Welcome!

February 28, 2015

LIVE

Gauges – Now – Reigate radar

Weather

Today  –  Yesterday  –  Trends – Forecast

Extremes

Month  –  Year  –  All time

Online

Map  –  MetOffice  –  Twitter

Data

RGS data  –  WU data

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 :-)

Dust!

April 14, 2015 — 1 Comment

Saharan dust reaching the UK gets in the news quite regularly, usually unfavourably in connection with pollution events. Desert dust is one of several types of minute particles, called aerosols, that are emitted into the atmosphere including salt, carbon and volcanic ash. Human made aerosols, such as CFCs and sulfate aerosols from the burning of fossil fuels, are infamous for destroying the ozone layer and causing climate change but 90% of atmospheric aerosols have natural origins and they all contribute in major ways to the global weather machine. The aerosols constantly floating around in the air that we breathe are made up of a complex mix of particles clumping together in an invisible soup that we are unaware of most of the time.  On occasions dust concentration becomes “thick” enough to become visible and reveals itself as haze. Here is some information about desert dust to help get to grips with this impressive weather phenomenon and hopefully clear the air of those mysterious arid particles!

Where does it come from?

Dust is naturally lifted into the atmosphere from deserts and is an important component of global weather and climate processes and nutrient transport to ecosystems.  Global dust emissions into the atmosphere are estimated to be up to 5000 million tons per year and emerge from numerous arid and semi-arid source regions.  For comparison, the Icelandic volcano, Eyjafjallajökull, erupted 250 million tons of volcanic ash during the eruption in 2011 but the average global annual volcanic output of ash has been estimated as an average of only 150 million tons per year, so desert dust usually dwarfs volcanic ash in the atmosphere.

global dust source regions

global dust source regions

The biggest global source of atmospheric dust is the Sahara Desert, a huge area of sand dunes, stone and gravel plateaus, dry valleys and salt plains creating nearly 5 million sq km of potential dust producing terrain.  Within the Sahara Desert the Bodele Depression in Chad is thought to contribute half of all Saharan dust.

How does dust get into the air?

Dust is lifted by strong surface winds produced at different scales, from small local convective processes such as dust devils to meso-scale convective systems such as large thunderstorms through to regional scale frontal depressions. Importantly, rainfall in arid areas contributes to available dust by causing flash floods that wash fine debris into river and lake beds. These rivers and lakes then dry out and provide an important source of desert dust when the wind blows. A good example is the Bodélé Depression in Chad, which is part of the dried out Lake Chad.  This area has dust storms on average of 100 days per year and can loft 700,000 tonnes of dust into the atmosphere every day.

http://en.wikipedia.org/wiki/Bod%C3%A9l%C3%A9_Depression

Meso-scale convective weather systems in deserts can cause strong cold downdrafts of out-flowing evaporatively cooled air descending from cumulonimbus storm clouds that can entrain particles and lift them vertically into powerful upward thermals. Sandstorms known locally as haboobs are created in this way and appear as frightening “Hollywood”style dust fronts in Africa, Australia, China, the USA and recently in the film Interstellar.

haboob dust storm formation

haboob dust storm formation

Dust can also be lifted from the surface by powerful winds covering a large area associated with troughs and fronts sweeping across, or near to, desert regions.  One such wind is called the Sirocco which occurs in eastward tracking Mediterranean lows where the warm sector produces strong southerly winds which can bring dusty conditions into Europe especially in Spring and Autumn.

2015-04-11_10-44-42

Various other synoptic scale meteorological scenarios bringing European / UK dust events are discussed below.  Once elevated, coarse dust (sand) falls out nearest to the origin but fine dust (clay), less than 0.002mm in diameter, can be lifted high into the troposphere, up to 10km, where it can remain aloft for weeks and be driven thousands of miles across oceans by jetstreams.  Saharan dust routinely travels to the Caribbean in the summer on an easterly jetstream.  Dust is eventually deposited in light winds, usually in anticyclonic high pressure systems, or is washed out in rainfall.  In this way some 40 million tons of dust is transported from the Sahara and deposited in the Amazon rainforest every year.

2015-04-13_14-14-24

desert dust entrainment and transport

There are broadly two types of dust storm.

  1. Dust plumes have a streaky linear point pattern of dust emerging from a point source and spreading into a cone.
  2. Dust fronts are walls of dust rising on an extensive, frequently curved path.

2015-04-13_14-54-30

Desertification of environments in China and Africa seem likely to be increasing the area of global dust producing regions and potentially making the planet more dusty. However, it is not certain whether global atmospheric dustiness will increase or decrease due to expected climate change in source regions like North Africa.  The world has certainly been more dusty in the past. It is understood that during past glacial periods (last glacial maximum 18,000 years before present) water was locked up in glaciers creating drier conditions particularly in periglacial mid-latitudes.  In Europe, China (Yellow River) and the US (Idaho, Washington, Iowa and Mississippi), huge areas of wind-born dust deposited thick aeolian sediments, one of which extends across the North European plain which now forms very fertile soil called loess.  Loess has become some of the most productive agricultural terrain in the world.  “Dust to dust” seems more apposite than ever considering our reliance on natural dust transport for our food.

2015-04-14_11-21-47

loess hills, fertile farmland courtesy of dust!

What are the impacts of dust on climate and environment?

Dust plays various complex roles in atmospheric processes, including modification of solar energy receipt, temperature, cloud formation and influencing rainfall.  Dust also has impacts on ecosystems and human activities which can be beneficial or detrimental and even hazardous. So, what can atmospheric dust do exactly?

DUST…

  • absorbs and scatters incoming sunshine causing surface cooling
  • increases cloud condensation nuclei enhancing rainfall or…
  • increases cloud condensation nuclei enhancing condensation of small droplets which stay aloft so reducing rainfall
  • causes “blood / red / mud rain” events creating dirty cars and windows
  • neutralizes acid rain: dominant minerals in dust are usually >pH7 and include acid neutralizing carbonates
  • imports important beneficial minerals and nutrients such as nitrates, phosphates, iron, calcium, silicates etc to ecosystems like the Amazon rainforest. 200 million tons of fertilizing dust is transported from Africa to the Amazon each year of which about 40 million tons is deposited directly into the forest ecosystem: this is possibly the main nutrient source for the forest. Marine ecosystems also benefit from dust inputs e.g. stimulating growth of phytoplankton and subsequent food chain.
  • imports pernicious alien spores and soil fungus to coral reefs potentially causing coral death events
  • reduces Atlantic hurricane formation: enhanced dust from the Saharan Air Layer (SAL) over the Atlantic during the hurricane season has been correlated with reduced numbers of hurricanes, possibly due to the dust reducing sunshine which suppresses Atlantic sea surface temperatures in the hurricane development zone.  The EUMETSAT satellite image below shows a dust veil (pink) killing off convection cells (brown and green) as it moves across the Atlantic towards the Caribbean.  The Saharan Air Layer is a hot, dry and dusty stream of upper air emanating from West Africa, especially during summer.  The SAL could also inhibit convection, and hurricane formation, by creating an inversion preventing updrafts necessary to kick-start tropical storms.

2015-04-14_17-18-04

  • Dust also impacts human activities and health.  Severe dust storms impact activities requiring good visibility such as air travel and some sports. It can also carry organisms such as spores, fungus, bacteria and viruses which could introduce disease far away from the origin of the dust.  Serious cardiovascular and respiratory problems might also be aggravated by fine airborne dust. http://rt.com/news/smog-britain-sahara-pollution-981/

So, dust is clearly a critical part of the weather machine and can bring both benefits and problems.  The next section attempts to explain how desert dust can get all the way to Britain from the Sahara.

What weather patterns typically bring dust to the UK and Europe?

2015-04-11_09-36-51chart1

In Winter, the subtropical Saharan HIGH pressure is strong with winds wanting to spill away in all directions, potentially carrying dust.  However, with a more southerly jetstream and visits by low pressure systems, the Mediterranean is often unsettled and wet during winter. Despite the Sahara being dusty in winter, dust events extending to Europe in winter tend to be restricted because particles are washed-out by winter rainfall before it gets very far north.

2015-04-11_09-36-51chart3

The transitional seasons of Spring and Autumn can produce the most significant dust episodes in Europe. The desert heats up and dries out creating ideal conditions for dust to be elevated by strong winds.  Low pressure can still dip south on meridional jetstreams and create Genoa low pressure which typically increases wind speeds across North Africa.  A cooler Mediterranean Sea surface temperature means that less convection occurs and creates less wash out opportunities as any dust travels north. Therefore, springtime is potentially more dusty for Europe given the right conditions.

2015-04-11_09-36-51

In Summer the Mediterranean  HIGH pressure develops as a semi-permanent feature.  This inhibits transport of dust from the Sahara.  Nevertheless, occasional heat lows over Iberia or cut-off lows can create the right southerly wind on a Spanish Plume to bring dust as far as the UK.

2015-04-14_22-45-40

Saharan dust moves north associated with Spanish Plume

Dust events in Europe vary in scale and can occur at any time of year but it seems usually and most effectively in transition seasons, especially Spring. In 1901 an historic dust event created the first recorded “blood rain” across Europe.  In this single dust event, well documented, some 50,000 tonnes of dust was deposited across Europe (this would have required a 250km long convoy of 2500 20-tonnes lorries to transport). It has been estimated that dust build up across Europe is 4-5mm per century.

Some case studies of European dust events

Here are some examples of past European dust events showing the synoptic evolution of how dust gets to Europe. Note that meso-scale convective systems (MCS) typically producing dust storms in the Sahara are sub-synoptic and sometimes the dust lofting event barely shows up on these charts.  Nevertheless, the synoptic patterns transporting the dust into populated parts of Europe are well illustrated in these examples.

European Dust March 2014

This slideshow requires JavaScript.

European Dust April 2011

This slideshow requires JavaScript.

So what is the future for dust?

There is no certainty on the impact of climate change on the future of dustiness in the atmosphere.  There have been press articles suggest there is increasing Saharan dust emission due to population increase, intensive farming and land degradation in North Africa.

“There has been a dramatic increase in some aspects of dust flux [emissions], which have doubled over the last 50 years. Population pressure alone is likely to exacerbate the problem and if current trends continue the amount could double again over the next 50 years,” said Dr Bryant, a Reader in Dryland Processes at the University of Sheffield.

Nevertheless, the impact of these activities is not certain and others suggest dust emissions are not increasing. For example, despite human desertification and degradation of semi-arid environments causing increased potential source areas of dust, it appears that the most significant dust source globally, the Sahara  desert, has not in fact been perturbed by human activities since the major dust sources are mostly in uninhabited areas and in true-deserts.

The IPCC predict that North Africa will get drier and therefore presumably more dusty.  However, models suggest that specific dust source regions could become wetter.  There are significant uncertainties over African dust and climate change and there seems to be no clear correlation over recent decades between measurable climate change and dust load in the atmosphere. Models cannot agree on rainfall changes in North Africa.

Here are some links for further information on dust…

aerosol http://www.nasa.gov/centers/langley/news/factsheets/Aerosols.html

excellent detail:  http://www.goes-r.gov/users/comet/EUMETSAT/at_dust/print.htm#page_1.3.0

Greek forecasting dust: http://forecast.uoa.gr/dustindx.php?domain=med

Barcelona dust forecast centre http://dust.aemet.es/forecast

satellite dust over Western Europe: http://oiswww.eumetsat.org/IPPS/html/MSG/RGB/DUST/WESTERNEUROPE/index.htm

cross-sections of dust across Europe: http://charadmexp.gr/instruments/16/

ecosystem impacts http://gallery.usgs.gov/videos/223#.VS1CZfnF_To

http://earthobservatory.nasa.gov/Features/Aerosols/

http://www.independent.co.uk/environment/saharan-dust-six-things-you-didnt-know-about-red-dust-storms-9232879.html

more info: https://books.google.co.uk/books?id=RyYyWQrn-D8C&pg=PA288&lpg=PA288&dq=dust+events+europe&source=bl&ots=EAi_LZA045&sig=7lvbivQoXeOXESgsqRaI8ERn27w&hl=en&sa=X&ei=h78rVf-2AeWQ7Abqx4GgDg&ved=0CDgQ6AEwAw#v=onepage&q=dust%20events%20europe&f=false

notes

pollution event Friday

pollution event Friday

High pressure bringing fine sunny weather to much of Britain is shifting over the continent and set to bring a brief high or very high pollution event across the SE of the country on Friday due to a subtle shift in wind direction.  Slack air residing over Benelux countries for several days is set to move our way.

Belgium, Luxembourg, Netherlands and Northern France are populated, urbanised and industrial areas of Europe and the longer an air mass sits over such places the more pollutants it picks up.  The air over Benelux has been sitting slackly over these locations collecting pollution for several days (see back-trajectory chart below).  This air is forecast to travel over to the SE of the UK through Thursday bringing the highest levels of pollution to our area by on Friday morning.

Friday can be a particularly bad day for pollution events because of the build-up of local pollution through the week (which has also previously been correlated with greater rainfall on Fridays due to additional condensation nucleii being present in the atmosphere).  This local pollution will add to the toxic mix of imported long range air pollution expected to arrive here on Friday morning. Fortunately it is due to be replaced fairly quickly by fresher cleaner westerly Atlantic air by Saturday.

The situation is worse on Friday because of a temperature inversion at low levels persisting during the morning (see above right).  Temperature inversions often occur in high pressure situations overnight when air near the surface cools  by radiation.  The cooler air near the ground is unable to mix vertically because it is trapped by a warmer layer of air aloft, this is especially common on cool mornings and can be observed when mist or fog lingers at the surface.  It is such trapped surface air that can build up most polluted conditions and causes most health impacts.  In the most persistent inversions this can cause a condition known as “fumigation” (see below).  In addition, at higher atmospheric levels a plume of dust whipped up from the Sahara is likely to increase any haze observed.

Air pollution can be produced locally by vehicle exhaust pollution, industry and agricultural activities (lofting of slurry products and ammonium based fertilizers for example).  Long range air pollution transport is imported from elsewhere.. non-dom pollution!  Ozone pollution at the surface is not produced directly but is the product of a chemical reaction with polluting gases such as Nitrous Oxides and strong sunlight.  This creates ozone gas which, at the surface, is a pollutant.  Fortunately, ozone levels are not expected to be high on Friday but particulate matter certainly is.

Fortunately this is a brief pollution event because the wind is due to shift quickly to the SW/W through Friday which will introduce fresher, cleaner Atlantic air by Saturday after the passage of a cold front.

cooler fresher air by Saturday

cooler fresher air by Saturday

A bit more detail on air pollution: Air pollution is a mix of tiny particles and gases such as sulphur dioxide, nitrous oxides and ozone.  Particulate matter has sources that can be natural, such as volcanoes, dust from deserts, fire, spores and pollen.  Particles from natural sources are usually bigger in size and are less damaging to human health than smaller human-made / anthropogenic particles from the burning of fossil fuels (such as coal), other chemical reactions and agricultural activities.  Exposure to these smaller PM2 or PM2.5 particles can be more hazardous to human health because they can contain toxic heavy metals, can be soluble and, due to their tiny size, can be breathed deeper into the lungs and enter the cardio-vascular system causing health risks such as lung cancer, asthma and even heart attacks.   There is “no safe threshold” identified by DEFRA for these smallest of deadly particles PM2 so any exposure is worth avoiding.

DEFRA advice on how to respond to pollution events is listed below.  Importantly, avoid unnecessary exercise, breathe through your nose if possible and stay indoors if you are in a vulnerable group (children, elderly and people with lung problems).  In poor air quality conditions you might notice your mucus build up more than usual and perhaps coughing too as a response.

Exposure risks to air pollution

Exposure risks to air pollution

The outlook for Reigate is for the weekend to be a little more unsettled and cooler as a cold front passing through on Saturday, but little rain is expected to reach us in the SE. Sunday looks dry too but rain further north might slip south so watch this one.  Then a return to high pressure into next week and more dry conditions for the first half of the week. It looks like the second half of April could turn more unsettled, but nothing dramatic is on the cards locally.

2015-04-08_09-51-39

useful websites

http://uk-air.defra.gov.uk/

http://aqicn.org/map/europe/

http://laqm.defra.gov.uk/documents/air_quality_note_v7a-%283%29.pdf

http://londonist.com/2015/04/air-pollution-warning-friday-will-see-high-levels-in-london.php?utm_campaign=coschedule&utm_source=twitter&utm_medium=Londonist&utm_content=Air%20Pollution%20Warning:%20Friday%20Will%20See%20High%20Levels%20In%20London

http://www.telegraph.co.uk/news/earth/environment/11529007/Time-to-see-red-over-air-pollution.html#disqus_thread

http://www.thurrock-today.co.uk/news.cfm?id=30345&headline=New%20air%20quality%20health%20fear

2015-04-07_11-05-58

March 2015 Reigate summary

Reigate March 2015 weather summary

  • Average Temperature 6.9C
  • Tmax 14.4C
  • Tmin -1.9C
  • total rainfall 23.8mm
  • max wind gust 46mph
  • sunshine 143.2 hours

Anticyclonic conditions controlled a good chunk of March bringing a lot of dry weather to Reigate.  Total Reigate March rainfall of 23.8mm is around half of that expected from the long term average for March recorded since 1873.  Whilst March rainfall this year was low it was not outside the “normal range” with mean monthly rainfall for March in South East England being 49.8mm and the mode (most frequently occurring) at a relatively dry 38.7mm.  March 2015 turns out to be the 36th driest year since 1873, the lowest being 1929 with a paltry 2mm of rainfall.  So March 2015 was dry but not super-dry!

With high pressure around March was sunny and not terribly windy, especially through the middle month.  Mid-Month the temperature dipped notably (see top chart) as a slack easterly set up with the high moving over Scandinavia.  It was not a beast from the east because the continent was warm and there was no really cold air available.  The slack conditions finished at the end of March, however, as a significant NW gale blew across the UK.  This broke our daily wind run record at 305.4 miles.  Wind run is the “fetch” or distance that the wind has travelled passing a point during the day.  Imagine a balloon drifting in the wind during the day… how far would it travel? This is the wind run.  The NW wind also turned lorries over on motorways further north, gusting over 70mph in places.  In Reigate the max gust was 46mph, pretty strong but with no leaves on the trees it did not cause damage.

143.2 hours of sunshine in Reigate exceeded the long term average.

March will be remembered for the “deep partial” solar eclipse that crossed much of the UK on 20 March.  Sadly for us in Reigate and much of the SE we saw nothing of it except a gathering gloom and slight dip in temperature.  More on our eclipse observations here: http://rgsweather.com/2015/03/21/reigate-some-eclipse-effects-on-weather/

2015 overall is above the long term CET average but not by much. Nevertheless, as a moderate El Nino is set to start properly this summer it is likely that 2015 will be break more temperature records globally as a hot year overall for the planet.

2015-04-07_11-58-30

All our weather data can be downloaded from the data page here.

MetOffice March summary 

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

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

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.

2015-03-20_21-20-06

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).

2015-03-20_21-28-25

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.

2015-03-24_18-09-58

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.”

2015-03-24_18-11-02

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.

2015-03-15_09-29-41

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!

_48471968_banner

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… :-)

2015-03-14_20-16-43

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.

2015-03-14_14-51-20

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.

2015-03-14_14-52-35

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.

2015-03-14_16-59-58

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.

 

2015-03-12_21-10-33

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