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!
Archives For stratocumulus
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.
The best view is often up (or down if you are in the International Space Station)… so we are starting a “cloud collection”!
Stratocumulus clouds over Earth Jan 4 2013 and over Reigate January 13 2013 9am: spread-out cloud masses which can cover the sky: fairly flat cloud bases, often “rolling” (undulatus billows): low altitude clouds, usually occur at heights above 1500 feet to 6500 feet. Indicate stable atmosphere i.e. not much change in temperature with height (sometimes even indicate a temperature inversion where temperatures increase with height at a certain altitude), so not much upward convection, so the clouds don’t rise in height, so rarely cause much precipitation. Stratocumulus often tell you that the weather is not going to change much for some hours, watch out for them thickening up or rising in height. The photo shows “stratocumulus perlucidus” which are slightly broken up, allowing sunshine between the gaps. They differ from cumulus clouds by usually being clumps joined together, whereas cumulus clouds are separate and detached (caused by convection). It might also be true to say that any low altitude cloud that is clumpy, joined together and has a fairly bubbly top … is usually stratocumulus!!
Weather: 1.9°C, cloud height approx 2000 feet; wind direction north-east 5-10 mph.
Stratocumulus clouds are frequently ignored or considered boring. We think they are worth a second look!
Please join in our cloud collection by sending your own photos of clouds:try and name the cloud type if you can and tell us something about the location and weather at the time.