We have a boat on beautiful Smith Mountain Lake Virginia. Many times in the summer there is a curious phenomenon. Normally with high pressure building in, fair weather, and a very calm day; when the sun sets, the winds start picking up from the south, sweeps across the main channel [1 mile wide), and slams the boats at the dock with 4 ft waves. It starts small at sunset and steadily increases to about 2am where it rapidly subsides and becomes very calm. There is a small mountain (about 1500 ft elevation) on the south border of the channel. I was wondering if there is a meteorological explanation.

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Craig Jones
MIKE MOSS SAYS:     Craig, You're almost certainly seeing the effect of "mountain winds,"

sometimes called "drainage" or "density" flow, and a subset of "katabatic winds" which means wind blowing downhill. In this case, the circulation is associated with air immediately adjacent to the mountainside becoming cooler and more dense than air at the same elevation farther away from the ridge (due to radiative cooling of the ground which then cools the air in contact with it). The cooled air near the mountainside then drains downslope and is replaced by warmer air aloft flowing in from over the lake, which is then cooled by the mountainside and so on. The process continues until a sufficient layer of cool, dense air builds up over the lake to balance out the density of the air on the side of the mountain, and the winds die back off. Just looking at a topo map and aerial photo of the lake, I'd expect most of the lake to experience a southeast or south-southeast wind initially, which may trend toward the south. Of course, there may be some localized channeling effects I wouldn't be aware of. This process occurs with any mountain-plains or mountain-valley system, and reverses during the morning to midday period as the surface is heated by sunshine, producing "valley winds" or "anabatic flow" uphill. The winds can become especially strong across a lake or other water body that is immediately adjacent to a mountain, due to the lack of friction associated with the water surface.

This should be a more prevalent phenomenon on clear nights with fairly low humidity (more efficient/rapid mountainside cooling), and much less likely when skies are overcast through the evening and nighttime hours. I would also think the effect would be stronger toward the end of summer and early fall when the lake water temps should be warmer relative to air temps, as opposed to spring and early summer.


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