General DiscussionThe major wind systems of the earth determine much of the large scale oceanography with which we are familiar. The local winds modify the ocean and the overlying atmosphere on a minute-to-minute and day-to-day basis.
SE Indian Ocean
Gulf of Oman
Katabatic winds are created when relatively dense air flows down a mountain slope as the result of radiative cooling atop the mountain. The simplest katabatic wind is a breeze that rolls down a mountain at night, either as a drape of cool air in the tropics, or as a channeled stream of cool air whispering down a valley (Defant, 1951).
In southern Europe, katabatic winds are widespread and frequent, and have acquired local names that denote over which sea or valley they blow, or their direction. The bora is the cold, gusty northeasterly wind that blows down the slopes of the Karst Plateau and along the eastern shores of the Adriatic Sea. At Trieste, the bora has been known to overturn vehicles and sweep people off their feet. Its mean speed in the winter is 52 kilometers an hour, with gusts up to 100 kilometers an hour. The very low temperatures of the bora keep it extremely dry, and even after it has crossed the Adriatic and into the Ionian Sea, its temperature and humidity remain far lower than normal.
Across West Africa, in all seasons other than the summer, the hot dry harmattan blows from the Sahara Desert to carry dust far over the Atlantic Ocean. Though mainly the derivative of the northeast trade winds, the harmattan may be enhanced in the fall and winter by an easterly moving low-pressure system. Under such conditions, it is called the leste in the Canary Islands, where the humidity may drop to less than 20%.
The best-known and most common local wind is the sea breeze. It is a diurnal wind that develops as the air over coastal lands begins to rise in response to daytime warming. Maximum wind speeds usually occur in the midafternoon, along with the greatest penetration inland and extent seaward. The vertical depth of the sea breeze is initially only a few feet, sometimes building in thickness to 6,000 meters by evening. Its influence offshore is up to 70 kilometers, but that distance is highly variable, depending on the configuration of the coast, the topography of the adjacent lands, and the local vegetation. The direction of the breeze begins normal to the coast, but as the day wears on, the sea breeze rotates to an oblique angle under the influence of the Coriolis effect.