There are few clouds as pleasing to watch develop as the Cumulus congestus. From below, the darkening base of this shower cloud looks ominous and brooding but, from a distance, it has the appearance of a gigantic inflatable creature: its crisp, voluminous shape swelling into the middle atmosphere, as if it is being pumped up, from behind the scenes, by some enormous, invisible dad.

The largest of the four possible species of Cumulus, congestus can develop from the smaller humilis and mediocris species when the atmospheric conditions are ‘unstable’. This means that the way the air temperature changes with altitude tends to encourage the rising column of warm, moist air at the centre of the cloud to keep lifting higher and higher. Such unchecked convection makes the cloud swell to formidable proportions. While it has the same crisp, cauliflower summit as the smaller fair-weather Cumulus, the congestus species is so tall that it readily produces sizeable showers.

This stage of Cumulus growth is when it is at the point of maturing into a different cloud type. Once the top of the cloud begins to ‘glaciate’, its droplets freezing into ice crystals, the crisp, sharp edges of its summit soften and become more blurred. This is the point at which the cloud has officially turned into a Cumulonimbus storm cloud.

Such a fully-fledged storm cloud is the stage in the cloud’s development that tends to get all the attention – what with its heavy downpours, its spreading canopy of ice crystals and dramatic thunder and lightning. But we rather prefer the congestus stage just before it becomes such an ostentatious individual. We like to gaze up and chart the cloud’s progress, blow by blow, as this brother to the fair-weather Cumulus swells into adulthood.

 

Current Cloud of the Month:
March 2010

Previous Clouds of the Month:
February 2010
January 2010
Cloud Reflections (December 09)
Numbers in the Clouds (November 09)
Sun Pillar (October 09)
Convection Clouds (September 09)
‘Pile d’Assiettes’ (August 09)
Cumulus congestus (July 09)
‘Asperatus’ (June 09)
Clouds at Night (May 09)
Sundogs (April 09)
Diamond Dust (March 09)
Cloud Streets (February 09)
Crepuscular Rays (Jan 09)
Valley Fog (December 08)
Cloud Shadows (November 08)
Contrails (October 08)
Mamma (September 08)
Kármán Vortex (August 08)
The Summertime Halo (July 08)
The Nor’west Arch (June 08)
Microbursts (May 08)
Irridescent Clouds (April 08)
Northern Lights – Aurora Borealis (March 08)
Ice halos (February 08)
Lightning (January 08)
Roll Cloud (December 07)
Banner Cloud (November 07)
Stratocumulus (October 07)
The Unclassified Cloud (September 07)
Alexander’s Dark Band (August 07)
Fumulus Snail (July 07)
Distrail (June 07)
Altocumulus undulatus (May 07)
Cumulonimbus capillatus (April 07)
Lacunosus (March 07)
Horseshoe Vortex Cloud (February 07)
Jet-Stream Cirrus (Janurary 07)
Altostratus/Altocumulus/Altowhateveritis (December 06)
Anti-Crepuscular Rays (November 06)
Stratocumulus (October 06)
Altocumulus (September ’06)
The Kelvin-Helmholtz Wave Cloud (August ’06)
The ‘Brocken Spectre’ (July ’06)
‘Whale’s Mouth’ (June ’06)
Noctilucent (May ’06)
Cirrus (April ’06)
Cap Cloud (March ’06)
Fallstreak Holes (February ’06)
Nacreous (January ’06)
Cirrostratus (December ’05)
Tuba (November ’05)
Virga (October ’05)
Cirrocumulus (September ’05)
Altostratus (August ’05)
Cumulus (July ’05)
Mamma (June ’05)
Pileus (May ’05)
Lenticularis (April ’05)
Stratus (March ’05)
Cumulonimbus (February ’05)
Contrails (January ’05)


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