What is WIND? and Types of WINDS

Air may not seem like anything at all; in fact, we look right through it all the time, but during a windstorm, air really makes its presence known. Wind is able to lift roofs off buildings, blow down power lines and trees, and cause highway accidents as gusts push around cars and trucks.

Wind is moving air and is caused by differences in air pressure within our atmosphere. Air under high pressure moves toward areas of low pressure. The greater the difference in pressure, the faster the air flows.

The Fastest Winds

In 1934, on the roof of a little wooden building atop Mount Washington, in New Hampshire, an instrument to measure wind speed, called an anemometer, made history. It recorded a wind speed of 231 miles per hour (mph) during a huge spring storm, the fastest wind gust ever recorded with the instrument!

More recently, sophisticated Doppler radar has been used to measure winds, recording a wind speed of 318 mph in an Oklahoma tornado in 1999. That’s faster than the top speeds of Japanese bullet trains and over three times quicker than the fastest baseball pitch.

Describing Wind

Wind is described with direction and speed. The direction of the wind is expressed as the direction from which the wind is blowing. For example, easterly winds blow from east to west, while westerly winds blow from west to east. Winds have different levels of speed, such as “breeze” and “gale”, depending on how fast they blow. Wind speeds are based on the descriptions of winds in a scale called the Beaufort Scale, which divides wind speeds into 12 different categories, from less than 1 mph to more than 73 mph.

To understand what makes the wind blow, we first need to understand what atmospheric pressure is. Pressure at the earth's surface is a measure of the 'weight' of air pressing down on it. The greater the mass of air above us, the higher the pressure we feel, and vice-versa. The importance of this is that air at the surface will want to move from high to low pressure to equalise the difference, which is what we know as wind.

So wind is caused by differences in atmospheric pressure - but why do we get these differences? It's down to the rising and sinking of air in the atmosphere. Where air is rising we see lower pressure at the earth's surface, and where it's sinking we see higher pressure. In fact if it weren't for this rising and sinking motion in the atmosphere then not only would we have no wind, but we'd also have no weather.

Small scale winds

This rising and sinking of air in the atmosphere takes place both on a global scale and a local scale. One of the simplest examples of a local wind is the sea breeze. On sunny days during the summer the sun's rays heat the ground up quickly. By contrast, the sea surface has a greater capacity to absorb the sun's rays and is more difficult to warm up - this leads to a temperature contrast between the warm land and the cooler sea.

As the land heats up, it warms the air above it. The warmer air becomes less dense than surrounding cooler air and begins to rise, like bubbles in a pan of boiling water. The rising air leads to lower pressure over the land. The air over the sea remains cooler and denser, so pressure is higher than inland. So we now have a pressure difference set up, and air moves inland from the sea to try and equalise this difference - this is our sea breeze. It explains why beaches are often much cooler than inland areas on a hot, sunny day.

Large scale winds

A similar process takes place on a global scale. The sun's rays reach the earth's surface in polar regions at a much more slanted angle than at equatorial regions. This sets up a temperature difference between the hot equator and cold poles. So the heated air rises at the equator (leading to low pressure) whilst the cold air sinks above the poles (leading to high pressure). This pressure difference sets up a global wind circulation as the cold polar air tries to move southwards to replace the rising tropical air. However, this is complicated by the earth's rotation (known as the coriolis effect).

Air that has risen at the equator moves polewards at higher levels in the atmosphere then cools and sinks at around 30 degrees latitude north (and south). This leads to high pressure in the subtropics - the nearest of these features that commonly affects UK weather is known as the Azores high. This sinking air spreads out at the earth's surface - some of it returns southwards towards the low pressure at the equator (known as trade winds), completing a circulation known as the Hadley Cell.

Another portion of this air moves polewards and meets the cold air spreading southwards from the Arctic (or Antarctic). The meeting of this subtropical air and polar air takes place on a latitude close to that of the UK and is the source of most of our weather systems. As the warm air is less dense than the polar air it tends to rise over it - this rising motion generates low pressure systems which bring wind and rain to our shores. This part of the global circulation is known as the mid-latitude cell, or Ferrell Cell.

Another important factor is that the coriolis effect from the earth's rotation meaning that air does not flow directly from high to low pressure - instead it is deflected to the right (in the northern hemisphere - the opposite is true in the southern hemisphere). This gives us our prevailing west to southwesterly winds across the UK.

TYPES OF WINDS

1)Planetary winds

The general distribution of winds throughout the lower atmosphere is known as planetary winds. Confined within some latitudinal belts, these winds blow rather regularly throughout the year and are basically controlled by the latitudinal pressure belts. The main planetary winds are (i) the North-east and the South-east Trade winds (ii) the Temperate Westerlies and (iii) the Polar Easterlies, which blow from the polar high pressure area to the temperate low pressure area.

Trade winds: Trade winds blow in a belt lying between 5°N-30°N in the northern hemisphere and 5°S-30°S in the southern hemisphere. From the equator ward side of the Sub-tropical highs in the Northern hemisphere air flows towards the Equatorial low but it is deflected right according to Ferrle’s law and as a result instead of blowing as northerly wind, it bends westward to become North-east trade winds.

In the Southern hemisphere winds originating from the Sub-tropical high pressure and blowing towards the Equatorial low pressure are similarly deflected westward to become the prevailing South-east trades.

Trade winds are noted for their consistency, both in force and direction in many areas especially over open seas and derive their name from the nautical expression ‘to blow trade’ meaning ‘to blow along a regular track’. Zones of sub-tropical highs in latitudes about 30°-35°N and 30°-35°S are areas of descending air and are characterize by calms light variable winds, comparatively dry air and quiet, stable weather conditions. These zones of latitudes are called Horse latitude.

The Westerlies: The Westerly winds blow across latitudes 35°-60° of both hemispheres. The air streams flowing pole wards from the Sub-tropical high pressure areas deflects eastward in the Northern he sphere to form South-westerlies. Similar winds in the Southern hemisphere are known as North-westerlies. Unlike the trade winds, the westerlies are very variable in force and direction especially in the Northern hemisphere.

In the Southern hemisphere, on the other hand, the Westerlies blow with great strength and regularity throughout the year over the almost uninterrupted expanse of ocean and have given the name Roaring forties to the region specially between latitudes 40°S and 50°S. Sometimes the name is applied to the winds themselves as they give a roaring sound on account of high speed.

The Polar easterlies: The Polar easterlies blow from the Polar high pressure area to the Temperature low pressure area. On their equator ward journey they are deflected westward to become North easterlies in the Northern hemisphere and South easterlies in the Southern hemisphere.

2)Monsoon winds:

The type of wind system in which there is a complete or almost complete reversal of prevailing direction from season to season is known as the monsoon winds. The largest and best developed monsoonal area in the world is the SE Asia including India. During the summer (April to September) the interior of the continents is intensely heated. This creates a low pressure into which winds are drawn from over the cooler surrounding oceans.

In India South-west monsoon wind blows in summer. As the air from over the oceans is very moist, it results in heavy rainfall. During the winter (October to March) the continental interior becomes much cooler than the surrounding oceans; the wind direction is thus reversed, blowing from the continental high pressure to the low pressure over the oceans. This creates the NE monsoon over India, which is generally a cool and dry wind.

3)Cyclone and Anticyclone:

A cyclone is a region of low atmospheric pressure and associated storm system, of which there are two types. The first, characteristics of temperate latitudes, is now usually referred to as a depression. The second in general covers a smaller area and typical of the tropics, is usually called a Tropical cyclone. The Tropical cyclones are generally violent with wind speeds as high as 120-280 km/hr.

Anticyclone is a region of atmospheric high pressure and associated with light winds. Both cyclones and anticyclones are manifested by circular oval or wedge shaped arrangement of isobar with lowest pressure at the centre in case of cyclones and highest pressure at the centre in case of anticyclones. The law governing here known as Buys Ballot’s law, states that in the northern hemisphere winds move in an anticlockwise direction around the centre of low pressure and clockwise around centers of high pressure; in the southern hemisphere the reverse is true. Both cyclones and anticyclones move as systems, cyclones moving much faster than the anticyclones. As a result the wind directions at a place shift with the passage of a cyclone or an anticyclone.

4)Local winds:

Winds, caused by local factors and confined to a limited area compared to planetary winds, are called local winds. Some well known examples of local winds are given below:

Land breeze: It is a common local wind that affects only coastal area. During the night the land becomes very much cooler than the sea as land is quickly chilled than the sea. The air adjacent to the surface is also chilled with the result that there is a marked high pressure over land. Thus the cooler, heavier, denser air over the land flows towards the sea and land breeze occurs.

See breeze: Unlike land breeze it blows the opposite way from the sea to the land. During the day land becomes quickly heated compared to sea with the result there is a marked low pressure over the land. Thus air is drawn into the land from the comparatively high pressure area of the adjacent seas or oceans. The southerly sea breeze in summer is highly welcome in Kolkata.

Fohn: Fohn is a warm dry wind that blows down the valleys of the north facing slopes of the Alps and is most common in spring and autumn. It occurs when a depression to the north of the Alps draws air from the south over the mountains.

Chinook: Chinook is a warm dry southwesterly wind, similar to the Fohn in origin and character. It need be pointed out here that the Chinook or the Fohn is relatively much warmer than the air into which it is invading but in the absolute sense it may be even cold at times below freezing point of water.

Tornado: Tornado also sometimes known as twistu in the U.S.A. is a violently rotating storm in which winds whirl around a small area of extremely low pressure.

Some other important local winds: 

Harmattan is a very dry and dust laden West African wind that blows south-east from the Sahara to the coast especially between Octobers to February. Its incursion into the coast of Gulf of Guinea gives relief to stifling humidity.

The sirocco, a hot dry south wind blowing from the Sahara to the Mediterranean, is most unpleasant. In Egypt this wind is called khamsin.

The mistral and bora are cold north winter winds. Mistral is experienced in southern France, especially in the Rhone delta and bora blows down the mountains to the east coast of Adriatic Sea and North Italy.

Norwester is a squall occurring during hot season (April-June) in North India and may bring rain or hail.

The Loos are hot and dry summer westerly winds that sweep the Upper and Middle Ganga Plains.