Archives For April 2015

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.

2015-04-27_22-22-51

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).

2015-04-27_19-24-24

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.

2015-04-27_19-28-07

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

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: https://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