Atmospheric Pressure and Winds

Both A and R are true but R does not explain A.

Explanation — Variations in atmospheric pressure, caused by differences in temperature and air density, produce the horizontal movement of air called winds. Winds also transport heat and moisture from one region to another, influencing temperature, humidity, and precipitation. However, this describes the effects of winds and does not explain why winds are formed.

Pressure Gradient

Reason — Pressure gradient is the decrease in atmospheric pressure per unit distance in the direction where the pressure decreases most rapidly. It determines the speed at which air moves from a region of high pressure to low pressure.

Sub-Tropical High Pressure Belts

Reason — Sub-Tropical High Pressure Belts lie at about 30° North and South of the Equator, where the ascending Equatorial air currents descend and pile up, creating an area of high pressure. This belt is also called the Horse latitude.

Equatorial Low Pressure Belts

Reason — The Equatorial Low Pressure Belt extends from about 5°N to 5°S of the Equator. Intense heating by the vertical rays of the Sun causes the air to expand and rise, creating a low-pressure area. Since the rising air produces calm conditions with little or no horizontal wind, this belt is known as the Doldrums.

They have varying velocity and are irregular

Reason — Trade Winds are permanent (prevailing) winds that blow steadily from the Sub-tropical High Pressure Belts towards the Equatorial Low Pressure Belt. They have a fixed direction and fairly uniform velocity. They are called North-East Trades in the Northern Hemisphere and South-East Trades in the Southern Hemisphere.

Alps : Foehn :: Rockies : Chinook

Reason — Foehn is a strong warm and dry wind which descends the northern slopes of the Alps, where the air gets compressed and warmed and reaches the valley bottom as a dry, hot wind. A similar wind moving down the western slopes of the Rockies in the USA and Canada is known as Chinook, which literally means 'snow eater'.

Tornado

Reason — A tornado is a tropical cyclone which occurs over land. It looks like a dark, funnel-shaped cloud extending downward from the base of the thunderstorm, with low pressure in the centre and very strong winds blowing inwards at speeds as high as 500 to 800 km per hour.

Southern Hemisphere

Reason — In an anticyclone, there is high pressure in the centre and winds blow gently outwards. These winds are clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. As the winds in the picture are blowing outwards in an anticlockwise direction, the anticyclone is occurring in the Southern Hemisphere.

Jet Streams

Reason — Jet streams are the concentrated bands of rapid air movement found at the tropopause and the stratosphere, located at 10-15 kilometres above the surface of the earth. They are formed near boundaries of adjacent air masses with significant differences in temperature.

All of the above

Reason — All the given statements are true about jet streams. The position and strength of the jet stream helps meteorologists to forecast future weather events; the airline industry flies aircraft within the jet stream so that flight time and fuel consumption get reduced; and precipitation usually increases in California and Europe when the jet stream moves farther south or becomes stronger.

eye

Reason — The centre of a cyclone is a calm region called the eye. It is surrounded by a ring of violent winds and heavy rain known as the eye-wall. Within the eye, winds are light and there is little or no rainfall, resulting in a brief period of calm weather.

mistral

Reason — Hurricanes, typhoons, and willy-willies are different names for tropical cyclones in different regions of the world. Mistral, however, is not a tropical cyclone; it is a cold local wind that blows from the Alps across France towards the Mediterranean Sea.

Flying along a jet stream reduces flight time.

Reason — The major jet streams in the Northern Hemisphere are westerly winds that blow from west to east. Therefore, a flight from London to Delhi benefits from the jet stream and takes less time, whereas a flight from Delhi to London flies against it and takes longer. Airlines use jet streams to reduce flight time and fuel consumption.

The four main pressure belts of the earth are-

1. Equatorial Low Pressure Belts
2. Sub-tropical High Pressure Belts
3. Circum-polar Low Pressure Belts
4. Polar High Pressure Areas

These belts are located between 60° and 70° in each hemisphere. Due to the earth's rotation, the wind surrounding the Polar region blow towards the equator. Centrifugal forces operating in this region create the low pressure belt appropriately called Circum-polar Low Pressure Belt.

The Coriolis Force is caused by the Earth's rotation. It deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Its effect is zero at the Equator, increases with latitude, and is maximum at the poles.

The three chief types of wind are-

1. Permanent or Planetary winds
2. Periodic winds
3. Local winds

Certain winds reverse their direction periodically with season and are called periodic winds. These winds blow at regular intervals or in regular cycles. They are winds that result from localised differences in pressure and temperature.

There are certain winds in different parts of the world which flow in comparatively small area and have special characteristics. These are called local winds.

Two local winds are Loo and Mistral.

Two types of variable winds are-

1. Cyclones
2. Anticyclones

They are called variable winds because they do not blow in any definite direction and their direction varies with the movement of the pressure system. Their speed also depends on the intensity of the depression.

Cyclones require a sea surface temperature of about 26°C to develop and maintain their strength. They also need a high relative humidity. These conditions are only found in the tropical oceans during summer. Therefore, cyclones are frequent in summer in the tropical regions.

Cyclones are called tornadoes in North America, hurricanes in south-east Caribbean region, typhoons in eastern China, Japan and Philippines islands and cyclones in the Bay of Bengal and Arabian sea.

Two chief characteristics of anticyclones are-

1. There is high pressure in the core and low pressure around it. Winds blow gently outwards.
2. These winds are clockwise in northern hemisphere and anticlockwise in the southern hemisphere.

Pressure gradient is defined as the decrease in pressure per unit distance in the direction in which the pressure decreases most rapidly.

Contrasts in temperature cause changes in air density, which are responsible for variations in pressure. These variations cause horizontal movements of air called winds.

Coriolis force is the force generated due to rotation of the Earth on its inclined axis. Due to Coriolis force, moving objects (winds and water currents) tend to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflection is least at the Equator and greatest at the poles.

Altitude refers to the height of a place in relation to sea level or ground level.

Monsoons are periodic seasonal winds blowing in the regions of South East Asia and Northern Australia. They develop because of differences in heating conditions of the continent and the oceans.

Atmospheric pressure refers to the force per unit area exerted against a surface by the weight of the air above that surface.

The factors affecting atmospheric pressure are:

1. Altitude — Atmospheric pressure decreases with increase in altitude at the rate of about 1 cm of mercury for every 110 m of ascent. It is highest at sea level.
2. Temperature — Atmospheric pressure decreases with increase in temperature because warm air expands, becomes lighter, and rises, creating low pressure.
3. Water Vapour — Water vapour reduces atmospheric pressure because it is lighter than dry air. Hence, humid air is less dense and exerts lower pressure than dry air.
4. Rotation of the Earth — The Earth's rotation, together with unequal heating of the Earth's surface, helps in the formation of the planetary pressure belts by influencing the movement and distribution of air.

(i) The Westerlies in the Southern Hemisphere blow with greater force because there are fewer landmasses and more open oceans, resulting in less friction and stronger, more continuous winds.

(ii) There is a seasonal shifting of pressure belts due to the inclination of the Earth's axis and the revolution of the Earth around the Sun, which causes the overhead position of the Sun to shift between the Tropic of Cancer and the Tropic of Capricorn. As a result, the pressure belts migrate northward in June and southward in December, returning to their normal positions during the equinoxes.

(iii) As altitude increases, atmospheric pressure decreases because the air becomes less dense. The lower layers of the atmosphere are compressed by the weight of the air above them, making them denser and exerting greater pressure. On average, atmospheric pressure decreases by about 34 millibars for every 300 metres of ascent.

Below labelled diagram shows the pressure and wind belts of the earth:

The three chief types of winds are-

1. Permanent / Prevailing / Planetary Winds — Permanent winds blow throughout the year from one latitude to the other in response to the latitudinal differences in the air pressure. For example, Trade winds, the Westerlies and the Polar Easterlies.

2. Periodic Winds — Certain winds reverse their direction periodically with season and are called periodic winds. For example, Land breeze and sea breeze, summer monsoon and winter monsoon.

3. Local winds — There are certain winds in different parts of the world which flow in comparatively small area and have special characteristics. These winds are called local winds. For example, loo, foehn and chinook and mistral.

Some of the important types of local winds are-

1. Loo — A hot, dry wind that blows over the northern plains of India and Pakistan during summer afternoons. Its temperature usually ranges between 45°C and 50°C and it may cause sunstroke.

2. Foehn and Chinook — Foehn is a warm, dry wind that blows down the northern slopes of the Alps, helping to melt snow and hasten the ripening of grapes. A similar warm, dry wind blowing down the eastern slopes of the Rocky Mountains in the USA and Canada is called the Chinook.

3. Mistral — A cold, dry wind that blows from the Alps across France towards the Mediterranean Sea, often causing temperatures to fall below freezing.

(i) The Earth's rotation gives rise to the Coriolis force, which deflects moving air. As a result, winds are deflected to the right of their direction of motion in the Northern Hemisphere.

(ii) Temperature and pressure are inversely related because when air is heated, it expands, becomes less dense, and exerts lower pressure. When air cools, it contracts, becomes denser, and exerts higher pressure.

(iii) Humid air is lighter than dry air because water vapour (molecular weight 18) is lighter than the average molecular weight of dry air (about 29). As water vapour replaces heavier gases in the air, humid air becomes less dense and therefore lighter than dry air.

The Coriolis Effect (or Coriolis Force) is the apparent force caused by the rotation of the Earth on its axis, which deflects the direction of moving air.

It causes the planetary winds to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis effect is zero at the Equator and maximum at the poles.

Tropical cyclones-

1. Tropical cyclones develop over warm tropical seas.
2. They cover a comparatively small area and generally move from east to west.
3. Before the cyclone arrives, the weather is hot, humid and calm, with a sudden fall in atmospheric pressure.
4. As the cyclone approaches, strong winds, thick clouds, heavy rainfall and poor visibility occur.
5. The eye of the cyclone is a calm region with light winds and little or no rainfall.
6. After the eye passes, violent winds resume from the opposite direction, accompanied by heavy rain, before gradually weakening.

Temperate Cyclones-

1. Temperate cyclones develop over both land and sea in the temperate regions.
2. They cover large areas (about 1,000–2,000 km) and generally move from west to east.
3. Wind speed is moderate, though it may become strong near the fronts.
4. Different parts of the cyclone have different temperatures because of the presence of warm and cold fronts.
5. The approach of a temperate cyclone is marked by a fall in atmospheric pressure, wind shift, and often a halo around the Sun or Moon.
6. Light rain or drizzle begins ahead of the warm front and may later turn into heavy rainfall.
7. Rainfall is generally light to moderate and may continue for several days.

Jet streams are narrow bands of very fast-moving winds found near the tropopause, at an altitude of about 10–15 km above the Earth's surface.

The significance of jet streams is as follows-

1. They steer weather systems and help meteorologists forecast weather.
2. They are used by aircraft to reduce flight time and fuel consumption.
3. They influence the distribution of rainfall by directing storms and weather systems.
4. They have a major influence on global weather patterns and can contribute to floods, droughts, and other extreme weather events.

(i) Doldrums extend from about 5°N to 5°S of the Equator. Due to the vertical rays of the Sun, intense heating causes the air to expand and rise by convection, creating a low-pressure belt.

(ii) The Equatorial region receives vertical rays of the Sun throughout the year, causing intense heating. The heated air expands, becomes lighter, and rises continuously, creating low atmospheric pressure throughout the year.

(iii) The Circum-polar Low Pressure Belt is formed where the warm Westerlies meet the cold Polar Easterlies. The warm air rises over the cold air, creating a zone of ascending air and low atmospheric pressure.

Below labelled diagram shows a cyclone in the Northern Hemisphere:

Two situations when I physically felt the effect of the atmospheric pressure are:

1. I felt discomfort when I travelled by plane for a vacation. It happened because when travelling in a plane, changes in altitude cause variations in atmospheric pressure. The pressure inside the cabin differs from the pressure at ground level. These pressure differences can lead to discomfort, particularly in the ears, sinuses, and other air-filled cavities, which can be alleviated by equalizing the pressure.
2. On the vacation, I visited Ladakh. There my nose started bleeding due to decrease in atmospheric pressure at high altitude. As blood pressure exceeded the atmospheric pressure, it resulted in nose bleeding.

Cyclone Fani (2019)

Origin — Cyclone Fani developed from a low-pressure area in the Bay of Bengal in late April 2019. It intensified into an Extremely Severe Cyclonic Storm and made landfall near Puri, Odisha, on 3 May 2019.

Effects — It brought heavy rainfall, strong winds, and storm surges, causing extensive damage to houses, roads, power lines, agriculture, and infrastructure. Timely evacuation of millions of people helped reduce the loss of life, though there were casualties and heavy property damage.

Cyclone Yaas (2021)

Origin — Cyclone Yaas developed from a low-pressure area in the Bay of Bengal in May 2021 and made landfall near Balasore, Odisha, on 26 May 2021.

Effects — It caused heavy rainfall, strong winds, storm surges, and coastal flooding in Odisha and West Bengal, damaging houses, crops, and infrastructure. Large-scale evacuation reduced casualties, but there was widespread disruption.

Cyclone Tauktae (2021)

Origin — Cyclone Tauktae developed from a depression over the Arabian Sea in May 2021 and intensified into an Extremely Severe Cyclonic Storm before making landfall in Gujarat on 17 May 2021.

Effects — It caused heavy rainfall, strong winds, and storm surges along the coasts of Kerala, Karnataka, Goa, Maharashtra, and Gujarat, resulting in damage to infrastructure, loss of life, disruption of transport, and suspension of fishing and offshore operations.