The mid-level cloud known as ‘Altocumulus’ often takes the form of smooth rounded masses that look like a tray of bread rolls. But occasionally, as with the fine specimen shown above, this cloud can have a more ragged appearance with sharp turrets sprouting from its cloudlets. It is then known as Altocumulus ‘castellanus’, since its tops resemble the crenellations of castle battlements. When these airborne defenses proliferate across the morning sky, they suggest meteorological skirmishes to come.

Altocumulus castellanus can appear when the air is ‘unstable’ in the middle of the troposphere (this is the lower part of our atmosphere – in which weather happens). This means that the air temperatures up at that middle level will tend to encourage the vertical growth of clouds. As the sun rises through the day, it can cause low Cumulus to develop on thermals of air floating from the warmed ground. If these Cumulus grow tall enough to reach the unstable air in the mid troposphere, they can continue to build into enormous, fierce Cumulonimbus storm clouds, releasing barrages of hail, thunder and lightning.

With the distant storms still rumbling at day’s end, how obvious it seems in retropect that all those Altocumulus castles could never have have remained at peace.

Current Cloud of the Month:
August 2010

Previous Clouds of the Month:
July 2010
June 2010
May 2010
April 2010
March 2010
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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