Factors Affecting Wind Motion and Classification of Winds

Wind is horizontal movement of air, in contrast to currents, which imply a vertical movement of air. Winds are the mechanism by which uneven distribution of pressure is balanced globally.

Also, winds are an integral part of thermodynamic mechanism of atmosphere by which heat, moisture and other properties are transferred from one place to another.

Factors Affecting Wind Motion:

Since pressure differences are mainly caused by unequal heating of the earth’s surface, solar radiation may be called the ultimate driving force of the wind. If the earth were stationary and had a uniform surface, air would flow directly from high pressure areas to low pressure areas. Because none of these conditions exist, the direction and speed of wind are controlled by a number of factors. These are pressure gradient, the Coriolis effect, the centripetal acceleration and friction.

1. Pressure Gradient Force:

This is the force generated due to the differences in horizontal pressure, and it operates from the high pressure area to a low pressure area. Since a closely spaced gradient implies a steep pressure change, it also indicates a strong wind speed. The wind direction follows the direction of change of pressure, i.e. perpendicular to the isobars.

2. Coriolis Force:

Due to the earth’s rotation, winds do not cross the isobars at right angles as the pressure gradient force directs, but get deflected from their original path. This deviation is the result of the earth’s rotation and is called the Coriolis Effect or Coriolis force. Due to this effect, winds in the northern hemisphere get deflected to the right of their path and those in the southern hemisphere to their left, following Farrel’s Law. The Coriolis force changes wind direction but not its speed. This deflection force does not seem to exist until the air is set in motion and increases with wind velocity, air mass and an increase in latitude. (Fig. 2.16)

3. Centripetal Acceleration:

Due to inward acceleration of air towards the centre of rotation on the rotating earth, it is possible for the air to maintain a curved path (parallel to the isobars), about a local axis of high or low pressure. It is known as centripetal acceleration.

4. Frictional Force:

The irregularities of the earth’s surface offer resistance to the wind motion in the form of friction. This force determines the angle at which air will flow across the isobars, as well as the speed at which it will move. It may also alter wind direction. Over the relatively smooth ocean surface, the friction is minimum, so the air moves at low angles to the isobars and at a greater speed. Over uneven terrain, however, due to high friction, the wind direction makes high angles with, isobars and the speed gets retarded.

Classification of Winds:

Based on their characteristics and their variations over time and space, winds may be broadly divided into three types.

1. Primary or Prevailing Winds:

These are the planetary winds which blow extensively over continents and oceans. The two most well- understood and significant winds for climate and human activities are trade winds and westerly winds.

The trade winds are those blowing from the sub-tropical high pressure areas towards the equatorial low pressure belt. Therefore, these are confined to a region between 30°N and 30°S throughout the earth’s surface. They flow as the north-eastern trades in the northern hemisphere and the south-eastern trades in the southern hemisphere. This deflection in their ideally expected north-south direction is explained on the basis of Coriolis force and Farrel’s law.

Trade winds are descending and stable in areas of their origin, and as they reach the equator, they become humid and warmer after picking up moisture on their way. Also, the trade winds from two hemispheres meet at the equator, and due to convergence they rise and cause heavy rainfall. The eastern parts of the trade winds associated with the cool ocean currents are drier and more stable than the western parts of the ocean. (Fig. 2.17)

The westerlies are the winds blowing from the sub-tropical high pressure belts towards the sub­polar low pressure belts. They blow from south­west to north-east in the northern hemisphere and north-west to south-east in the southern hemisphere. The westerlies of the southern hemisphere are stronger and persistent due to the vast expanse of water, while those of the northern hemisphere are irregular because of uneven relief of vast land-masses.

The westerlies are best developed between 40° and 65°S latitudes. These latitudes are often called Roaring Forties, Furious Fifties, and Shrieking Sixties—dreaded terms for sailors. The poleward boundary of the westerlies is highly fluctuating. There are many seasonal and short-term fluctuations. These winds produce wet spells and variability in weather. (Fig. 2.17)

2. Secondary or Periodic Winds:

These winds change their direction with change in season. Monsoons are the best example of large-scale modification of the planetary wind system. Other examples of periodic winds include land and sea breeze, mountain and valley breeze, cyclones and anticyclones, and air masses.

Monsoons were traditionally explained as land and sea breezes on a large scale. Thus, they were considered a convectional circulation on a giant scale. The Asiatic monsoons are characterised by seasonal reversal of wind direction. During summer, the trade winds of southern hemisphere are pulled northwards by an apparent northward movement of the sun and by an intense low pressure core in the north-west of the Indian sub­continent.

While crossing the equator, these winds get deflected to their right under the effect of Coriolis force. These winds now approach the Asian landmass as south-west monsoons. Since they travel a long distance over a vast expanse of water, by the time they reach the south-western coast of India, they are over-saturated with moisture and cause heavy rainfall in India and neighbouring countries.

During winter, these conditions are reversed and a high pressure core is created to the north of the Indian subcontinent. Divergent winds are produced by this anticyclonic movement which travels southwards towards the equator. This movement is enhanced by the apparent southward movement of the sun.

These are north-east or winter monsoons which are responsible for some precipitation along the east coast of India. The monsoon winds flow over India, Pakistan, Bangladesh, Myanmar (Burma), Sri

Lanka, the Arabian Sea, Bay of Bengal, south­eastern Asia, northern Australia, China and Japan. Outside India, in the eastern Asiatic countries, such as China and Japan, the winter monsoon is stronger than the summer monsoon. (Fig. 2.18)

The land and sea breezes affect only a narrow strip along the coast. During day-time, the land gets heated rapidly and develops a low pressure area over it. The warm air starts rising from there. The sea, on the other hand, is relatively cooler. It develops a higher pressure area which comes under the influence of divergent air flow.

This air moves towards the low pressure area developed over the land and replaces lighter, warmer air there. The sea breeze thus moderates the warm coastal areas during the day. These conditions are reversed during night. The land sends back heat through terrestrial radiation more rapidly than the sea and gets cooler than the neighbouring sea. A reverse air-flow from land towards sea is developed. This is land breeze. (Fig. 2.18)

Valley breeze and mountain breeze are mountainous counterparts of land and sea breezes. During day time the slope of the mountain is heated more than the valley floor. As such the air from the valley flows up the slope. This is known as valley breeze. After sunset, the pattern is reversed. Rapid loss of heat through terrestrial radiation along the mountain slopes results in sliding of cold dense air from higher elevations to valleys. This is called mountain breeze. (Fig. 2.18)

3. Tertiary or Local Winds:

Local differences of temperature and pressure produce local winds. Such winds are local in extent and are confined to the lowest levels of the troposphere. Some examples of local winds are discussed below.

In the plains of northern India and Pakistan, sometimes a very hot and dry wind blows from the west in the months of May and June, usually in the afternoons. It is known as loo. Its temperature invariably ranges between 45°C and 50°C. It may cause sunstroke to people.

Foehn is a hot wind of local importance in the Alps. It is a strong, gusty, dry and warm wind which develops on the leeward side of a mountain range. As the windward side takes away whatever moisture there is in the incoming wind in the form of orographic precipitation, the air that descends on the leeward side is dry and warm. The temperature of the wind varies between 15°C and 20°C. The wind helps animal grazing by melting snow and aids the ripening of grapes. Similar winds in USA and Canada move down the west slopes of the Rockies and are known as chinooks. It is beneficial to ranchers east of the Rockies as it keeps the grasslands clear of snow during much of the winter.

During winter, areas adjacent to highlands may experience a local cold wind which originates over the snow-capped mountains or highlands and blows down the valley. Mistral is one of the local names given to such winds that blow from the Alps over France towards the Mediterranean Sea. It is channelled through the Rhine valley. It is very cold and dry with a high speed.