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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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Today, it snowed over Reigate!  The sky was actually seen falling at lunchtime.  Well, OK, more accurately, cirrus cloud was seen all over the sky at midday and wonderful virga fall streaks trailed behind . Virga is a beautiful treat for cloud watchers and today was an extra-special display.  Virga is any form of precipitation (snow, ice, rain), usually falling from medium or high cloud, that evaporates before reaching the ground.  So… it snowed over Reigate today but 10km up and it never hit the ground.

Whilst it was a warm day down in Reigate (100m, Tmax 25ºC), at 30,000 feet (10,000 m) it was a chilly -45ºC. A moist humid layer of air at 10,000 m moved in from the west filling the sky with cirrus clouds which are formed from ice subliming in humid high altitude air. Cirrus is a thin wispy cloud but can precipitate i.e. “rain”. The ice literally falls from those thin cloud veils. The cirrus today were being carried along by a brisk 40 mph westerly wind. The ice rapidly fell into slower wind speeds below: at about 5000m the wind was only 20mph. Hence, the cirrus appeared to leave veils of trailing ice particles: VIRGA.  Some were probably initiated as contrails which broke up in the brisk winds. A much lower layer of cumulus appeared from local convection at about 1000 m, where the air temp was still 20ºC.

Here are the charts for humidity at 10,000 m (300hPa) and 5000 m (500hPa): note the very humid air aloft and the very dry air at 5000 m.  The ice never of course hit the ground because it was evaporated / sublimed at great height.  Only passing aircraft might have experienced any virga precipitation.

virga conditions

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Reigate was one of the warmest places in the UK today.  The town bathed in air from North Africa at nearly 20ºC.  The strong SW wind driven by a 100mph jetstream aloft along a decaying front brought some superb mid and high level clouds across Reigate.  A variety of unusual clouds were visible.

Cirrocumulus is a high level cloud and unusually covered much of the sky from early on this morning. The light dappled mini-cumulus flecks 30,000 feet in the sky were beautifully fibrous in appearance.  Occasionally the cirrus rolled into parallel lines converging on the horizon: cirrus radiatus.

Virga is another unusual site for cloud spotters.  These are ice crystals falling from cirrus or altocumulus: this precipitation evaporates before reaching the ground so appears as a tail streaking out from underneath cirrus or altocumulus.  Several virga could be seen during the day but tend to disappear quite quickly.  

Unusual altocumulus lenticularis wave clouds rolled in at the end of the day on a decaying front and even deposited a few drops of rain momentarily.  Virga making it to the planet surface!  Altocumulus lenticularis are the clouds occasionally mistaken for UFO’s!  They are usually formed by high winds blowing over hills or mountains and creating wave like clouds in the air stream; our lack of high mountains makes them rather rare in the SE.

Although not as warm as today, mild and breezy conditions will persist for many days now. It will not always be dry but less than 10mm of rain is forecast for Reigate over the next 8 days so any fronts reaching us will be weak.

The pressure pattern and direction of our weather will be the “reverse” of those cold days of March: south westerly and westerly winds from the Atlantic will prevail most of the time over the next few weeks.  LOW pressure systems are set to cross to the north of the UK and HIGH pressure will sit to the south: this will keep the jetstream more or less persistently over the UK and drag in milder but moist air, especially for the west. Reigate and the east of the UK will be drier and can expect daytime temps around 15C, cooling towards the end of this week.

Polar Continental (Pc) air is most common in winter as HIGH pressure forms over cold northern continental interiors and pushes out freezing air to mid-latitudes.  In summer, when it does occur, Pc brings dry stable and warm conditions to the UK as the continents warm up.  Pc has been an unusually frequent visitor this March and effectively reversed our usual south westerly prevailing wind. As Spring sunshine warms the surface and Atlantic LOW pressure systems edge closer to the UK next week dragged by a more northerly migrating jetstream, we can be assured that moist maritime air will be making a return and any remaining incursions of polar continental air will increasingly lose their frequency and ferocity, Russia has to warm up sometime!
air masses UKMOBefore we bid “farewell” to the freezing Polar Continental air until next winter it is worth remembering the good times.  Pc has occasionally brought crystal clear skies with excellent visibility and dramatic views of the sky both day and night (as anyone staying up to see the ISS will testify). The long picture series shows Cumulus Congestus building over Stratford on Avon last week and an unusual Pileus Altocumulus Lenticularis veil forming over the dramatic rising thermals. Pileus is a fleeting, ephemeral cloud type and forms as convective up-draughts in the cumulus force upper winds over the rising congestus, just like air being forced to rise over a mountain range.  Moisture in the air condenses, or sublimes into ice, and forms a beautiful veil called Pileus.  The photos were taken over just two minutes and then the Pileus melted away.  Pileus is a beautiful cloud but has a darker side because it sometimes forms above rising nuclear mushroom clouds and volcanic eruptions.

Cloud streets, lines of Stratocumulus, were also a feature of the easterly winds: where an isothermal “cap” (temperatures staying the same with increasing height) kept a lid on rising thermals and clouds remained flat and formed lines in the airstream.  Cloud streets seem to urge us to follow them, pointing the way to something important over the horizon.  Finally, the “sundog” (mock sun) was another fleeting feature of polar continental air, though not exclusive to it: apparently only 5/100 people have ever seen a sundog, so here is a picture of one in case you haven’t caught one yet.  They occur as low-angled sunlight refracts through hexagonal ice crystals.

Pc air wasn’t all as beautiful as this of course: freezing grey blankets of dull stratocumulus dominated the weather for days in the south east and deposited icy snow grains right through to Friday.  Nevertheless, I do hope you had the time to look up and admire the best of the Polar air show this March.  So, Polar Continental may crack our cheeks and rage and blow but we’ll kind-of miss it… won’t we?  “Adieu, adieu, adieu… remember me.” Exit Ghost of Pc! (The photos above were all taken along the Stratford canal last week, the statue is William Shakespeare in Bancroft Basin).

n.b. March summary for Reigate coming soon!


Today, a classic Arctic airmass brought almost every type of cumulus cloud over Reigate. Arctic air starts off very cold somewhere near the North Pole but quickly moves south over progressively warmer oceans and land surfaces. This makes it “unstable”, which means any heating of the ground by the sun will allow relatively warm bubbles of air to rise rapidly from the surface as thermals. Thermals will continue to rise so long as the air around them is COLDER than they are, which is pretty much forever in an Arctic air mass! When moisture in the thermals condenses, towering cumulus clouds form and showers become likely. Today these showers fell as snow despite the air temperature being +4.5°C! The dry Arctic airmass with a low relative humidity (56%) coupled with the rapid lapse rate (drop in temp with height) allowed snow forming at high altitude not to melt or sublime before it hit the surface. Cumulus don’t usually live long after sunset … they decay and die quickly once their supply of thermals is cut off.  Like many things of beauty, cumulus clouds are short lived so make the most of them when you see them next time!  The photos above show the wonderful cumulus clouds which formed over Reigate today, numbered in order of appearance: how many did you spot?  Tomorrow might be similar, so keep an eye out again…the best view is often up!


Anti-cyclonic stratocumulus cloud continues to blanket Reigate and the SE of England in a cold, gloomy weather underworld.  A temperature inversion (where temperatures rise with height, instead of fall) at 1000 metres is trapping any rising air which inhibits convection spreading out the cloud mass to cover the entire sky: good to reduce rainfall, bad for any views of the sun!  Regrettably no light at the end of the tunnel yet: the north-easterlies circulating round the HIGH pressure to the North are likely to continue for much of the week and bring in blankets of stratocumulus cloud from the North Sea. This spectacular view from the EU Meteosat satellite today at midday clearly shows the cloud blanket voering SE England from 22,300 miles away.
Temperatures will struggle to exceed 5C in the days to come; this time last year temperatures were well above 12C and even hit 17C on a few days.


The cold easterlies predicted have created interesting and unusual cloud patterns across the country. The satellite photo from today shows ripples in the breaking cloud to the west of the hills in Wales and the SW.  The easterlies driving the cloud are part of the cold sinking dry air created by the anticyclone to the north east of the UK.  The easterlies are  forced to rise over the hills and then sink the other side (the “lee-side”), creating this wavy cloud pattern.
Another feature of this cold snap is illustrated in the video below which shows how temperatures have changed over the last few days.  Notice how inland temperatures drop significantly lower than anywhere near the sea. The sea stays warmer for longer during winter as it takes 5x longer to cool down (and warm up) than land surfaces.  The video shows temperatures rapidly falling to low temperatures inland but coastal areas are moderated by the warmer seas surrounding the British Isles.

The cold easterly winds and some light snow flurries will continue over the weekend as a weak short-wave trough (a “blip” in the isobars surrounding the HIGH) passes to the north of our region.  The very cold pool of upper air is drifting away to the south, so temperatures will recover very slowly through to Monday.  However, spring is certainly on-hold at the moment and, despite being a mostly dry week,  temperatures will remain lower than average in the continuing easterly air flow.  Watch next weekend as a low passes to the north of the UK which could bring a “last hurrah” for real winter conditions as a cold northerly is possibly on the cards.  Longer range forecasts give little cheer for us in Reigate and the South East as HIGH pressure seems to stick around to the North and bring in Channel lows to the south of the country bringing wet and cool weather.

How much colder is it 100 metres above you, right now? We tested this today by driving up the North Downs which stand 100 metres above Reigate, Surrey town centre.  A Kestrel 3500 was used to collect the data from the hill and temperatures were given a good while to stabilize.  The RGS weather station sits at the foot of the same hill at 96 metres. So, the (rather un-scientific!) results at 2.15pm were as follows:

Reigate 96m: Temp 3.1ºC; dew point 2.5ºC
Reigate Hill 200m: Temp 2.1ºC; dew point 1.5ºC

So Reigate Hill is a whole degree colder than the town, the wind chill was -1C. This was noticeable also in the heavy sleet on the hill.
The decrease in temperature with height is called “lapse rate” and it, usually, continues to drop for another 10,000 metres, the top of the troposhere (“weather atmosphere”). The 1C drop per 100 metres today is a steep lapse rate. We would normally expect around 0.6C drop per 100m, called the environmental lapse rate. The freezing level is currently only 300 metres above Reigate town centre and, with temperatures falling tonight the freezing level will almost certainly arrive at the surface.  The clouds were interesting: thick nimbostratus formed a solid cloud base at 300 metres; amazing shreds of fractus cloud formed on the stiff SE wind blowing up the hill.  Fractus or thicker Pannus clouds are those whispy shreds which appear under rain clouds and indicate deteriorating weather.

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Thursday update: situation on Sunday – Tuesday is mostly GRIM, wet, miserable and cold but details are still uncertain regarding snowy-ness here in Reigate: could be lots or just sleet and rain mix.  Snow up north definitely but SMALL changes in location and speed of LOW centre and wind direction will make a BIG difference to what weather we get in Reigate. Stay tuned for latest! (nb re-reading my post here: “mild” means “relatively mild” at Tmax 4°C Sunday, Monday “cooler” means Tmax 2°C: so cold, wet and miserable right through from Sunday am!)

The Arctic wind built threatening, angry stratocumulus clouds over Reigate early today (pictured) and some built into perky sleet showers by the end of the day.  The rest of this week should be brighter, clearer, frosty and less windy but sadly no sign of Spring yet – models show cool conditions continuing next week. Light snow is possible in Reigate on a decaying occluded front over Thursday night.  The Arctic wind will shut down in the next few days as a HIGH pressure builds from the SW.  Significant snow is a possibility in the first part of next week anytime from Sunday night through to Tuesday for Reigate and SE England as winds swing Easterly associated with a trough slipping SE. It’s tricky to forecast whether it will be snow, sleet or rain and where exactly it will fall at this stage but ,despite this vague detail, models do agree that a SLIDER LOW will move down from the NW and bring, initially warmer southerlies and rain on Sunday, but then drag in some cool Easterlies which could cause a spell of snow, possibly for the rush hour on Monday.  Early next week will be cold and snow / sleet showers threaten anytime. It’s a tricky forecast because the temperature of these Easterly winds is now creeping up as Spring approaches and surface temperatures will hover at the upper threshold of snowfall (2° or 3°C) so that it could easily fall as sleet or rain. Places further east and higher up on the Downs are more likely to get snow rather than sleet: it is that close. Follow 1-10 on the slides to check the causes of this potential snowy event: it is quite different from our Skyfall last month.