Archives For altocumulus

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!