Tertiary circulation: local winds
The tertiary atmospheric circulation is at the smallest level of the atmospheric circulation. Within the primary atmospheric circulation systems are the secondary circulation systems and within the secondary circulation systems are yet another smaller scale circulations of air which operate over relatively short distances, usually less than 160 km and over short periods of time.
The Tertiary or local atmospheric circulation systems may be thermally direct, such as, the convection cells, which arises from differential heating of the Earth’s surface having energy transfer quite similar to the single cell model of the primary atmospheric circulation system. They may also operate as modifications of established larger scale airflows, which are called local winds. The topography of the ground surface greatly modifies the characteristics of the air flowing across it. In this case, kinetic energy is derived directly from the regional wind.
The tertiary atmospheric circulation system comprise squall lines, thunderstorms and associated phenomenon like tornadoes and water spouts and local winds (dust devils, dust storms, microbursts, land and sea breezes, mountain and valley winds, föhn winds, depression winds, convection winds)and urban heat island circulation.
Land and Sea Breezes
Coastal areas often experience land breezes and sea breezes. They are caused due to diurnal differential heating of land and sea. During the daytime, the land warms up more than the water. Thus, there is low pressure over land and high pressure over water body. Over the land, as the warmed air rises, creating a low pressure, air from the surrounding ocean which are comparatively cool and moist blows into the land to replace the air that is rising. This flow of air is called a Sea Breeze. During the night, the land cools more rapidly than the water. This not only produces a local High Pressure but conserves heat and remains warmer for a longer period. The resultant energy and high pressure induces a flow of air from land to sea. This flow is called a land breeze.
Sea and land breezes are common in the tropics during most of the year and at middle latitudes during the summer. Sea breezes usually produce a line of clouds just inland along a tropical coastline or above tropical islands.
The influence of sea breeze is over a small area and does not exceed 25 km from the Coast. Its speed is between 8-30 km/ph and is generally stronger in tropical than temperate regions. The coastal resorts are most benefitted by these winds.
The life of the fishermen in the tropical coastal area hinges on these winds. During the night time they are blown seaward by the land breeze while during the day time the sea breeze blows them back towards the coast.
Mountain and Valley Winds
On clear nights, long wave radiation loss from the mountain ridge will lead to considerable cooling of that surface and its overlying air. Cooling of the valley sides and floor will be much less marked because of the radiation exchanges between the two valley walls and the floor. As a result, the cooler denser air of the top will start sinking to the floor following the mountain slope (hence slope winds), moving down as low generally smooth flow. This is the katabatic wind. Steep slope accelerate katabatic flow. Along the edge of massive Greenland and Antarctica ice sheets, katabatic winds frequently exceed 100 km-1/h. Although the speed of the flow depends on the angle of slope and the roughness of the surface, it is found to be approximately proportional to the square root of the temperature difference between the top and the bottom of the valley. Katabatic winds are frequently experienced in certain areas and are given local names. In the Adriatic Sea, a katabatic wind forms between north and east, which usually blows in winter and is known as Bora. The Mistral, which blows down the Rhône valley and over the Gulf of Lyons is another Katabatic wind. Where such winds reach the sea, being cold and dry, they increase the density of water through cooling and evaporation and thus, promote convective mixing of the water in addition to mechanically induced turbulence.
During sunny days in summer, the higher parts of mountains, the slopes facing the Sun, in particular, become appreciably warmer than the adjacent valleys. This is because the air above the mountain slope, besides being heated by insolation, is also warmed by conduction more effectively than the valley floor. Thus, the air just above the valley top becomes warmer than the air at the same level over the valley itself. This causes convectional activity, leading to light and irregular drift of air up and along the mountain slope. This is known as anabatic wind.