‘Convection clouds’ are ones that form and grow by the process known as convection. Cumulus and Cumulonimbus are examples of this type of cloud.

Convection in the atmosphere is the way air floats upwards on account of being warmer than the surrounding air. You can see this sort of movement close up when a shaft of sunlight picks out the tiny motes of dust suspended in the air of your room. Since the dust reveals the invisible swirling currents of the air, it lets you see the air near to a radiator rising upwards. The radiator warms the air beside it, making it expand and become less dense than the air further away. This causes the air near the radiator to gently float up towards the ceiling. The same convection currents, on a much larger scale, drive the formation and development of convection clouds like the large Cumulus shown above.

Out in the atmosphere, patches of sun-warmed ground can act as enormous radiators that set the air rising. These convection currents are known as thermals, and produce fair-weather Cumulus clouds. But convection can also produce clouds when the Sun is not shining. This happens when a huge region of colder air from the poles collides with a region of warmer air from the tropics. When this happens, some of the colder air can ride up over the warmer. The result is in an unstable region of the atmosphere that produces large convection clouds. The less dense, warmer air below rises rapidly through the denser, cooler air above. And whenever air rises like this it cools. This means that some of the invisible water vapour it contains can condense into droplets that appear as cloud.

Since they tend to form rapidly in the rising columns of air, convection clouds are ‘optically dense’. They contain a lot of very small droplets, the surfaces of which scatter the sunlight more than in clouds made of fewer, larger droplets or ice crystals. For this reason, convection clouds often look bright white on the sides facing the Sun and dark grey on the sides away from it.

 

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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