Valley Fog Breakup

This loop of 5 visible satellite images shows how fog in a river valley can break up at times. Intuitively, it would seem that the fog in a valley should "burn off" from the edges, leaving the center of the valley as the last place to clear out. But as the loop shows, many of the valleys in southern British Columbia did just the opposite on 13 June 2001.

In the first frame, all of the valleys are socked in with morning fog. If you watch closely, you'll see that on subsequent images, a thin black line appears in the center of the valley fog. This shows the fog burning off first in the center of the valley. As the morning progresses, the black line widens until the outside edges of the valley are the last to burn off.

The reason for this is due to the complex mountain-valley circulations that develop in the morning. As the sun rises, it begins to warm up the top of the mountains that are above the fog. This warm air rises (shown by the squiggly lines pointing upward). This draws air from the valley up the walls of the valley (small green arrows) to replace the rising air. But now there is an absence of air in the middle of the valley. To compensate for this, the air above the valley (blue stable core) sinks. Sinking air warms, which dissipates the fog in the center of the valley first.

Figure courtesy of Whiteman, C. D., 2000: Mountain Meteorology: Fundamentals and
Applications, Oxford University Press, New York, 355 pp.