(Click image to enlarge) Photographed from a 300-ft aerostat balloon over the Potomac River, Washington DC, USA. © Curt Westergard.
See this photo in the Cloud Gallery here. Visit his web page here.
During the longer autumn and winter nights, the ground has more time cool than at other times of the year. This is particularly the case when it is a starry night, devoid of cloud cover, for the ground loses (or ‘radiates’) its heat out to space much more readily without any blanket of clouds to keep it in. When such a situation coincides with no more than the slightest breeze of moist air, conditions are ideal for the formation of ‘radiation fog’.
This is when the low air is cooled by the plunging ground temperatures to such an extent that some of its moisture condenses into droplets. It is a ground-level Stratus cloud that consists of chilled air, and so has a tendency to sink into and run down valleys, in much the same way that rainwater runs down them as rivers. If fog gathers at low ground like this, it is known also as ‘valley fog’.
And that is what they call the beautiful fog shown above, flooding down the course of the Potomac River early in the morning, near Washington, DC, in the US. Forming further upriver, towards the the Blue Ridge Mountains, the low-level cloud creeps silently through the night to find its level in the lower reaches of the river valley. It is little wonder, therefore, that this city district, past which the river flows, has the nebulous name of Foggy Bottom.
Current Cloud of the Month:
‘Asperatus’ (June 09)
Previous Clouds of the Month:
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)
To
contribute a Cloud of the Month,
go to our Photograph Submissions
page.
