On May 30 2017, a severe cyclonic storm ‘Mora’ crossed the Bangladesh coast in the forenoon, with a peak surface wind speed of 130 kmph (IMD, 2017). The cyclone claimed 9 lives in Bangladesh along with massive damage to property.

Mizoram reported heavy rainfall accompanied with strong gusts of wind that is reported to have damaged about 80 buildings, most particularly in Siaha district, where damages were reported at the district headquarters and the district hospital buildings. Cyclone Mora exhibited an example of the devastation that can be caused by strong wind flows.

The destructive effects of certain types of wind types, such as in hurricanes, are symbolically characterized in the Hindu myth of the wind God Vayu, who is seen as a destructive and intemperate God – a benevolent God in the earlier Vedic era. This change in the status of the God could possibly be attributed to the milder wind flows in the Steppes, from where the Aryans are said to have originally migrated. India’s coastal and tropical climate with devastating cyclones could have impacted the God’s impetuosity, although the matter is subject to speculation.

The Tower of the Winds, as homage to the various wind Gods in ancient Greece, was used to note the seasonal changes in wind patterns. The ancient Greeks had four wind Gods according to the four seasons, namely, Boreas (winter), Zephyros (spring), Notus (summer) and Euros (autumn). The divine keeper of the winds was Aiolos. Roman mythology retained the wind Gods of the Greeks, along with their designation to certain seasons.

In scientific terminology, winds are flows of gases that make up the atmosphere. It can be of many types and is attributed to the nature of forces producing them. Their scale and spatial dimensions, geophysical location, velocity and also their effects thus constitute the subject matter of study.

Early assumptions of winds were based on their utilities, such as for transport, or for mechanical uses such as in windmills, along with their general impact on the weather and climate. However, it was only after the development of the natural sciences that scientific assessments of wind patterns and mechanisms were analyzed.

However, certain types of winds are also utilized as wind energy for beneficial uses such as sailing, which served to establish trade routes due to global wind patterns, in windmills and for generating electricity.

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Mechanism of Wind Flows

Winds are horizontal movements of air in the atmosphere, in contrast to currents, which are vertical movements. Winds usually occur due to uneven distributions of pressure at a global climatic or local scale that wind movements act to balance.

A universal rule of thumb is that whenever an area of high air pressure exists adjacent to an area of low air pressure, the difference in pressure causes wind flows from the high pressure area to the low pressure area. Certain factors contribute to affect wind motion in terms of direction and speed. These are the pressure gradient, the Coriolis effect, friction and centripetal acceleration.

The pressure gradient force is the force generated when pressure differences impacts the intensity of wind flows from areas of high pressure to those of low pressure. The more closely spaced the pressure gradient, the more pronounced the pressure change, resulting in wind types with higher wind speeds.

The wind flows in the direction of the change in pressure, which is perpendicular to the isobars (areas with same air pressure), as shown in the Fig 1.

Fig 1: Wind flow perpendicular to the isobar due to the pressure gradient

However, the flow of wind is not exactly perpendicular to the isobar, but deviates somewhat, and this is due to the forces exerted by the Earth’s rotation on its axis from the west to the east. This deviation of wind flows due to the earth’s rotation is called the Coriolis effect.

In the Coriolis effect, according to Ferrel’s law, the winds in the Northern hemisphere get deflected to the right and in the Southern hemisphere get deflected to the left. The effect changes the direction of winds but not their speed. However the deflection tends to increase with an increase in wind velocity, mass and latitudinal position on the Earth, as shown in Fig 2.

Fig 2: Increase in amount of Coriolis effect with decrease in latitude

As the air rushing in towards the low pressure area moves towards the centre of rotation, and due to the Coriolis effect, the winds follow a curved path around a local axis that can be of high or low pressure. This phenomenon is due to the force of the centripetal acceleration of winds. The winds might also form different wind types due to another force – friction – that acts to offer resistance to wind motion due to the nature of the earth’s surface.

This frictional force can determine the angle of wind flows, wind velocities, as well as determine the direction of wind flows. Thus for example, while over the ocean, the lack of resistance due to friction can produce high surface wind speeds (P. Tiwari, 2017). The four constitute the four principal factors that contribute to the occurrence of winds and their various types, which shall be discussed below.

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Primary or Prevailing Winds

Primary or prevailing winds are types of winds that are consequent to global wind circulation patterns. Other than transporting warm, cold and moist air worldwide, winds can transport even airborne pollutants all over the globe.

The uneven heating of the Earth combined with the various factors mentioned above contribute to certain global systems of wind patterns whereby winds flow in consistent, steady flows. The Coriolis effect is a major determining factor in determining the direction of prevailing wind patterns globally, whereby winds flow eastwards or westwards to a degree determined by their latitudinal position. There are four principal wind types that qualify as primary or prevailing winds.

Trade winds – They are so called due to their usage by mariners in history in trade through sea routes due to their predictability and reliability. Also called the tropical easterlies, the direction of wind flows in trade winds are generally east to west and the wind flows in these types of winds are located between 0 to 30 degrees latitude at both hemispheres. In the Southern Hemisphere, they flow from southeast to north west, and in the Northern Hemisphere, they flow from northeast to southwest.

Mid-Latitude Westerlies – Occasionally referred to as just the westerlies, these wind types flow from west to east at between 30 to 60 degrees latitude in both hemispheres. The westerlies exhibit more variances and anomalies and are thus considered less reliable than the tropical easterlies. Both the westerlies and the  tropical easterlies contribute to impacting the flow of ocean currents.

Polar easterlies – These wind types flow east to west at the polar regions of the earth between 60 to 90 degrees latitude. The air carried by these winds is not as moist and is in fact cold and dry because of the low temperatures in these regions, particularly during the winter season. Sometimes however, the wind direction gets slightly deflected to flow southwestwards in the Northern hemisphere and northwestwards in the Southern Hemisphere (A. Harris, 2017).

Jet Streams – 9 to 16 km above the surface of the earth are jetstreams, which are ribbons of very strong winds reaching speeds of upto 200 mph that affect weather systems across the globe. They are caused by temperature differences between the tropical and polar masses of air. Jet streams can quickly connect one weather event in the globe with another and can lead to explosive deepening of depressions and other such air pressure changes (Met Office, UK, 2015).

There are mainly two jet streams – the polar jet stream and the subtropical jet stream. The polar jet stream flows over the polar and mid-latitudinal regions and the sub-tropical jet stream flows between 30 degrees North and South latitudes of the equator. The subtropical jet stream is weaker than the polar jet stream and is most activated over the western Pacific.

The Tropical Easterly Jet stream helps in the occurrence of the Indian summer Monsoons by providing areas northwards of the Indian Ocean with a deep layer of warm air and areas southwards with cold air.

Synoptic Winds

Large-scale events such as warm and cold fronts constitute weather phenomena that produce synoptic winds. The types of winds constituting synoptic winds include the geostrophic wind, the cyclostropic wind and the gradient wind.

In the Northern Hemisphere, due to the Coriolis effect wind flows are clockwise around high pressure areas and anti-clockwise around low pressure areas. When the wind flows are nearly parallel to the isobars due to the pressure gradient force and the Coriolis effect in balance, these wind types are called geostrophic winds.

These can be gradient winds which have strong curving motion influenced by the Coriolis effect with centrifugal force. When the wind flows are overwhelmed by centripetal force without much influence of the Coriolis effect, such types of winds are called cyclostropic winds. Cyclostrophic winds are responsible for producing extreme weather systems with circular wind flows such as cyclones and tornadoes.

Mesoscale Winds

Synoptic winds are forecastable. However there are other winds that arise and fade over short time periods and small-scale geographical locations such as thunderstorm winds that are difficult to predict. These types of winds are called mesoscale winds.

Microscale Winds

Microscale winds are minute both spatially and temporally compared to the aforementioned wind types and can range from a few meters and a few minutes. Larger wind patterns might consist of many microscale winds.

Local Winds

Certain local winds can also occur due to temperature disturbances. Land and sea breezes are due to differential heating. In locations where land and sea are in close proximity, land breezes flow from land to sea at the night-time and early morning due to quick release of warm air by land and slow release by sea at night. The reverse happens in the daytime to the evenings when sea breezes flow from sea to land due to warm air rising inland that is replaced by the cool sea breeze.

Another sort of local wind types are mountain and valley breezes. In a similar mechanism to land and sea breezes, where a upslope breeze known as valley breeze sweeps up mountains at the start of days due to warming of the mountain slopes. This returns to form mountain breezes by the afternoons as warming of the valley air leads to breezes flowing from the mountain slopes.