Atmospheric Pressure and Winds

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

Explanation — Contrasts in temperature cause changes in air density, which are responsible for variations in pressure. These variations cause horizontal movements of air called winds. Winds transport heat and moisture from one region to another and thus help in the occurrence of precipitation and affect both temperature and humidity.

Pressure Gradient

Sub-Tropical High Pressure Belts

Equatorial Low Pressure Belts

They have varying velocity and are irregular

Alps : Foehn :: Rockies : Chinook

Tornado

Southern Hemisphere

Jet Streams

All of the above

eye

mistral

Flying along a jet stream reduces flight time.

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.

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.

Due to the rotation of the earth on its tilted axis, the wind starts deflecting to its right in the Northern Hemisphere. In the Southern Hemisphere, it starts deflecting to its left from its original path.

Thus, a wind blowing from the north becomes north-easterly in the northern hemisphere and the wind blowing from south becomes south-easterly in the southern hemisphere.

The Coriolis effects is greatest near the poles and zero at the equator because of the earths spherical shape. It is proportional to the angle of the latitude.

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.

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

The factors that affect Atmospheric Pressure are:

1. Altitude — Atmospheric Pressure decreases with increase in altitude at rate of 1 cm of mercury for every 110 m of ascent. Atmospheric Pressure is highest at sea level.
2. Temperature — Atmospheric Pressure decreases with increase in temperature.
3. Water Vapour — Water Vapour concentration affects atmospheric pressure because molecular weight of water (18 g/mol) is less than the average molecular weight of air (about 29 g/mol). Therefore, humid air is less dense and exerts less pressure than dry air.
4. Rotation of the Earth — Due to rotation of earth, bulk of the air at the poles is thrown away towards the Equator. This creates low pressure in Equatorial regions as they are hot whereas at the poles, the cold heavy air sinks down and creates high pressure. In fact, temperature and rotation of the earth together contribute to the formation of world pressure belts.

(i) The Westerlies in the Southern Hemisphere blow with greater force than those in the Northern Hemisphere because there is absence of landmass in the southern hemisphere.

(ii) The causes of shifting of pressure belts are inclination of Earth's axis and change in relative position of Earth with respect to the Sun -

1. At the time of Summer solstice the Sun is vertical over the Tropic of Cancer and therefore, all the pressure belts except the northern polar high pressure belt shift northward.
2. After Autumn Equinox, there is southward migration of the Sun which becomes vertically over the Tropic of Capricorn at the time of Winter Solstice and therefore the pressure belt shifts southward except in the southern polar high pressure belt.
3. The Sun is overhead over the Equator at the time of Spring Equinox and Autumn Equinox and hence all the pressure belts occupy their normal positions.

(iii) As we go higher upwards, the atmospheric pressure decreases at an average rate of 34 millibars per 300 metres of latitude. It is because the air in the lower layer of the atmosphere is denser than at higher level. The air is denser in lower layer because of gravitational pull of earth and also because the lower layers are compressed under upper layers of atmosphere.

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

2. Foehn and Chinook — Foehn is a dry, hot wind which blows in the valley on the northern slopes of Alps. The temperature of the wind is from 15°C to 20°C. The wind is of use for melting snow and it hastens the ripening of grapes.

Similar kind of wind in the USA and Canada move down the west slopes of the Rockies and are known as Chinook.

3. Mistral — Mistral is a local cold wind that blows from the Alps over France towards the Mediterranean Sea. Mistral brings down the temperature below freezing point.

(i) The rotation of the earth produces another force other than the pressure force which is called the Coriolis force. It causes deflection of winds. Thus, the winds are directed to the right of their flow in the Northern Hemisphere due to Coriolis effect.

(ii) Temperature and pressure are inversely related to one another as atmospheric pressure decreases with increase in temperature. This is because when the temperature rises, air expands. The molecules of air move far apart and hence exert less pressure. On the contrary, with decrease in temperature, the air gets compressed and the space between molecules decreases and exerts more pressure on the region.

(iii) Humid air is lighter than dry air because the molecular weight of water is less than the average molecular weight of air. When water evaporates and enters the atmosphere as a gas, the water vapour molecules take the place of other gas molecules in the air. So, the volume of wet air weighs less than an equal volume of dry air.

Coriolis effect or Coriolis force is the force generated due to rotation of the Earth on its inclined axis. Due to Coriolis effect, planetary winds 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.

Tropical cyclones-

1. Tropical cyclones are mainly developed over the sea.
2. They are usually limited to a small area and usually travel from east to west.
3. When there is a tropical cyclone the air is still, but the temperatures and the humidity are high. There is sudden drop in air pressure.
4. When the front of the vortex arrives, there are strong winds and thick clouds, then the winds become violent with great speed. Dense clouds and heavy rain reduce visibility.
5. The eye of the cyclone heralds a calm condition.
6. When the rear of the vortex arrives, there are violent winds, thick clouds and heavy rains. The wind blows in the opposite direction.

Temperate Cyclones-

1. Temperate cyclones are produced both on land and on sea.
2. They occupy areas measuring a couple of thousands of kilometres and move from west to east.
3. In a temperate cyclone, the wind speed is low.
4. All sectors of the cyclone have different temperatures.
5. There is not a single place where the winds and rains are inactive.
6. Direction of winds rapidly change at the front.
7. The approach of a temperate cyclone is marked by fall in temperature, fall in the mercury level, wind shifts and a halo around the sun and the moon.
8. A light drizzle follows which turns into a heavy downpour.
9. They cause light rainfall which continues for many days.

Jet streams refer to the concentrated bands of rapid air movement found at the tropopause and the stratosphere, located at 10-15 km above the surface of the Earth.

The significance of jet streams is as follows-

1. Most weather systems are moved forward with the jet streams. The position and strength of the jet stream then helps meteorologists forecast future weather events.
2. The airline industry consistently uses the jet stream for its flights, which reduces the flight time and aids in reduction in fuel consumption.
3. Precipitation usually increases in California because the polar jet stream moves farther south and brings more storms with it. In addition, precipitation often increases in Europe because the jet stream is stronger in the Northern Atlantic.
4. Whatever the position of the jet stream, it has a significant impact on the world's weather patterns and severe weather events like floods and droughts.

(i) Doldrums extend from 0 to 5° North and South of Equator. Due to the vertical rays of the sun here, there is intense heating. The air expands and rises as convection current, causing a low pressure to develop here. Hence, Doldrums is a low pressure belt.

(ii) Equatorial regions have low atmospheric pressure throughout the year because atmospheric pressure decreases with increase in temperature. This is because when the temperature rises, air expands. The molecules of air move far apart and hence exert less pressure. On the contrary, with decrease in temperature, the air gets compressed and the space between molecules decreases and exerts more pressure on the region.

(iii) In the sub-tropical region the descending air gets divided into two parts. One part blows towards the Equatorial Low Pressure Belt. The other part blows towards the Circum-Polar Low Pressure Belt. This zone is marked by ascent of warm Sub-Tropical air over cold polar air blowing from poles. Due to earth's rotation, the winds surrounding the Polar region blow towards the Equator. Centrifugal forces operating in this region create the low pressure belt, known as Circum-polar low pressure belt.

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

Origin — Cyclone Fani originated from a low-pressure system that formed in the Bay of Bengal in late April 2019. It intensified into a severe cyclonic storm and made landfall near Puri, Odisha, India, on May 3, 2019.

Effects — Cyclone Fani caused widespread destruction in Odisha and parts of West Bengal. It resulted in heavy rainfall, strong winds and storm surges, causing significant damage to infrastructure, including buildings, roads and power lines. The storm led to the evacuation of millions of people from coastal areas, minimizing the loss of life. However, there were still casualties and extensive damage to property and agriculture.

Cyclone Yaas

Origin — Cyclone Yaas originated from a low-pressure area in the Bay of Bengal in May 2021. It rapidly intensified into a severe cyclonic storm and made landfall near Balasore, Odisha, on May 26, 2021.

Effects — Cyclone Yaas brought heavy rainfall, strong winds, and storm surges to coastal areas of Odisha and West Bengal. It caused significant flooding, damaging houses, infrastructure, and agriculture. The storm led to the evacuation of millions of people from vulnerable regions, minimizing the loss of life. However, there were still reports of casualties and widespread disruption to daily life.

Cyclone Tauktae

Origin — Cyclone Tauktae originated from a depression over the Arabian Sea in May 2021. It rapidly intensified into an extremely severe cyclonic storm and made landfall near Gujarat, India, on May 17, 2021.

Effects — Cyclone Tauktae brought heavy rainfall, strong winds and storm surges to the coastal areas of Gujarat, Maharashtra, Kerala, Karnataka and Goa. The storm caused widespread damage to infrastructure, including buildings, power lines and communication networks. It resulted in casualties and displacement of people along with significant disruptions to transportation and economic activities. The cyclone also affected offshore activities, including fishing and oil drilling operations.