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June 27, 2014

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Reigate Grammar School weather station, Surrey, SE England. School run website for local weather info and amateur weather observations and educational posts on weather. SkyWarnUK storm spotting. RiverSearch monitor for River Mole, Surrey drainage basin. Forecasts for educational and local information purposes. Town location automatic VP2 wx updated to our own website pages every 15 minutes and “live” wind readings are updated every 5 seconds.  Data to UK Met Office weather observations website (WOW) and to US based Weather Underground (WU). 

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Quick June summary for weather in Reigate: data only this month.

Tmax 25.6c

Tmin 6.2c

Precipitation 30mm

Max Gust 21mph

Sunshine 175 hours

Analysis and more weather posts after the summer holidays.  Thanks for reading!

The “Spanish Plume” forecast to arrive from Friday and into Saturday is a special weather set-up for producing thunderstorms in the UK, as seen below.

2014-06-07_08-06-18

It is a rather complex weather pattern which needs some special ingredients to prime the atmosphere but also some fine tuning of local conditions to cook-up any big thunderstorms.  Like all thunderstorms, Spanish Plume storms are based on convection which means that thermals of air can rise uninhibited through the atmosphere creating tall deep cumulonimbus clouds.  Unfortunately, thunderstorm formation is notoriously difficult to forecast accurately, often being altered by local factors below the resolution of most forecast models or subtle changes in wind patterns which can subdue expected instability even up to the last moment.  Some places may see a tumultuous thunderstorm of epic proportions while other places not far away will see very little impressive action or nothing at all.  So an understanding of the plume might help figure out how much you can expect from any convective action forecast with a Spanish Plume event!

For a wonderful time lapse of the plume passing over the UK watch this… https://www.flickr.com/photos/tupperware_pilot/14185145588/

Read below to find out the ingredients for a Spanish Plume but, if you are in a rush, then read the BBC weather summary here (it’s a bit shorter!) :-)

The large scale “synoptic” ingredient that is priming the atmosphere for the possibility of storms is a moderately deep (for June) low pressure that has arrived from a cool NW direction, and will sit in the Atlantic to the SW of the UK and west of Spain.  This LOW pressure system has cool moist unstable Atlantic polar maritime air wrapped round numerous fronts. The low pressure system provides the main synoptic ingredient to kick off a Spanish Plume.  It spins winds anti-clockwise around the central low and this means that, if the LOW is deep enough (which this one is for June at an unseasonable 988mb) and located in just the right place, powerful upper winds driven by a lively jetstream will reach the UK from a warm southerly direction ahead of any fronts as the LOW inches towards the UK.  HIGH pressure over the Mediterranean will also help to push winds out of North Africa, over Spain, into France and finally reach Britain across the English Channel.  This southerly wind is what is known as a “plume”.  It is characteristically a dry airmass, often loaded with Saharan dust, that arrives in the UK at relatively high levels, above around 1000m, as it rides over local boundary layer weather nearer the surface.

By itself, a warm dry wind wafting up from Spain will not necessarily create any big bangs.  Other, smaller scale ingredients (herbs and spices if you like) are required to finally cook-up a good thunderstorm, with perhaps large hail, cg (cloud to ground) lightning and thunder and even funnel clouds or a tornado. Thunderstorm triggers are needed to push the air UP.  Thunderstorms require some critical ingredients: heat, moisture and lift. Additionally, convergence of winds, cool or warm water bodies (the Channel), orographic influences (hills) and wind shear and any nearby FRONTS can also contribute to the thunderstorm cook-up.

Perhaps the most important factor in thunderstorm formation is TIMING.  The precise arrival time and combination of unstable air masses, fronts and moisture is what makes or breaks storms. Any element that is premature or delayed can be the death of expected storm formation. The best example of this is whether unstable air arrives during a sunny warm day or at night, when surface based heating is absent.  In the case of the Spanish Plume event 7 June, the unstable air arrived at night over a comparatively stable boundary layer surface air mass.  The only storms that resulted were elevated thunderstorms embedded in the unstable upper air ahead of the cold front (which oddly brought warmer conditions later in the day as cloud cleared and sun came out but too late to coincide with the impressive instability of the plume).

Here is an overview of the other main ingredients as it applies to the Spanish Plume like the one on 7 June 2014 over the UK:

Heat! the SE of England is due to get warm or even hot on Saturday with Tmax temps well over 25c in sunny spots.  This will warm the air at the surface and, like a hot air balloon, these air “parcels” will want to rise (called lift). Warm air is also being moved into the country from the south by a process called advection.  Charts showing the heat energy providing the potential for air to rise are called CAPE: convective available potential energy.  In the UK we are pleased with CAPE values of 200-500j/Kg for some thunderstorms.  On Saturday we might expect values of up to 3000j/Kg. The other measure used to assess thunderstorm potential is the LIFTED INDEX (LI) this measures the difference in temperature between a parcel of air lifted to 5000m and the temperature of the air around it.  Negative figures show the air is buoyant and ready to rise.  Values of -8 or -9 are unusually low and show a very unstable airmass with the potential for plenty of lift!

In a Spanish Plume event the upper air contains enough energy and moisture to produce elevated thunderstorms even in the absence of surface heating: this means moderate thunderstorms can occur at night and with extensive cloud cover. The morning moderate storms experienced in the SE on Saturday 7 June were all elevated thunderstorms because extensive cloud cover throughout the morning meant an absence of surface based heating to kick off more purposeful convective activity.  Here’s an excellent blog explaining elevated thunderstorms compared to surface thunderstorms that would occur due to surface heating.

Moisture! local SE winds are set to be quite humid with a relatively high water content: high dew points illustrate this with some reaching 20c on Saturday… a muggy humid day. This moisture will be required, of course, to form clouds.  To form really big clouds you need a lot of water in the atmosphere.  Once water vapour starts to condense it releases latent heat and this heat gives additional lift to convection and feeds thunderstorm formation.  Interestingly and perhaps counter to what might be expected, a dry plume of air mid-way up through the atmosphere is also an important ingredient to the production of big thunderstorms.  Drier air at mid-levels aloft gives the atmosphere added instability for the production of thunderstorms.  The reason for this is that dry air cools more rapidly with height than moist air (because rising moist air releases latent heat when clouds inevitably form and this additional heat reduces the rate of cooling of saturated air with height).  The rate of cooling with height is called the “lapse rate” and the lapse rates or temperature gradient on Saturday is steep. So moisture is a critical ingredient to storm formation because it controls instability and cloud formation.

The skew-t charts show temperature change with altitude.  They are called Skew-t because the temperature lines are skewed off the vertical slightly.  Whilst they are initially odd to look at, focus on the red and blue and dashed lines: if the red and blue lines are close together it means the air is saturated (cloudy).  If they are far apart then the air is dry. If the red line skews to the right then this is known as an inversion where temperatures can increase or stay the same with height.  Such an inversion will prevent thermals rising and the formation of clouds from surface convection, a critical ingredient for big thunderstorms.

more analysis of Skew-t

more analysis of Skew-t

The other ingredient the skew-t chart shows above is plenty of wind shear with height, in this case speed shear.  Wind shear is the change of wind speed and/or direction with height.  Increasing wind speed with height has the effect of dragging air off the ground like a hoover. On Saturday there is a jetstream moving overhead during the day and this will cause divergence aloft and encourage more air to lift off the surface. Strong directional wind shear, when winds turn at angles through the atmosphere and when different winds directions meet around fronts, is also a trigger for tornadoes.  The UK quite regularly has tornadic conditions in lively convective thunderstorm weather but the ingredients for tornadoes are a even more fickle!  Our tornadoes are considerably less powerful than those in the USA but can be of great interest and still cause damage by uprooting trees, damaging rooves and chimneys and even tipping cars.

Lift! like a hot air balloon, a warm bubble of air will only rise if it is warmer than the air around it.  On Saturday morning a CAP will exist in the atmosphere that will prevent air lifting far off the surface, thus preventing extensive vertical lift. The cap is known as a temperature inversion and the cap on Saturday is pretty solid, so little convection is expected early on, except possibly in unstable upper atmospheric layers which might cause elevated thunderstorms.   During the day, if there are suitable breaks in cloud cover, the sun will heat the surface and this will start to break down the cap.  A strong cap has the effect of building energy and heat below and, if the surface heats up sufficiently then the cap can be broken suddenly.  This usually happens in the afternoon when a sudden explosive thunderstorm could be produced.  In the mid-west of the USA tornado chasers call this process “busting the cap” and it produces the possibility of extreme weather.  On charts the cap is shown as convective inhibition that acts against convection but, at the same time, can be an ingredient for extreme weather.  Another mechanism that is capable of lifting air rapidly is the proximity of fronts which can mechanically lift the air as different air masses converge.  On Saturday, a slow moving cold front is forecast to be located along a north/south axis through the Midlands and S England, moving gradually east… this could be the focus of most activity if the cap remains solid further east.

Convergence of winds! winds converging at the surface are another local or regional factor that enhances convection. Converging winds at the surface tend to slow down and pile up, like lorries slowing down uphill and causing congestion.  When surface winds pile into each other (converge) they can only go one way… UP!  Convergence often occurs at the coast where winds coming from the sea slow down due to friction over the land.  This is one reason why the south coast often produces thunderstorms or enhances them as they cross the Channel (so long as the Channel sea surface temperature is warm enough – another story!). The other map below shows areas where there is a sufficient combination of wind shear, heating and energy to possibly start to rotate a thunderstorm: rotating thunderstorms are called supercells and are capable of producing tornadoes.  These are the “big daddy” of thunderstorm cells and would be an awesome site for anyone able to catch a photo before or after the inevitable heavy rain, hail, lightning, darkening skies and thunder!

So the action of winds converging at the surface and diverging aloft or doing the reverse, is important to thunderstorm activity. The charts below (website ) are a cross-section through the atmosphere from S-N and W-E across the UK before and during the plume.  The streamlines showing wind vectors illustrate the turbulent nature of the air during the plume.  There are elements of both lift and subsiding air at different elevations showing the complexity of the plume event.

Even with so many ingredients primed for thundery weather here in Reigate on Saturday it still remains a risk potential rather than a certainty.  Local factors might inhibit convection, too much cloud in the morning could reduce surface heating and the cap may not be broken, so no thunderstorms! It’s a matter of waiting to see the conditions nearer the time to make a final forecast and often it remains uncertain until the very day.  Nevertheless, there is a possibility of explosive convective thunderstorm action on Saturday, mostly in the Midlands and the East, if the CAP is BUST!  If this occurs then instability of the atmosphere will rapidly build extremely tall cumulonimbus clouds up to 10,000 metres tall.  The UKMO issued an uncharacteristically early warning of heavy rain for Saturday on the basis of this Spanish Plume event. Others will doubtless follow, but it is safe to ignore the daft scaremongering of the Daily Express but do keep a watchful eye on any dark clouds!

Although an accurate forecast is beyond the scope of this post and best left to the professionals, one broad indicator of possible storm pattern over the course of the day is set by convergence of winds and the likely position of the front as it progresses west to N/east.  The first plume of instability arrives in N France later on Friday and some might arrive S England Friday pm with low risk thunderstorm activity imported across the Channel from France (as it happened this occured only in the SW).  The charts for Saturday below show how the main location of biggest storm activity could move broadly from west to NE during Saturday and clear off into the N Sea overnight. (as it happened the front moved much quicker and clear S England and most parts of Central England by lunchtime leaving a bright afternoon with bubbly Cu).

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The animation here also shows the total rainfall expected to accumulate during the 48 hour plume event.

Whatever happens, do watch out for altocumulus castellanus in clear skies or asperatus undulatus (pic above) or shelf clouds and cumulonimbus developing ahead of storms as the plume arrives on Saturday and then watch for any cumulonimbus clouds exploding if the cap is bust later on!  Please send in your photos of any interesting weather phenomenon to RGSweather : on twitter and facebook and email.

Comments and any additional information always welcome!

 

sources: many thanks to these sites:

  • weather online
  • netweather.tv
  • lightning wizard
  • manunicast (new)
  • RASP BlipMap
  • RGSweather
  • nullschool wind flow charts
  • meteosat dundee uni sat pics
  • metbrief UKMO synoptic charts

 

Reigate weather in May 2014: summary

  • Tmax 26.3c
  • Tmin 1.5c
  • Total Precipitation 69.2mm
  • Total sunshine hours 150.5 hours

May was a month with some convective interest but no explosive thunderstorms for Reigate despite one or two possible moments with CAPE values exceeding 1000 kj.  The rapid in-situ development of an impressive Cumulonimbus over the town on a southerly flow on 22 May proved to be interesting for cloud watchers and delivered a well structured gust front but only one crack of thunder and some fleeting cg lightning.

Elsewhere in Europe record breaking rainfall over the Balkans caused tremendous floods and there were numerous funnel clouds and spouts from France through to Italy in unusually convective conditions.

Here are some thundery pics of the synoptic situation in late May.

More pics showing the synoptic situation during thundery epsiodes across the UK and Europe.

April 2014: quick monthly weather summary from our weather station in Reigate, Surrey, UK.

Another relatively dry month with 43.6mm of rain falling on Reigate, about the same as April 2013. Please note that we are still calibrating the automatic tipping bucket rain gauge which is possibly under-reading by around 10%.  Calibration is a tricky affair and we want to get it right.  We are supplementing all data by using monthly totals using our CoCoRahs manual rain gauge which is, by default, a more accurate measure of total rainfall. Raw data is available on our data page here.

There were no dramatic warm-ups or cool-downs in temperature during the month.  April 2013 saw a fairly significant warm-up from mid-month but April 2014  flat-lined with no especially warm or cold temps.  It was noticeable that air temps never dipped below freezing.  This allowed the average monthly temp to climb higher than April 2013 overall, despite a lower Tmax.

Tmax 20.4c

Tmin 1.8c

Average temp 11.2c

Total rain 43.6mm (CoCorahs)

Sunshine 138.7 hours

A high pressure dominated the South of England for the middle of the month and reduced rainfall totals.  Later in the month April showers delivered moderate rainfall totals.  There were no significant thundery episodes.

April is the month when many trees come into leaf in the UK. Below are the school lime trees in the playground taken at either end of the month.

April is also a month of lengthening days and misty mornings with heat building at the surface triggering fluffy cumulus clouds in the afternoon.  Some of these produce enough instability to produce “April showers” in cool maritime airstreams with warming at the surface.

Away from the UK, the US experienced a significant tornado outbreak at the end of April when an active cold front swept clean across the entire country and triggered 133 identified tornadoes in the Mid-West / tornado alley and at least 40 deaths.  The Mayflower tornado (EF4) on 27 April in Mississippi tore through the state leaving a 41 mile trail of destruction and 16 deaths.  Tornado damage cost over US $1 billion. http://www.weather.com/news/tornado-central/tornado-scars-april-outbreak-20140507

Excellent article here on the US tornado outbreak April 2014.

In stark contrast, but related to extreme weather elsewhere, the drought in the SW of the USA continued through the month.

Other weather news includes a potentially mega outbreak of the El Nino this year.  The El Nino is a warming of the eastern Pacific Ocean waters.  This huge ocean warming is a major fly-wheel of global climate and the ENSO is a significant gear change that impacts weather systems globally.  The warming is natural (nothing to do with AGW) but has major impacts on weather extremes around the world and could, potentially, make 2014/2015 the warmest year ever. More detail here on El Nino from the great weather guru Gav at http://youtu.be/VJXpvv0P2pw

The map below shows the usual weather relationships expected during the course of a major El Nino event (ENSO).  Note that the UK and Europe does not experience a known / correlated weather impact: i.e. an El Nino does not have a regular impact on our weather.  However, this does not mean we will not experience some impacts from this major global weather gear change. All things considered, we are lucky in the UK to have less severe weather than many countries around the world, albeit this can be a little frustrating for weather enthusiasts who like a bit of exciting non-injurious weather occasionally!

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Google Earth image of rivers through LGW

Google Earth image of rivers through LGW

With options for a second runway at Gatwick airport now firmly in the public eye again, this post explores flood control engineering and water management currently in place at the airport.  The post adds detail to the previous article on flood control in the River Mole basin and should provide a fuller understanding of flood control on the River Mole including management of water flows through Gatwick airport and any impact on downstream discharge. The previous post on the causes and management of flooding more widely in the River Mole drainage basin can be found here.

Gatwick Airport is built across the low relief, impermeable clay flood plain of the Upper River Mole drainage basin. The solid geology is mostly impermeable Lower Cretaceous Weald Clay with some sandstones overlain by recent drift deposits of river gravels and alluvium deposited by the four rivers.  This mostly impermeable surface gives the Mole a naturally rapid (“flashy”) response to rainfall events as less precipitation is able to infiltrate into rocks and instead flows as surface runoff into streams and channels across the clay plain delivering water to channels rapidly and raising discharge quickly after storms, an altogether more speedy process than soil throughflow which dominates in more permeable drainage basins.

Three tributaries of the River Mole drain into this basin within or near the boundaries of the airport: Crawters Brook, Mans Brook and Gatwick Stream.  All of these rivers have been extensively modified, diverted and culverted during the history of the airport development and both up and downstream during the wider urbanisation of the area most particularly in Crawley, Manor Royal industrial estate and Horley.  Most significantly, the Gatwick Stream was culverted underneath the South Terminal during original airport construction in 1958.  More recently, the River Mole was diverted in 2000 further north and west to accommodate the North Terminal.  A second runway will require further diversions and channel modifications of streams and rivers and further significant attenuation schemes to manage additional runoff from any new runway.

This article outlines how Gatwick airport manages runoff and water flow to reduce the impacts of flooding and pollution not only within the airport boundary but also upstream and downstream of the airport.  It should be noted that the Upper Mole basin is an increasingly urbanised catchment with over 11% of the drainage basin covered in urban development.  Despite initial perceptions, Gatwick comprises only a small proportion of this total urbanised land cover in the River Mole basin, parts of which are expanding as a key growth area in the south east with the M25, M23 and London to Brighton railway providing excellent infrastructure for growing business and commercial activity and attractive locations for housing.  At less than 8 sq/km, the airport itself is currently around three times smaller than the urbanised area of Crawley (30 sq/km).  So Gatwick is just a part of an urbanising River Mole catchment and therefore shares responsibility for managing the river with other land users and the Environment Agency.  Despite the difficult hydrological siting of the runway on a flood plain and the negative press regarding winter flooding, Gatwick airport is managing water flows responsibly and behaving as a good neighbour to the local area by investing in the Upper Mole Flood Alleviation Scheme that will endeavour to control flooding, not just benefiting the airport but also reducing flood risk for Crawley and downstream to Horley, Reigate, Dorking, Leatherhead, Fetcham and Esher and the M25.

 

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All human development within flood plains from the smallest garden patio, private driveway or house extension to the largest scale road project, shopping centre or major airport, reduces the natural capacity of landscapes to moderate the flow of water into rivers.  Human development usually involves the removal of trees and vegetation and the addition of impermeable hard surfaces such as cambered roads, sloping rooves, guttering and drains that are designed to remove water as quickly as possible to avoid local flooding.  Such development prevents the infiltration of water into the ground which slows the movement of storm runoff into rivers by allowing some of the precipitation to “sink in” to the soil, thereby slowing down flow reaching the river.  Trees and vegetation also intercept rainfall and transpire water from the soil and these processes also slow the movement of water into channels or removes it from river basins altogether by the natural process of evapo-transpiration from vegetation.

The time it takes for water to enter rivers is called “lag time”: the time between peak rainfall intensity and peak discharge.   Lag time tends to be longer in naturally vegetated lowland basins and those with permeable geology.  In developed river basins with extensive urban surfaces the lag time tends to be shorter.  Rivers with short lag times are sometimes called “flashy” because they respond rapidly to rainfall events and flood more easily.  Storm runoff reaches the river quickly across impermeable surfaces and this shortens the lag time and exacerbates flooding.  All development has a tendency to speed up the transfer of runoff into rivers.  Airports are large open areas, the runways alone covering 4km2, with extensive impermeable surfaces.  Such wide open impermeable surfaces could therefore have significant impacts on local river discharge unless runoff was carefully managed.   Like all airports in the UK, London Gatwick works closely with the Environment Agency to ensure runoff and pollution are carefully controlled.  The shortening of lag times through the rapid removal of rainfall from urban areas can cause flood problems downstream as water ingresses into channels more quickly and this is why Gatwick “attenuate” runway runoff by the addition of holding or balancing ponds.

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Each airport is unique in how the site creates challenges for water managers.  Gatwick was built in a low flood plain which clearly presents unique flood and pollution control challenges particularly now due to climate change and increased rainfall intensity.  Nevertheless, all airports present one challenge or another with regard to their site.  Heathrow is located on permeable gravels and these present challenges for controlling pollution entering the water table. Other major UK airports located further away from fluvial flood risk are presented with the challenge of shedding excess surface water from heavy rainfall efficiently into balancing ponds and water courses with sufficient capacity to remove pluvial flood water.  So Gatwick is by no means unusual in having its own unique challenges presented by a flat low lying site.

“Balancing ponds” are the main engineering device for “attenuating” runoff from airports.  Balancing ponds are designed to delay or “attenuate” runoff from runways by holding back discharge from reaching the river directly from the runways and hard surfaces inside the airport boundary.  Balancing ponds are large, sophisticated engineering structures and are one of the most extensive spatial land uses for many airports, including Gatwick.  A glance out the window on take-off or landing from LGW will show keen-eyed observers numerous balancing ponds and channels in amongst trees, tracks and open spaces around the runway perimeter. These ponds are linked into a complex gravity-fed system designed to transfer runoff and separate clean from contaminated water and treat it and store it, before eventually returning the water to local rivers in a controlled manner or recycling it for other purposes within the airport itself.

All eight of the Gatwick balancing ponds have been designed to control the rate of runoff so that flows are equivalent to the runoff from a grassy or “greenfield” surface.  In short, from the point of view of precipitation, Gatwick airport acts hydrologically like a vegetated grassy surface because rain falling over the runways flows into balancing ponds that regulate runoff reaching the River Mole at a discharge equivalent to water flowing off a greenfield.

For every square metre of additional concrete built within the airport boundaries the airport is obliged to construct additional attenuation to meet this “greenfield” attenuation requirement. This will also be the case for any second runway.  It is worth considering that this obligation to strictly control and continuously monitor runoff is not the case for all development outside the airport boundaries, for example additional private driveways, housing and roads.

The control system of water management at Gatwick Airport permits regulated discharge of clean water into the River Mole from each of the eight balancing ponds depending on the rate of flow of the river.  At times of high flow more water is discharged than in periods of low river flow.  At Pond D, which receives the bulk of cleaned water from runway runoff (see below), a maximum discharge into the river of 1680 litres per second (1.6m3/sec 1.6 cumecs) is possible.  The discharge of the River Mole entering the airport boundary during times of flood often exceeds 10 cumecs and can exceed 20 cumecs at Sidlow, near Reigate.  The addition of 1.6 cumecs, regulated to slightly precede the natural flood peak, is therefore of little significance to the overall flood conditions of the river.  Despite some local myths, there are no “flood gates” at Gatwick airport or manual or automated systems which could ever allow significantly sudden discharges beyond a maximum of 1.6 cumecs to be discharged at times of high flow into the River Mole and therefore exacerbate flooding downstream of the airport.

There is little evidence from hydrographs that there are any spikes or discharge peaks outside the expected normal hydrograph curves for high intensity rainfall events, particularly those associated with the floods during the past very wet winter 2013-2014.  A sudden discharge of water of any significance would show up on Environment Agency hydrographs and, monitoring these during the course of the winter, there was no clear evidence of unexpected peaks or surges outside the normal response to rainfall along the river downstream of the airport.  The winter 2013-2014 yielded 250% more rainfall than average for the area and the Gatwick holding ponds and water management system remained within the design capacity.

On its course through the airport, the River Mole is owned and managed by London Gatwick.  The airport has the responsibility to manage the riparian zone (flood plain) and the channel to maintain efficient discharge. River levels and pollution levels are constantly monitored by means of gauges and biochemical oxygen demand.  The River Mole enters a culvert built in the 1950’s underneath the west end of the runway.  The river was diverted north of the runway in 2000 to allow for the North Terminal.  The new channel for the River Mole around the airport is entirely artificial but has been carefully designed as an attractive wooded park-like area with public access being maintained along much of the stretch around Povey Cross bridge for recreational enjoyment, even inside parts of the airport boundary.  Gatwick Operational staff  walk the entire airport stretch of the Mole twice a year to monitor the state of banks and spot any disruption to the efficient flow of the river along the artificial flood plain. The channel itself has been designed as an efficient shape to discharge flows effectively and an artificial flood plain has been built to safely allow for discharges that exceed bankfull stage (the natural maximum discharge for the channel beyond which floods across the flood plain occur).

Gatwick Airport, just like other extensively urbanised land uses such towns like Crawley or Horley or industrial estates like Manor Royal business park, cannot be solely capable or responsible for controlling flooding on the River Mole.  These land users might endeavour to limit flooding locally to acceptable regulated levels but complete prevention of flooding is not possible.  A holistic approach using a variety of hard and soft engineering techniques across the whole drainage basin is likely to be most successful in controlling floods and the embryonic Upper Mole Flood Alleviation Scheme (UMFAS) is just such an example. Nevertheless, even with UMFAS, some flooding along the River Mole will continue to occur across the flood plain, an area which includes the airport, during times of exceptionally high rainfall totals and during high rainfall intensities (the latter becoming particularly more common).  These weather events are still “natural” on a river basin scale and, whilst airports can attenuate flow off runways by careful management, they cannot completely prevent floods from occurring that are caused by intense rainfall across an entire drainage basin.  Overwhelming discharges will inevitably overwhelm flood plains should they exceed the design capabilities of the engineered defences.  Nevertheless, it is still the responsibility of major land users to control floods and reduce impacts to acceptable levels and to protect key infrastructure and this is what Gatwick is doing by investing in modern flood control schemes both within the airport boundaries and across the whole Upper Mole catchment.

What follows are some details about Gatwick Airport engineering schemes designed to control water flows in and around the airport.

2014-04-02_19-48-46

2014-04-22_16-56-53

 

Pond M – is a relatively new balancing pond.  Its role is to attenuate airfield runoff and complete initial treatment of contaminated runoff.  Especially during wet periods, it transfers a controlled discharge of clean water to the River Mole by allowing outlets in a chamber to overflow clean water into the river after passing through “interceptors” that remove any remaining aviation fuel or silt.  Clean water from Pond M is discharged into the River Mole from a controlled overflow shown below. The volume of water discharged into the river at this location is designed to be small even at times of peak rainfall intensity.  During rainfall there is discharge at greenfield rates into the River Mole from this balancing pond.  Summer runoff from runways is cleaner so more water is discharged into river as there are no anti-icing used.  In winter more of the water from Pond M is contaminated and so enters the transfer system to Pond D for further treatment.

Prior to entering balancing pond M, runoff from the runway passes through a sensor that continuously measures biochemical oxygen demand (BOD) and detects anti-icing chemicals  and other contaminants and diverts contaminated water with BOD >10mg/l to the contaminated holding pond for storage.  This is then transferred to Pond D for tertiary treatment such as removal of oil / aviation fuel and silts and thence contaminated water is sent to pollution lagoons for further treatment prior to entering the Crawley Water Treatment processing plant east of the runway near Tinsley Green.

aircraft washing creates contaminated waste

aircraft washing creates contaminated waste

There are two aircraft stands where aircraft washing is permitted.  These two stands drain to a treatment plant which removes heavy metals released during aircraft washing including cadmium. Within Pond M catchment there are 2 aircraft stands.

2014-04-20_18-05-29

 

Pond A - is a balancing pond located at the outflow of the River Mole from its culvert under west end of the runway. Pollution monitoring also takes place here of all water shed from the west end of the runway. After flooding of the runway in 1967 the culvert was found wanting so a “syphon” was added. A syphon is essentially an additional channel which provides more capacity at times of peak flow to reduce the risk of flooding on the runway.

 

 

Pond D – this is the central “hub” of water control and treatment at Gatwick Airport and sits just behind the North Terminal.  Pond D receives all dirty contaminated water from LGW runoff from each of the balancing ponds e.g oil / aviation fuel and anti-icing chemicals. D pond also drains the eastern area of airfield and both terminals.  Pond D is sited approximately 50m above sea level and a levelling gauge indicates the height of the water in the pond.  During the 23-24 Dec winter flood Pond D was at its highest ever level at around 55m asl and was above the level of the culvert feeding it.

Three massive archimedes screws lift water from Pond D, aerating it at same time, up to separation ponds for clean and contaminated water.  Each screw can lift 840 litres per second.  Gatwick is permitted only to use two screws at any one time, the third being present in case of mechanical failure of the others.  Two screws are therefore said to be “on duty” whilst a third is always on stand-by.  During peak storm runoff the two screws are able to lift an absolute maximum of 1680 litres of water per second from pond D to flow through two parallel separators. Biochemical Oxygen demand (BOD) is monitored continuously to check the status of water entering the ponds and separate accordingly into clean or contaminated ponds.

Clean water enters a separate pond where it is aerated to reach a required standard before being discharged into the River Mole just downstream of Povey Cross Bridge via a surface spillway built to allow water to flow naturally, under gravity and unimpeded out of the pond once a certain level is reached during flood conditions. There are no gates and there is no impounding of large volumes of water that could then be “released” to cause a sudden flood peak in the Mole downstream.  The additional discharge into the River Mole from Gatwick airport from Pond D cannot exceed 1.6 cumecs because the two screws jointly have a maximum capacity of 1.6 cumecs.

During “normal” river flow and “average” rainfall conditions there is little or no discharge of clean water into the River Mole.  The rate of discharge into the River Mole from Pond D depends on the rate of flow of the river itself.  When the River Mole is at high flow conditions more airport water is permitted to be discharged from the clean pond.  Rates of discharge are set by the Environment Agency and automatically controlled by automated systems at Pond D.

The contaminated water from Pond D is further cleaned of silt by use of interceptors and any oil or aviation fuel is removed by running absorbent filaments through the surface of the water.  This is then extracted by means of mechanical pressure.  The process is continuous.  The water from this pond is then pumped along a 4km pipe to pollution lagoons outside the eastern boundary of the airport and thence finally to the Crawley water treatment works where it joins the water arriving Crawley which is is cleaned to a standard non-injurious to fish prior to being released into the natural water course of the Gatwick Stream which joins the River Mole near the Hookwood roundabout.

Gatwick airport contaminated runoff is therefore treated through the Crawley water treatment works and is the same quality.  The flow of contaminated Gatwick runoff permitted through Crawley treatment works is 50 litres per second or 0.05 cumecs. The attenuated discharge of runway runoff in the Crawley treatment works represents no significant “additional” runoff other than what would have reached the Gatwick Stream or Mole in natural conditions.

With the maximum discharge of clean water at Pond D at 1680 litres per second (1.6 cumecs) and the rate of flow through Crawley treatment plant at 0.05 cumecs, the additional flow to the river network is comparatively small.  The Gatwick Stream at flood peak can achieve ten times this flow.

 

A new facility at Pond D also extracts clean water for use in the airport fire hydrant network around the perimeter of the runway, increasing the capacity for the airport to attenuate runoff by recycling water within the airport boundary.

All pumping stations relating to water management across LGW have back-up facilities in case of power outage, especially critical during times of flood.

The whole system of water management is mostly gravity fed and fully automated.  Water quality is checked with 100’s of samples every month.  The Environment Agency attaches strict conditions to the quality of water entering the River Mole so that pollution discharges are minimised and that water entering the river is the highest quality possible and not injurious to fish.  Fines can be applied if the airport is found to cause pollution along the river, which has happened in the past but it is fortunately a rare event due to tighter controls and monitoring.

1958 Gatwick Stream culvert construction

1958 Gatwick Stream culvert construction

The culvert running underneath the South Terminal and railway station, built in 1958, unfortunately was not designed with sufficient capacity to accommodate the river discharge during times of extraordinary peak flooding along the Gatwick Stream (i.e. exceeding 15 cumecs).  The culvert can accommodate only peak flows during 1:50 year flood events.  Larger, less frequent flood events with higher discharges cannot be accommodated by the present culvert.  It is not possible to rebuild the culvert due to the extensive airport, road and railway developments that have taken place above it, so a modern attenuation scheme upstream of the culvert has been constructed and is due to be finished in August 2014.  The Gatwick Stream scheme is an airport initiative that complements the wider Upper Mole Flood Alleviation Scheme (UMFAS).

  • Details for the wider Upper Mole Flood Alleviation Scheme (UMFAS) have been posted here before, find details here.
  • LGW contributed £4 million to the UMFAS inc Worth Farm, Tilgate Lake (both completed), Clays Lake (due to start in September)
  • Gatwick Stream scheme estimated cost £12 million (completion August 2014)
New flood attenuation scheme

New flood attenuation scheme

The new Gatwick Stream flood alleviation scheme will be open to the public and appear as a rather unusual park-like layout not dissimilar to “Teletubby Land”!  Grassy mounds with numerous oak trees have been retained to conserve local bat populations and increase the park-like appearance of the area.  Footpaths will encourage the public to use the area for recreation.  Due to the proximity to the end of the runway, large flocks of birds will be discouraged from the site to reduce the risk of birds striking aircraft.  This will be achieved by pumping out any water residing in the basin into the stream to maintain a dry environment thus discouraging flocks of wetland bird species, for example.

The Gatwick Stream has been diverted and modified into a new meandering natural-looking channel course.  Fish have been re-stocked.  Flow will be monitored continuously at the South Terminal culvert entrance.  If the Gatwick Stream discharge exceeds the 1958 culvert maximum of 15 cumecs then the gates will inch down and attenuate the river flow which will spill over into the newly created basin.  Flood waters will fill the basin but will be allowed to discharge naturally as soon as the flood peak has passed.

The park will be open to the public.  Interpretative signs will hopefully be in place to explain the scheme and link to the UMFAS and Gatwick flood and water management and wildlife conservation to present flood management in the Upper Mole Basin.   This seems to be an excellent opportunity for Gatwick to be on the front foot in terms of displaying a modern flood alleviation scheme put in place on schedule and with public and wildlife conservation interests included.  This could be a triumph of modern hydrological engineering made necessary by past mistakes (i.e.1958 culvert being built too small)! It combines modern soft approaches to flood management, such as reintroducing meanders, conserving wildlife and allowing natural wetland environments to attenuate discharge, while also including the hard engineering necessary to protect critical national infrastructure.  The scheme should also benefit Horley residents by reducing flood risk downstream.  This demonstrates the importance of Gatwick as an international facility requiring protection and the seriousness with which the airport takes its responsibility to control flooding both within the airport and for residents downstream in times of climate change.

Car parks are self-balancing

Airport parking is another large scale land user and comprises further extensive impermeable hard surfaces that could potentially increase surface runoff and create flood risk.  Deep ditches surround these car parks and these were designed to reduce the need for balancing ponds.  The ditches themselves provide self-balancing around each car park.

Long term car parks at the North Terminal discharge water from the ditches into a balancing pond known as Dog Kennel Pond. Complex control systems handle pond levels and during intense rainfall some car park runoff is discharged into the River Mole near Povey Cross Bridge. The discharge is cleaned using interceptors and so is considered to be a clean flow as it matches the same quality of water that would runoff from any road surface.

2014-04-21_12-30-18

 

Y shaped and new round pollution lagoons:

The final piece of the Gatwick water management jigsaw is a 4km pipeline from Pond D that transfers polluted water to the pollution lagoons clearly visible on maps and Google Earth.  These lagoons are netted to prevent harming wildlife.  The lagoons further treat by aeration contaminated water prior to transferring it to the Crawley Treatment plant from whence it is discharged into the Gatwick Stream along with waste treatment from Crawley.

The final destination for most contaminated airport runoff water is Crawley treatment works which receives airport water from the pollution lagoons nearby which, in turn, received their water from the 4km pipeline from Pond D. The maximum rate of treatment for airport water through the treatment works is 50 litres per second (0.05 cumecs) which, again, is of little significance to the Gatwick Stream at times of high discharge.

2013-12-30_09-05-58

Conclusions

Gatwick airport is unfortunately sited in an impermeable flood plain vulnerable to flooding.  In response, the airport has progressively built a sophisticated hard engineering water management system that collects, separates, cleans and discharges a controlled flow of water back into rivers via complex transfer through a series of balancing ponds and cleaning processes.  The system has also had to address past mistakes in engineering that proved wanting in times of more recent intense rainfall events.  The majority of contaminated water is discharged into rivers through Crawley Water Treatment works.  During periods of heavy or prolonged rainfall some clean water can be discharged from Pond M and Pond D directly into the River Mole.  The LGW water management system controls runoff into local streams and this effectively “attenuates” runoff to the equivalent of a grassy greenfield site from each of the eight ponds around the site.

Water management and flood control at Gatwick airport is more strictly regulated and more carefully monitored than many expanding urban areas in the same catchment, for example, in Crawley and Horley.  These urban areas, creeping inexorably onto flood plains can exacerbate flooding to a greater degree than the airport alone, though this would need more detailed investigation to “prove”. The causes of flooding in the Mole catchment are a combination of factors which were explored in previous posts on this blog found here.

Importantly, the management of water flow within Gatwick is sophisticated, automated and ongoing. There are no “flood gates” that can be opened to cause a sudden increase in discharge on the River Mole.

Inside the airport, the major flood event on 23-24 Dec 2013 was related to intense rainfall causing rain water to migrate through the maze of 1980’s airport piping, drains and ducting and, unfortunately, discharge through a wall directly into an electrical facility servicing the North Terminal.  The complex organic historical growth of the airport has created unique problems of management that require continuous investment and engineering to solve.

Finally, the airport has impressive “epic” engineering structures that control, separate, transfer and treat runoff from the runways continuously and discharge an attenuated flow of clean water into the River Mole and contaminated water via the Crawley treatment works.  Discharge is usually low but, it is worth remembering that all engineering structures are built to a design limit.  New water engineering schemes at Gatwick, such as Pond M, are being designed to protect against rare events of exceptionally high rainfall with return periods of at least 1:120 years.  Events yielding rainfall in excess of these design capabilities will still cause flooding both within the airport and downstream.  Plans to add another runway should include awareness of flood attenuation both within the airport boundary and for communities downstream.

http://www.gatwickairport.com/PublicationFiles/business_and_community/all_public_publications/Runway%202/Consultation_full.pdf

The River Mole could become a “beacon” example of modern holistic river management!  It could also become a vital green lung for this part of the South East alongside a major international airport.  As the airport hopefully continues to invest responsibly in both soft and hard management along the river course, as much of the river as possible should be made available for recreation and public access.  The River Mole VISION should be for a continuous green network of footpaths and attractive open spaces to exist along an ecologically rich riparian zone for local residents to enjoy: an unbroken network of paths from Rusper, through Crawley and Gatwick and onward to Dorking and through the Mole Gap into Leatherhead and Fetcham to Hampton Court!  Such a “Mole link” could enhance the natural diversity and varied landscape of the area as well as celebrating the vibrant economy, history and even enhancing local tourism.  This vision will require continued work with local authorities, land owners and the public to encourage a wider understanding of the key importance and benefits of our river network to the whole community and create a network of access agreements to join up this linear park.  Involvement of the public in how they might wish to use the river, particularly with any impact of a new runway,  could be a key to successful management of the River Mole into the future.

The Nutfield Marsh nature reserve, a wetland habitat designed, in part, to attenuate flood runoff on Redhill Brook by designing a natural wetland, could be a design blue-print for the Upper Mole.  This nature reserve has partly been managed by school groups from Reigate Grammar working with Reigate Area Conservation Volunteers.  The nature reserve has a role to play in reducing flood risk downstream on the Redhill Brook. Public access and enjoyment of these interesting locations will be both educational, healthy and yield a greater understanding of the need for careful management of rivers and flood plains as we likely face climate extremes and changing flood risks in the future.

The information in this post is based on a lengthy tour and investigation of the water management engineering in Gatwick airport especially designed for RGSweather and kindly hosted by John Barber (LGW Water Quality Manager) and Tom McShane (Project Manager).  All sites pertaining to water management around LGW were visited at the end of March 2014.  John and Tom imparted expert knowledge and this post represents a collaborative approach to publish, as far as possible, a clear and factually correct outline of flood control in LGW.  Comments / further information /updates always welcome.

links

http://www.airportwatch.org.uk/?p=21249

 

March 2014 was a contrast to last year.  The March average Reigate temp in 2014 was a warm balmy 8.7c, while 2013 it was only 3.5c.  March 2013 was cold, frosty and snowy, while March 2014 was warm and mostly dry and with good spells of sunshine.  March was also a welcome break from the winter storms as the jetstream broke up and meandered weakly north and south of the UK, rarely bringing in purposeful storms.

Like Reigate. the March Central England Temperature is 2c above the long term average at 7.6c.  This follows Jan and Feb 2014 being around 2c above average too.  This makes 2014 a significantly warm year so far for the UK.  Should these anomalies continue 2014 would be several degrees above average, making it a contender to be the warmest year on record.  However, this is extremely uncertain because we have a long way to go and the remaining months might work out being cooler!  It is quite likely that 2014 warmth will decline at some point, though when is not clear.

2014-04-01_22-57-19

 

Here are the raw figures for Reigate March 2014 (2013 in brackets).

Tmax 19.8c (16.1c)

Tmin -1.2c (-3.4c)

Average 8.7c (3.5c)

Total rain 29.9mm (67mm)

After the wild winter weather, March 2014 had no real weather drama in Reigate… it was calm with only moderate gusts of wind mid-month and mostly dry and quite sunny, with a total of 146.6 hours on our sunshine recorder.

Notable features of the month included several episodes of splendid altocumulus wave clouds.  These are caused when an upper air flows over hills or is caused to ruffle over an airstream below and the air starts to undulate.  In the upward undulations the air cools and condenses forming (more)  thicker cloud… the bands of cloud you see are usually perpendicular to the air stream.

A few interesting cumulonimbus showers with some thunderclaps occurred at times later in the month yielding some moderate spells of hail and also some fabulous double rainbows in the Spring sunshine.  The other notable feature came at the very end of the month when a Saharan dust storm covered the Southern UK and created a pollution incident exceeding 10/10 on the pollution scale in several parts of the UK including East Anglia.

 

The month was dry enough to allow some weather spring-cleaning.  We continued the process of calibrating the rain gauge using our own bespoke “trickle-ometer” courtesy of Ed in the physics department who developed our “calibratometer” to drip a tiny regulated flow into the VP2 buckets to get an accurate representation of a rainfall event without overwhelming the tipping buckets.  Results are not conclusive enough yet to warrant changing the VP2 settings… we need to run some more calibrations to calculate a precise % difference.  Meanwhile, we are using the CoCoRAHS rain gauge to check our readings and also a sister station in Reigate to cross-check measurements for the town to get a reliable average for rainfall.  Raw figures are available on the data page here.

As the wise calibration joke goes… “a man who has one watch knows the time, a man who has two watches is never sure”… the same is true for all weather instrumentation and calibration.

 

There is some interesting weather potential this week, but indications that the SE will see least of any drama which will be mostly further west. The well-established warm plume of S/SE wind from Africa/ Mediterranean and Spain has brought temps up to 19.4c in Reigate this weekend and 20c in London.  A warm sunny Saturday was especially pleasant.  The breezy S/SE wind is bringing Saharan dust falling over the UK, watch for this in any showers that might come our way on Monday.  Check your car for any dust.

Warm plumes from Spain can also introduce unstable moist air and these produce thunderstorms and showers when moist warm air converges or fronts  undercut the plume with Atlantic air creating lift. There is good potential for heavy convective downpours of rain this week due to these scenarios.  Whilst not necessarily a classic Spanish Plume the synoptic situation is very similar to May 1998 when large super-cell thunderstorms drifted to the North and caused torrential rain and flooding in parts of the North.

2014-03-30_18-37-25

Various indicators are used to establish the potential for heavy convectional rain and thunderstorms.  At times the charts show several of these indicators at unsually high levels for end of March / early April this week.  CAPE (convective available potential energy), LI (lifted index) and ThetaE (potential equivalent temperature) … these are all technical charts that are commonly used to assess how likely thunderstorms will be.  ThetaE rarely goes above 19c in the UK so temps of +12c in April are unusual so early. Unfortunately, the development of convective rainfall and thunderstorms is difficult for forecasters to predict for any one location for a particular time.  Thunderstorms and showers are, by nature, hit and miss affairs: one place might get large hail and a deluge while, a mile down the road might remain sunny and dry.  Nevertheless, watch out for some potentially heavy rain, especially if travelling this week.  Watch out for interesting cumulonimbus clouds too!

Total rainfall remains highest in the west and away from the SE where pressure remains higher and frontal action is more limited.  So Reigate may escape the worst of all this convective rain action but still worth keeping an eye out for rogue storms that may well come our way drifting most likely from storms in the Channel around mid-week.

Finally, models are hinting at high pressure building back in for the school holidays after this wet unsettled week. Whilst not yet represented on charts very convincingly, the models suggest a possibility of some reasonable holiday weather in the UK with pressure rising and temps above the 30 year average on the GFS ensemble mean.

 

 

 

Soil moisture: possibly the most under-rated meteorological measurement!  Rarely do weathermen get animated about the extent of wet sod across the country.  Nevertheless, soil moisture, usually measured in centimetres of water in the top two metres of soil or as % saturation (see maps below), has been found to control continental scale weather patterns, summer maximum temperatures and even heat waves and the extent of droughts.  So we ignore soil moisture at our peril, especially as soil moisture also controls vegetation growth and death and the ability of farmers to grow food.

How wet the soil obviously relates to how much it has rained recently.  During winter, in mid-latitudes, soils usually become increasingly saturated with a surplus of water building up as inputs of precipitation exceed evaporation which is reduced in the cooler months and shorter days.  During the summer, soils tend to become increasingly depleted of their moisture content as evaporation (output) exceeds precipitation (input).  This input and output of moisture forms an annual balance known as a soil moisture budget and is shown in the graph below.

2014-03-28_21-36-57

High rainfall during the winter builds up a lot of water in the soil.  In the spring time a high soil water content “uses up” more energy from the sun in the process of evaporation.  The more energy “used up” in evaporation, the more energy is lost from the system to produce sensible warming at the surface. Hidden energy, or latent heat, is required to change liquid water into water vapour.  So “latent cooling” reduces the amount of energy available to warm the atmosphere as long wave radiation.  So local temperatures can be depressed over areas of wet soil especially during a spring when wetter-than-usual soils might take a long time to dry out.  It might also be expected that, after a wet winter, there could be a cooler period until such time that the soil dries out locally and more energy becomes available to produce a sensible heat flux at the surface.

In fact, the effects of soil moisture go far beyond these micro-climatic changes and can have impacts that are continental in scale.  In a 2007 study (see below) it was found that 25% extra soil moisture could reduce continental Europe-wide temperatures by up to 2c from average summer maximums. Likewise, a 25% reduction in soil moisture could raise temperatures across continental Europe by 2c in the study period.  It was also discovered that higher winter and spring soil moisture could raise summer precipitation levels and change continent-wide pressure patterns.

The study from 2007 used reanalysis of computer weather models to investigate the impact of soil moisture on the European 2003 heat wave, the warmest for 500 years.  This heat wave killed over 20,000 people and caused crop damage.  The study found that by re-running computer models just with different soil moisture values, the maximum temperatures and heat wave intensity varied greatly.  Dry soils during the spring increased summer heat wave intensities while wet soils reduced the maximum temperatures.  The difference was significant, and in some localized regions the intensity of heat anomalies varied by 40% simply due to different soil moisture content at the outset of the model runs.  The largest differences were mainly located over central Europe.  It seems that differences in soil moisture have most impact across central continental Europe and progressively less impact on summer temps with increased distance further north.

Not only did soil moisture control temperatures, it also had a control over continental pressure patterns.  Dry soils built pressure through the middle troposphere, while wet soils could lower pressure.  This has numerous positive feedbacks: wetter soils reduce pressure which increases cloud formation and summer rainfall that enhance the wet soils.  Dry soils build pressure, reducing cloud formation, reducing summer rainfall, further drying out soils.

2014-03-28_22-22-50

So, in summary:

  • If the soil-moisture deficit is high, the dry soils raise the sensible heat flux, producing a deeper, warmer, drier low-level atmosphere: raising temperatures and enhancing surface heating and drying.  Increased drought risk.
  • If soil moisture is high, the latent heat flux by evaporation and transpiration dominates, enhancing cloud formation and a tendency for cooling.  lower temperatures and enhancing rainfall and further wetting of the soil.  Increased wet summer.

2014-03-28_21-46-22

 

The full article is here: Fischer_heat_waves_2007 (1)

A ridge of HIGH pressure brought excellent fair weather cumulus cloud formation over the SE today.  The sat pic shows cloud streets of cumulus over the SE where local convection, capped by an inversion at 5000 feet created beautiful fluffy bubbly cumulus for much of the day. This came after the “coldest” night of the year at -1.2c in Reigate and -6c in the far north of the UK under clear skies and polar air.

Another point to note was the opposite wind directions at different altitudes today: surface SE flows giving way to NW flows above 7000 feet.  The skew-t chart below illustrates these features of a super-lovely day here.  The fairweather Cu convection was surpressed later in the day as an advancing occluded front spread high level cirrus and cirrostratus across the sky thereby creating an enormous regional inversion of warmer upper air that effectively stopped further convection and the cumulus gradually faded with incoming cirrostratus and altostratus later in the day.

 

skew-t chart fair weather CU formation

skew-t chart fair weather CU formation

 

The rest of the week for Reigate deteriorates and becomes cooler and damper and more gloomy.  A large cut-off low is set to form over the continent, while a HIGH over Scandinavia will combine with this COOL-POOL to drag NE and easterly winds over the UK.

Usually such winds bring cool, damp and showery weather across the East and SE, including Reigate, with moisture picked up from the N Sea and temps from a cool early spring-time continent often creating layers of cloudy gloomy stratocumulus widely.  Thurs/Fri may see brighter spells with showers.

A LOW from Denmark mid-week is modelled to drift across the N Sea and this is likely to exacerbate the cold by bringing an extra-cool shot of cold continental air (-6 at 850hPa) over the UK mid-week: snow is possible over high ground of the Pennines and NE at this time.

Later in the week and at the weekend a subtle shift in the location of the Scandi HIGH and Euro-low could allow warmer SE winds sourced from the Mediterranean to filter into SE UK edging temps to the upper teens once again.  This is a fair way off still but will make the weekend a more pleasant prospect if this comes off.

Model predictions for the start of April look uncertain still with ifs-and-buts over how settled it will be.  Several sugges high pressure building at times early on but other models put a LOW over the UK for this period… so charts definitely worth watching for Easter school hols.

2014-03-24_22-21-06