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Olympic Games 1996
The Winds of SavannahThe conditions for sailing in the summer off Savannah are regularly amongst the best of the U.s. East Coast (far better than the "light and flukies" provided in the Chesapeake and Long Island Sound). Savannah's location typically under a subsidence inversion within the northwest quadrant of the Atlantic subtropical high, permits the development of a strong sea breeze, almost independent of the gradient wind direction. There is the problem of getting to the wind - 10-12 miles from the launching sites - but in the afternoon of a typical summer day, once there, one finds a lovely 14-16 knot sea breeze. Regime #1 - On fifteen days out of thirty when the gradient wind is from between west and southwest, a strong, pure southeast sea breeze appears. Regime #2 - On ten days out of thiry when the gradient wind is from between south and east, a mixture of gradient wind and sea breeze (associated with thunderstorms) develops. Regime #3 - On five days out of thiry when the gradient wind is from between northwest and east (associated with frontal passage or nearby low pressure), the gradient wind appears alone. The typical summer wind regime at Savannah represents the simplest possible type of surface air now - an onshore flow of cool marine air moving in response to the reduction in pressure associated with insolation (the heating of the earth's surface by the sun) - pure sea breeze, not requiring and not associated with a particular gradient wind.
Subsidence Inversions and Sea BreezesAbove the equator heated air rises high in the atmosphere and spreads both north and south before sinking back to the surface in huge subtropical masses. In summer in the absence of thermal or frictional turbulence over the thermally stable, relatively cold oceans these air masses form persistent localized high pressure systems and strong subsidence inversions (a layering of warm air over cold). The Atlantic (or "bermuda") subtropical high occupies the whole of the North Atlantic and typically prevents the polar front from moving south of Savannah. An occluded front (that may drift a little south or a little north, but crosses Georgia but once in a summer month) forms between the Polar air and the subtropical high, across the mid-North American continent from Colorado to New Jersey.In summer subsidence inversions form over large bodies of water because the water is colder than the overlying air and because over water thermal turbulence (the vertical mixing of heated air) is absent. Subsidence inversions are particularly strong under the subtropical highs because , when polar front disturbances are distant, mechanical turbulence is also minimized. As the air in contact with the cold surface gradually cools and contracts, the upper air sinks into the contraction. Because sinking causes compression and initiates adiabatic temperature changes, the sinking air becomes warmer. The lower (unmoving) air superajacent to the water becomes progressively colder, the upper (sinking) air becomes progressivelywarmer, and an increasingly strong inversion forms between them. During the day, as the high moisture content of the upper air results in a significant acquisition of heat from insolation, the temperature disparity increases nad, as sea breezes move the cold marine air inland, the disparity spreads shoreward. Consequently, during the day subsidence inversions become stronger and extend their peripheries over adjacent land. Although during the night they weaken, over land they are replaced by radiation inversions that reinforce their effects. Sea breezes receive a jump-start from these inversions. The sinking air generates high pressure over the sea that results in a surface outflow toward the land and the onshore flow beneath the inversion, protected from conflict with the gradient wind, moves shoreward unopposed. In the morning when thermal turbulence is still modest the inversion strengthens (because of increased heating of hte upper air) and extends inland as the marine air begins to move inland beneath it. In the near-shore areas thermal updrafts are halted at the inversion and a return flow (to replenish the offshore high pressure) develops above the onshore flow but beneath the inversion. This means that in the presence of a subsidence inversion the sea breeze will develop earlier and reach a greater strength with lesser amounts of insolation (or lesses amounts of available insolated surface) despite the presence of opposing or perpendicular gradient winds. As the onshore flow of marine air increases, cold air is carried farther and farther inland. But because its rate of advance is typically slowed by the intervention of poorly heating surfaces (such as water), the layer of marine air thickens and eventually rises to the inversion level. As the top of the cold marine layer merges with and extends the protective inversion, it facilitates the progression of the sea breeze front. The direct return flow which was so important n getting hte sea breeze started is now eliminated and the replenishment of the offshore high pressure becomes dependent upon the super-inversion flow of the gradient wind. Behind the front the inshore surface is cooled by the overflowing marine air. Here updrafts, cooler (after rising 1-2000 feet) than the air above, are unable to rise beyond the marine layer. But ahead of the front insolation is increasing, thermal columns are rising to ever higher levels, surface pressure is decreasing, and the marine air is drawn into the increasing low pressure ever more rapidly. The process by which a subsidence inversion facilitate the early development of a sea breeze is particularly successful along a coastline which provides a graduated interface between sea and land. In Finland as well as at Savannah the sea makes into the land in long reaches and the land makes into the sea in numerous low lying islands. From ten to twenty miles inland of hte "coast" there is nearly and equal amount of sea as land. This permits the subsidence inversion to move farther inland and to persist in greater strength throughout the night. Even ten miles from open water along such coasts on most mornings in summer the surface is calm - lies beneath the subsidence inversion, protected from the gradient wind. The first wind to appear is typically the sea breeze - and in these locales it is usually evident (at least in open water) by 1-:00 AM.
IYRU Olympic Update |
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