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Seasonal and Local Winds

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Seasonal and Local Winds

The planetary or the permanent winds described above are a response to the planetary’ or permanent pressure belts. Besides these winds seasonal and local winds blowing in response to seasonal or local pressure gradients are also experienced in many parts of the world. Monsoon winds provide the best example of the seasonal winds.

The Monsoon The word monsoon derived from the Arabic word ‘mausim’ means seasonal winds. In this system, the direction of the winds reverses seasonally. Though monsoon is typically considered a phenomenon of tropical South Asia, these winds are also experienced over parts of North America and Africa. Traditionally monsoon has been considered a result of the differential heating of land and sea In summer southern Asia develops a low’ pressure while the pressure over the sea is relatively higher. As a result, the air starts flowing towards land from the Indian Ocean. The winds coming from ocean carry moisture and thus cause rainfall in the summer season. This is know n as the southwest monsoon or summer monsoon. In winter, the pressure over land is higher than over the sea and consequently, the air starts flowing from land to sea. The air coming from land being dry, these winds do not cause rainfall This is called the northeast monsoon or winter monsoon.

This thermal theory’ of monsoon explains monsoon more as a regional phenomenon and fails to explain the total amount of energy involved in the global monsoon circulation. The modem meteorologists, therefore, seek the explanation for the phenomenon of monsoon on the basis of the seasonal shift in the position of the global belts of pressure and winds.

According to the dynamic theory, monsoons are a result of the shift of the inter-tropical convergence (ITC) under the mfluenee of the vertical sun. Though the average position of the ITC is taken as the equator, it keeps shifting northwards and southwards with the migration of the vertical sun towards the tropics during the summer of the respective hemisphere. During summer in the northern hemisphere in the months of May and June, the sun shines vertically over the Tropic of Cancer. Due to the northward shift of the zone of maximum heating and low pressure at this time the ITC also shifts northwards and approaches the Tropic of Cancer. The ITC being the zone of the lowest pressure in the tropical region is the destination of the trade winds blowing from both the hemispheres. With ITC situated close to the Tropic of Cancer the northeast trade winds are confined to an area extending to its north while the southeast trade winds blowing from the southern hemisphere have to cross the equator to reach this area of low pressure. However as the winds blowing from the southern hemisphere cross the equator, their direction is altered due to Coriolis effect, i.e., they are deflected towards their right and thus it gives use to the formation of a belt of equatorial westerlies blowing between the equator and the ITC. These westerlies in the months of May and June blow from the equator towards the ITC from southwest to northeast and they are called the southw est monsoon.

As the ITC again moves southwards at the end of the summer of the northern hemisphere the areas north of the equator which experienced the equatorial westerlies during the summer season come under the influence of the northeast trade winds. These northeasterly winds are called the northeast monsoons. The onset of the winter season in the northern hemisphere is the beginning of the summer season in the southern hemisphere and during this season the ITC shifts south of the equator and reaches as far south as the Tropic of Capricorn where the sun shines vertically at this time. In this season the northeast trades blowing towards the ITC have to cross the equator towards the south and as a result, they get deflected giving use to the equational westerlies in the southern hemisphere. These westerlies blow from the northwest to the southeast, replacing the trade winds of the southern hemisphere between the ITC and the equator They form the summer monsoon of the southern hemisphere. Thus due to the seasonal shift of the wind belts under the influence of the north-south migration of the vertical sun the areas situated in the tropical zone in both the hemispheres come under the influence of the trade winds during the respective winter and the equatorial westerlies during the respective summer season The direction of the winds is thus reversed seasonally and it makes up the monsoon system of these regions.

Though the dynamic theory provides a much better explanation of the system of monsoon as a circum-global the phenomenon, this theory does not negate the influence of differential heating of land and sea. There is no doubt that differential heating plays an important role in making monsoon much stronger in certain areas. It is this very factor that explains the extension of the southwest monsoon even to the north of the Tropic of Cancer in northern India.

Local Winds In certain regions, local winds are generated as a result of the influence of local pressure gradients and local terrain. One example of this is the simple system of land and sea breeze experienced in coastal areas. Due to differential heating of land and sea, the pressure over the landmass is lower than over the neighboring sea during the day. It creates a pressure gradient from sea to land and the wind follows this gradient during the day. The wind blowing due to this from sea to land is called the sea have. At night the land has a higher pressure due to a lower temperature than over the adjacent ocean and the wind starts blowing from land to sea. This wind blowing from land towards the sea at night is called a land breeze. The direction of winds under this system changes between day and night. Mountain and valley Minds also follow a daily alternation of direction. During the day the upper slopes of the mountains are warmer than the valleys and the air over the upper more exposed slopes getting heated up rises up. It causes an airflow upslope along the valley sides towards the mountain peaks. These winds are called valley Minds or anabatic Minds. When the upper slopes cool rapidly at night the cold air from the upper slopes moves towards the valleys thus producing mountain Minds. Under another system of local winds, called katabatic or drainage Minds, cold air flow s from higher to lower areas under gravitational force. The mountain winds can also be called katabatic winds due to the fact that they also involve a down-slope movement of cold heavier air under gravity. Such local winds occur in many mountainous areas and are known by various names, eg. , the Bora (north Adriatic coast) and the mistral (southern France). Southern California experiences a dry east wind {Santa Ana ) of desert origin. Blizzards of the Antarctic region are also winds of this type. Still other types of local winds, eg. John or Foehn and Chinook, result when strong regional winds passing over a mountain barrier are forced to descend on the lee side so that the air is dried and heated This is a result of adiabatic cooling and heating. Chinook, blowing eastwards down the slopes of the Rocky Mountains, leads to quick melting of ice in the central plains of USA. Due to this influence, the Chinook is often called snow eater A local wind called Punas blows down towards the western slopes of the Andes. It is a cold dry wind.

Some of the local winds carry the influence of their source regions and they know n for lowering or raising temperatures in the areas to which they blow. A hot dust-laden wind blows eastwards from Sahara across the Mediterranean and raises the temperature of the areas all through its journey. This hot dust-laden wind of Saharan origin, called Sirocco, may pick moisture while crossing the Mediterranean Sea and may strike the coastal areas of Europe as a warm damp wind causing much discomfort Another wind of desert origin blow s from the Sahara region towards the Iberian Peninsula. It is called Solano. A relatively cooler dry wind blowing over Sahara in winter season is called Harmattan. A local hot wind of desert origin blow s under the name loo towards east across the northern plain of India in summer season This dry hot wind raises the temperature significantly in the Cianga Valley up to as far as Bihar The cold winds affecting the north Indian plains in winter season in the wake of the westerly depressions are also examples of local winds carrying the influence of their source regions.

Variable Winds Cyclones and anticyclones are called variable winds. A cyclone involves a circular motion of air around a center of low pressure and the airflow in it is convergent. The winds in a cyclone blow in an anticlockwise direction in the northern hemisphere and in a clockwise direction in the southern hemisphere. As against the Cyclones, the anticyclones involve a divergent flow of air from an area of high pressure. The direction of winds in an anticyclone is clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.

Note: A more detailed description of cyclones and anticyclones is provided at a later stage.

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