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E. Linacre and B. Geerts |
1/'99 |
There are daytime breezes blowing onto the shores of large lakes. Dynamically these 'lake breezes' are the same as seabreezes occurring at the coast (1). Under suitable circumstances of intense insolation and little or no gradient wind, similar breezes are found are even small lakes. For instance,
an Australian salt lake only about 8 km across has winds converging across its edges in the early morning (when the water is up to 6 K warmer than the surrounding land) and diverging at noon, when the air above the water can be as much as 10 K cooler than the air at screen level over land (2).Other boundaries in surface conditions can induce similar, weak circulations: for instance the edge of snow-covered land or the edge of an area of fog or low stratus clouds. Similarly, weak 'vegetation breezes' are sometimes observed from large irrigated areas to the surrounding desert.
The main factor controlling these topographically controlled, thermally driven boundary-layer circulations is the degree of differential surface heating. For instance, lake breezes around Lake Erie in North America are typically weaker than those around lower Lake Michigan. Both lakes are about equal size, and the surrounding land cover is similar, but Lake Michigan is deeper and therefore has more thermal inertia: so its water is cooler in summer.
Except for very shallow lakes, the lake surface temperature remains constant within about 2K, day and night, even for small lakes. Likewise, green forests, swamps or crops use much of the net radiation energy for evaporation, not heating. Wet or green land adjacent to a lake may be considered as an extension to the lake, and on clear, calm summer days, a breeze may be felt at the perimeter of the wet/green area of rapid evaporation.
Another influence on a lake breeze is the shape of the shoreline. The PBL above a small circular lake, with a concave shore, carries less potential energy per unit length of shore. Convex shorelines (such as capes) experience stronger onshore breezes and weaker nocturnal offshore breezes.
Lake breezes may extend several hundred metres vertically. The larger the lake and the larger the temperature gradient, the deeper the lake breeze, but it is always confined to the depth of the PBL. It penetrates inland by the early afternoon to a distance which is usually less than the lake's width. Speeds of up to 5 m/s have been observed around the Great Lakes of North America.
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