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May 12, 2015

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Reigate Grammar School, UK. Local weather station and forecasts for education. Reporting on local and global weather and climate. 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, data back online :-)

May 19 thundery showers was not a “severe” weather day but 10mm of rain from 7 showers in 7 hours, several with some moderately intense rainfall rate (30-40mm/hr), small hail and episodes of thunder, was of interest locally and deserves a review of some charts leading up to the event.

Often our area of the SE seems to miss convective action but this time showers perked up as they converged across East Surrey and some developed into reasonably impressive cumulonimbus clouds (cb).  As a bonus mammatus clouds were spotted at the end of the day (see photo above). Below is a summary of some key ingredients for the day.

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Synoptic chart thundery 19 May 2015

Low pressure: This synoptic set-up saw an unusually cold plunge of upper air flood across the country during May18-19. The synoptic chart above shows the invasion of cold polar maritime air behind a cold front, itself not especially dynamic across the SE. The LOW over the North Sea dragged this unseasonably cool air from the NW: the basic ingredient for instability on this occasion.

meandering jetstream

meandering jetstream

Note the u-bend wiggle in the jetstream around the LOW pressure in the chart above. The northward limb of the jet on the inside of the U-bend can become the location for instability, though this was not a fast jetstreak occurrence.  The base of the trough also crossed the SE of the UK during the day encouraging lift.  Showers were forecast on the charts days before.

Steep lapse rates: Very cold upper air temperatures fell to below -30C at 500hPa (5500m) across the UK.  In Spring a cold airmass like this can become unstable over land especially if the sun warms the surface to create thermals of warm air able to lift through the cold environmental air. The 850hPa temperatures show comparatively warm air at 1500m which increases the lapse rate.  The morning of May 19 was sunny and the surface heated with 5 hours of May sunshine to a modest 14C, just enough to release thermals and rising parcels of air.  Convection requires heat as a key ingredient to steepen lapse rates and create thermals, rising parcels of air.

Low lifted index: LI forecast for Heathrow (below) showed a LI of -3.  The lifted index is a measure of instability in the atmosphere and shows how readily bubbles of warmed air will rise from the surface to a great height (convection).  It is the difference in temperature between the environmental air at 500mb level (around 5500m) and the temperature of the theoretically lifted bubble of air (parcel).

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lifted index (UK rarely exceeds -5)

Negative LI numbers are good for thunderstorms because they show that the parcel is significantly warmer than the air around it at altitude, which means that at 5km the air is still bouyant. http://www.weatheronline.co.uk/reports/wxfacts/Thunderstorm-Probability.htm

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Moderate CAPE: convective available potential energy is the energy available to push air vertically up and create those surging bubbles of cumulonimbus clouds.  The higher the CAPE the greater uplift potential (above).  Note it is potential energy and a high CAPE often confounds convective activity if other ingredients are lacking, such as heat or moisture.  19 May had marginally unstable values for the UK of 800j/kg.

In the USA Tornado Alley forecast CAPE values exceed 2,000 j/kg in super cell outbreaks.  CAPE on a skew-t chart is the area between the parcel trajectory and the environment temperature.  In the SE of the UK we often just get “skinny cape”, where rising parcels are only just warmer than the surrounding environmental air, whilst in Tornado Alley “fat cape” is frequent which indicate parcels much warmer than the surrounding air, increasing bouyancy.  http://www.weatheronline.co.uk/reports/wxfacts/CAPE—Convective-Available-Potential-Energy.htm

The colourful charts above from lightningwizard.com show streamlines of wind and air mass equivalent potential temperature.  Streamlines show where the airmass is coming from and, if they converge at the surface, can indicate potential for lift.  Theta e charts show moisture and temperature characteristics of airmasses.  Where there is a contrast of colours between high theta e and convergence at the surface and lower values and divergence of air at altitude this can bring on convective weather. The moisture chart shows that the airmass supplied sufficient moisture to the UK to provide for cloud formation and release latent heat.  Dew points stayed relatively low and this showed moisture (and heat) locally was lacking for bigger storms.  Compare these forecast charts with the actual occurrence of lightning on the day.

curved forecast shows showers

curved forecast shows showers

Forecasts for the day (Euro4) showed typical lines of rainfall accumulation which indicate showers.  The actual radar showed “popcorn convection” (@metmanjames) over a wide area drifting east.

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Recommend following @convectivewx for UK convective forecasts.  Their forecast from 3 days ahead was spot on.

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2015-05-11_10-22-47

April 2015 Reigate weather chart

April in Reigate was a mostly dry, very sunny, rather calm and relatively warm month.  However, the settled weather came at a price with a significant air pollution event on 10 April. Also, April showers were conspicuous by their almost complete absence so convective weather fans were left disappointed.

Here are some April details for Reigate:

  • Tmax 14.4C
  • Tmin -1.9C
  • Average Temperature 6.8C
  • Total rainfall 23mm
  • Max gust 46mph
  • sunshine total 136 hours (sunniest April on MetOffice records!)

April was 43% drier than average across SE England and this shows up in the diminutive 23mm of rain in Reigate.  Pressure rose early in the month and so April turned out 150% sunnier and about 1C warmer than average too. The central England temperature for April came out at 1.1C above long term average.

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It was the SUNNIEST April on record for the UK according to the MetOffice.  In Reigate we enjoyed 136 hours of sunshine in April.

sunny April 2015

sunny April 2015

The significant downside of this calm weather was high AIR POLLUTION. There was a high pollution warning for a time when easterly / south easterly winds brought high levels of PM2 particles into the SE and across London from Benelux countries.

Here are some photos from Reigate during April showing what a mostly very pleasant month it was.

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

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analysis chart shows HIGH edging out with moist Atlantic winds ready to pounce

The analysis chart above shows a weakening ridge of HIGH pressure over the UK being edged out north by a slow moving Atlantic LOW to the W/SW.  Reigate is still currently (Saturday am) in cool dull easterly winds generated by the HIGH but a significant switch in wind direction will take place over the next 12 hours into the bank holiday period as a wholly different mild and humid Sub-Tropical Atlantic air mass, with a source region round the Azores, takes hold from the SW.

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weather cross-section

A mild moist S/SW wind drives in from the Atlantic as the LOW edges north east tonight. The first mass of rain is edging onto radar from the SW and is expected to arrive in Reigate by around mid-late pm today.  Most rain is likely for places further north and west but the SE is still likely to pick up plenty of wet weather overnight with low cloud and rain into Sunday morning when it could turn heavy and showery for a while in the early morning as the trough passes directly overhead and pressure continues to fall. Things are expected to clear to brighter conditions later in the afternoon as pressure rises and winds turn more westerly. Cloud cover will hopefully break and cloud height will lift during the afternoon becoming more cumuliform.

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trough and fronts migrate north, showers follow

If the sun comes out then there could be a low risk of an odd heavy shower Sunday afternoon, possibly thundery, but these are more likely further north of our area where more unstable air makes progress across the Midlands and East Anglia.

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During Sunday winds will be occasionally blustery with moderate convective gusts possible, especially on hills and nearer the coast, and make the mild temperatures Tmax 16C feel considerably cooler. Temperatures overnight could hold up to a balmy 12-13C.

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Overnight Saturday-Sunday rain could linger as showers through the morning

Winds turn from SW to more southerly through Monday and pressure should up-tick slightly giving a mostly dry and warm day and less windy as things stand currently.  Troughs could progress east during Monday and build cloud and produce some showers.  More importantly there is a looming threat of something special for later Monday-Tuesday night.

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As the northern block (high over Greenland) holds on, the Atlantic LOW just west of Ireland will usher in a mild and moist S/SW flow of air from the continent.  An unstable LOW brewing in the topical Atlantic today (Saturday) is forecast to sweep up and intensify from Biscay later Monday and into Tuesday and this might bring heavy rain and winds to the south and SE and a possible thundery episode later Monday but more likely overnight into early Tuesday for SE.  The jetstream is dipping well south and is forecast to perk up and approach the UK from an unusually southerly direction by Tuesday.  If this happens the jetstream could deepen this low considerably, as modelled by some charts (latest UKMet shows 980mb).

Depending on the evolution we could find ourselves in the unstable left exit region of a jet where divergence aloft enhances convective action and creates heavy rain.  Warm air from the south will also contain more moisture.  A dry slot at mid-levels might also enhance instability (rising dry air cools more quickly increasing lapse rates and CAPE, enhancing lift).  High dew points near the surface temperature also encourage condensation and indicate extremely moist warm surface flows.

So all these ingredients stirred up could be a good recipe for some briefly moderate-severe weather in our region especially some briefly torrential rain, though totals are unlikely to amount to more than 10mm.  Gusty winds and gales near coasts could also accompany this system.  Latest metoffice chart shows pressure dipping to 980mb in the North Sea which is significantly LOW pressure for the time of year.

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coastal gales and convective gusts inland

However these episodes have a habit of tracking across Holland and merely clip Kent with thundery showers and miss us entirely.  Models also generally exaggerate these early on and then things flatten out nearer the time considerably.  Nevertheless, it is worth watching this develop as our first potential “warm plume” of the year.  If we take a direct hit the SE could have some heavy rain.

The GEFS summary below clearly shows the two main rainfall spikes tonight and Monday night.

2015-05-02_07-07-27

GEFS 850hPa temperatures and rainfall London

Later mid-week the LOW is expected to drift east across the UK bringing in a more westerly pattern so unsettled showery weather is likely for a while. Thereafter, a rise in pressure from a developing Euro high pressure may then take place from the south and settle things down for us in the SE, though this might only make faltering progress.

Hope you enjoyed this post. Comments are always welcome. Links to websites used to create our blog posts can be found on our links page.

2015-05-02_07-05-01

faltering pressure rise later next week

2015-04-27_20-21-59

cold plunge of polar air to end April

The newspapers have this colder than average week billed as a “polar plume”.  Cold air cannot really be said to “plume” like warm air (spanish plume).  Nevertheless, it is certainly cold up North with settling snow over the hills and wintry precipitation elsewhere too.

The GFS ensemble (several model runs combined at once) chart below shows that the colder than average (upper air) temperatures will last until the weekend at least.  Thereafter temperatures rise but note the rainfall spikes indicating unsettled conditions.

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upper air temperatures

For us here in the sheltered SE it is just cooler than average with patchy overnight frost and bright days with light showers Tuesday pm. A blustery cold front on Wednesday is likely to bring more purposeful rain for a while, as could further active fronts on Thursday when the trough axis moves through our region accompanied by the jetstream nearby to the south.  Friday is likely to see a transient high pressure ridge bringing settled weather and then a change in wind direction by the weekend.

The overall cool set-up is due to “northern blocking” which is when pressure builds over Greenland and the Pole and the, previously strong, Azores HIGH slackens off and nudges south. At the same time the usual Icelandic low pressure weakens or is dominated entirely by a HIGH pressure (see chart below).

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In this situation the jetstream works its way south of the UK.  The result is that cold polar air is able to leak south out of the polar regions and into the mid latitudes.

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The index shown above summarizes the overall pressure pattern in the Atlantic. It is called the North Atlantic Oscillation. The NAO measures the pressure difference between Iceland and the Azores.  It has been mostly positive this winter: meaning that pressure is LOW over Iceland and HIGH over the Azores.  Typically a positive NAO indicates a strong zonal jetstream and mild often wet conditions for Europe with relatively fast moving LOWS passing through.  The chart below shows how the NAO has gone negative recently and this indicates that pressure has built over the Pole, creating a blocking situation.

The charts above show theta-e temperature which shows cool airmasses over the UK clearly.  These are 850hPa temperature charts which represent temperatures at 1500m (1.5km) above the “boundary layer”.  This height is used to avoid disruptive temperature changes which occur nearer the surface caused by day and night, mountains and water bodies etc that upset the overall temperature pattern for analysis.  The situation shown below by this weekend is quite different, though still unsettled.  Note the warmer flow from the SW.

After the transient ridge on Friday it looks like pressure will fall into the bank holiday weekend as a low pressure nudges in from the Atlantic.

2015-04-27_22-38-16

pressure falls into weekend

The milder humid air brought from the SW by the Atlantic low could potentially cause some significant rain at times around the weekend and into early next week as this meets cold air over the country.  Despite the milder upper air arriving from the SW, it is unlikely the “milder” temperatures will be noticeable in such wetter and windier conditions. In winter this set-up could have brought big snow events but in early May it will simply bring rain. For the detail on timing and amount of weekend rainfall we will have to wait and see, but it certainly looks potentially quite wet, though models suggest pressure building briefly thereafter.

2015-04-27_20-10-05

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!  This post concentrates mostly on Saharan desert dust but volcanic dust is mentioned as a comparison and is worthy of a separate post at a later date.

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 mineral / desert dust emissions into the atmosphere are estimated to be up to 1500-1800 Tg/year (teragrams) per year and emerge from numerous arid and semi-arid source regions.  For comparison, the average global annual volcanic output of ash from average scale small eruptions has been estimated as an average of only 20 Tg/yr (20 million tonnes per year).  Less frequent, larger eruptions, inject much more ash into the atmosphere. The Icelandic volcano, Eyjafjallajökull, erupted 250 million tons of volcanic ash during the eruption in 2011. This was still small compared with the largest mega eruptions which blast huge volumes of volcanic dust higher into the atmosphere. Desert dust is usually swept up by winds only as high as the planetary boundary layer 2-4km (PBL).  So in average conditions desert dust usually dwarfs volcanic ash in the atmosphere, unless there is a colossal eruption. (1 Tg = 1 million tonnes )

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.

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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.

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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?

Over long time scales dustiness increases during cold climatic periods (glacials or ice ages). Evidence for this comes from ice cores in Antarctica and Greenland shown below.

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Reasons for increased dustiness during cold glacial periods includes:

  • Increase desertification (less rainfall generally in cold periods as more water locked up as ice)
  • Increased land area (due to falling sea levels, so more dust sources available)
  • Increased winds

Over shorter time scales, dust plays various complex and sometimes contradictory 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.

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  • 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.

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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

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European Dust April 2011

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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

http://www.hindawi.com/journals/isrn/2013/245076/

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.

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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.

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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).

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

another eclipse shadow

another eclipse shadow

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

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

Temperature

Temperature change

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

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.