOW over Scandinavia that eventually brought our modified “Arctic blast” through the last week of January.  As is normal for an Arctic airmass the SE of England away from the North Sea coast of Kent rarely sees any prolonged snowfall and this was the case.  An interesting feature developed on the 29/01/2015 within the Arctic airmass: a polar low may have spun up and moved South through the Irish Sea.  This was controversial and not accepted as a true polar low by everyone but it seemed to have many of the characteristics.  A post on this polar low can be found here.

 

The evolution of the January 2015 cold snap was interesting because it was initiated by an unusually cold and unstable NW airstream on the back of a deep low that crossed Scotland.  The NW airstream was unstable enough to bring the thundersnow event to Reigate. Oddly the original NW “blast” from Greenland pushed through so quickly that a lot of polar maritime returning and tropical maritime air was secluded in the low core over Scandinavia.  It was this secluded / occluded warmer airmass that a) probably contributed to the polar low feature and b) modified the Arctic blast and , at least initially, made it much less cold than is usual for such an airmass direction.  It took several days for any truly cold air to reach the SE of England and, even then, 850hPa temps never fell below -6 or -7C.

The CET for January was nearly 1C above average for the long term average, quite a lot more than the MetOffice.  As can be seen from the chart below this January was not exceptionally warm, being moderated by the cold snap at the end of the month.  Only 6 of the last 20 years have come out colder than the long term CET average.

2015-02-08_21-09-03

Globally January was +0.35C above the long term (30 year) average temperature.  The northern hemisphere was +0.55C above the long term average.

2015-02-08_21-10-04

 

http://www.metoffice.gov.uk/climate/uk/summaries/2015/january/regional-values

http://www.metoffice.gov.uk/climate/uk/summaries/2015/january