Archives For cumulus clouds

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!

showers over east coast 03-02-2013Today was bright and clear with a chilly northerly breeze, as forecast. Clouds developed throughout the day over Reigate but remained benignly flat anf fluffy, eventually joining up to form a thin broken blanket of stratocumulus through which the sun set spectacularly. Typically, in a northerly winter Arctic airflow, snow showers reach the East coast but fizzle-out and disappear inland. Today was an excellent example of this: watch the video. Places near the Kent coast and East Anglia saw wintry showers whilst places further from the sea, like Reigate, saw none at all.  A simple explanation goes like this:
Clouds form when air rises, cools and water vapour condenses into water droplets (clouds are tiny water droplets and/or ice crystals, not cotton wool!). The main driver of rising air is the heating of an airmass by a relatively warm surface. The North Sea is still quite “warm” at a balmy 7°C (note the nearby higher surface temperature in Margate 4.5°C) and this was sufficient to warm the chilly Arctic airmass travelling across it.  This warming caused convective uplift of warm bubbles of air (thermals) which formed shower clouds over the North Sea.  Think of the bubbles formed when a cold pan of soup is heated on the stove: they rise up, just like thermals rising through the atmosphere.  Rising thermals initiated cloud formationover the North Sea produced the shower clouds visible on the video marching across the East coast. The showers died-out inland because the land in January is colder than the sea (note the lower surface temp in Reigate 2.5°C) so warming is reduced and thermals are much less frequent.  Fluffy clouds may form but do not rise to any great height: these are called “fair weather cumulus”.   Showers and clouds tend to fade away in the evening because thermal activity is reduced.  (n.b: April showers occur for similar reasons: the stronger Spring sunshine warms the ground quickly and sets off rising thermals through airmasses which are still cool from the winter.  We must wait a few more months for the sun to get powerful enough for showers of this nature to kick-off inland: something to look forward to!)  Temperature changes with height are called lapse rates. Lapse rates control cloud formation and precipitation.  Try this site for more info.

The Arctic air mass will be replaced by warmer, wetter westerly winds tomorrow.  But don’t despair, if you enjoyed the fluffy cumulus clouds and outstanding visibility today then you will be happy to learn there is more to come this week!  Models agree that another long cold snap is due to return, initiated by an Arctic airmass on Wednesday.  This cold snap could last into mid-February with the possibility of an especially cold dip in temperatures next weekend.