• Our atmosphere envelops Earth with various gases, each possessing weight. The pressure exerted by the cumulative weight of these gases is termed atmospheric pressure.

Atmospheric Pressure

• Atmospheric pressure, or air pressure, is defined as the total weight of a column of air above a unit area at sea level. At sea level, this pressure is equivalent to a 76 cm high mercury column, denoted as 1013.25 mb.
• Maximum air pressure is experienced at sea level, where a 1 cm³ cube weighs approximately 1 kg (1034g). The unit of air pressure is the millibar (mb), with its SI unit being the Pascal.
• Air pressure is measured using various instruments, including the normal barometer, Mercury barometer (Fortin barometer), Aneroid barometer, and Micro barograph.

Barometer

• A barometer, invented by Italian scientist Evangelista Torricelli in 1643, is a device used to measure air pressure, also known as barometric pressure. As gravity pulls air toward Earth, it exerts pressure on everything it touches. Barometers are instrumental in quantifying this pressure.
• The horizontal distribution of mean sea level pressure is represented using isobars, connecting locations with equal pressure. When isobars cluster above the normal value in a circular manner, it indicates a high-pressure region.
• When isobars cluster below the normal value, it indicates a low-pressure region.
• A ridge occurs when high pressure extends outward into a predominantly low-pressure region, while a trough results from low pressure extending into a region of high pressure.
• An area of nearly uniform pressure situated between two highs and two lows is referred to as a col.
• Lines connecting places with equal barometric tendency, where the pressure rises or falls by the same amount in a given period, are termed isobars.

Isobar

• Isobars are lines linking places with equal pressure. The prefix 'Iso' signifies 'same,' so isobar denotes the same pressure. Close-together isobars on a map indicate significant pressure changes over a short distance, while widely spaced isobars suggest pressure changes over a larger distance
•  Wind moves from high-pressure areas to low-pressure areas. The distribution of air pressure is influenced by factors such as altitude, temperature, Earth's rotation, water vapor, and air circulation.
• The rate of pressure change per unit horizontal distance is termed pressure gradient.

Distribution of Air Pressure

• The distribution of air pressure can be categorized into two types: vertical distribution and horizontal distribution.

Vertical Distribution

• In the lower atmosphere, pressure decreases rapidly with height
• Atmospheric pressure decreases at a rate of approximately 1 millibar for every 10 meters increase in elevation, although this rate may vary.
• The pressure gradient is defined as the decrease in pressure per unit distance in the direction where pressure decreases most rapidly.

Horizontal Distribution

• The horizontal movement of air near the Earth's surface is referred to as wind. The study of the horizontal distribution of air pressure involves plotting isobars at constant levels.
• The horizontal distribution of atmospheric pressure is not uniform globally; it varies across different times and locations. 
• The key factors influencing horizontal air pressure include:
• The Temperature of Air Earth experiences uneven heating due to the unequal distribution of insolation. Differential heating and cooling of land and water surfaces create an inverse relationship between air temperature and air pressure, leading to regional pressure variations.
• The Rotation of the Earth The Earth's rotation results in the convergence and divergence of air. Convergence leads to low pressure, while divergence results in high pressure.
• Presence of Water Vapor Higher quantities of water vapor in the air contribute to low pressure, whereas lower quantities result in high pressure.

Pressure Belts

• Atmospheric pressure, influenced by latitudinal variation, temperature, and Earth's rotation, exhibits a general pattern across the globe.
• Earth can be divided into regions with broader pressure belts, despite regional anomalies or deviations.
• There are seven pressure belts globally, alternating between high and low pressure in the Northern and Southern hemispheres, along with a common equatorial low-pressure belt.

Four major pressure belts on Earth are outlined below:

Equatorial Low Pressure Belt (Thermal in Origin):

• Located on either side of the geographical Equator.

Extends between 5°N and 5°S latitudes, with a potential shift to higher latitudes during respective hemisphere summers.

• Represents the convergence zone of North-East and South-East trade winds.
• Also known as doldrums due to frequent calm conditions caused by intense heat leading to low pressure.

Doldrum:

• The equatorial low-pressure belt is located between latitudes 10°N and 10°S.
• Width fluctuates between 5°N to 5°S and 20°N to 20°S.
• Convergence of trade winds from the two hemispheres sub-tropical high-pressure belts results in calm air movement, earning it the name Doldrum.

Subtropical High Pressure Belt (Dynamic in Origin):

• Extends between latitudes 25° to 35° in both hemispheres.
• Convergence of winds at higher altitudes leads to the subsidence of air from higher altitudes.
• Characterized by anticyclonic conditions, causing atmospheric stability and aridity due to the descent winds contracting their volume.
• The zone is named horse latitude due to frequent calm conditions. Dynamic in origin.

Sub-Polar Low Pressure Belt

• Extends between latitudes 60° to 65° in both hemispheres.
• The sub-polar low-pressure belt is more developed and regular in the Southern hemisphere, while it is fragmented in the Northern hemisphere due to the dominant presence of water in the former.
• Well-defined low-pressure centers or cells are observed over oceans in the Northern hemisphere, such as in the vicinity of the Aleutian Islands in the Pacific Ocean and between Greenland and Iceland in the Atlantic Ocean.

Polar High Pressure Belt

• Persistent high pressure at the poles throughout the year due to very low temperatures.
• Both thermal and dynamic factors operate at the poles, with high pressure attributed to the extremely low temperatures.

Shifting of Pressure Belts

• Pressure belts typically shift with the position of the overhead Sun.
• In the Northern hemisphere, pressure belts move northward in summer and southward in winter.
• Diurnal changes in thermal conditions influence pressure conditions, leading to regional shifts in wind direction.

Atmospheric Circulations

• Atmospheric circulation refers to the large-scale flow of air, along with ocean circulation, facilitating the transfer of thermal energy to the Earth.
• The structure of atmospheric circulation remains consistent annually, despite variations in specific details. Atmospheric circulation is classified into three types:
• South Westerlies
• North-East Trade Winds
• Primary or General Circulation:
  • Primary winds, also known as planetary, permanent, global, invariable, or prevailing winds, blow constantly throughout the year.
  • Three types of primary winds: Trade Winds, Westerlies, and Easterlies.
  • Secondary Circulation This circulation is also Called seasonal, periodic, variable, and regional winds, these winds change direction with different seasons, exemplified by monsoons, characterized by a seasonal reversal of wind direction.
  • Tertiary Circulation: Formed due to local pressure gradients resulting from differences in the Earth's surface heating and cooling, tertiary winds, such as Harmattan and Chinook, blow only during specific periods in a small area, confined to the lower levels of the troposphere.

Atmospheric Tricellular Circulation:

• Explains the meridional circulation of the atmosphere, commonly known as the Tricellular Meridional Circulation. Surface winds move from high to low-pressure areas, while upper atmospheric winds move in the opposite direction, forming a cellular circulation system that also influences ocean water movement, affecting Earth's climate.

Three Cells Based on Thermal and Dynamic Factors:

• Hadley Cell: Located between 10° to 30° latitudes, a thermally induced cell with rising air along the Equator, leading to upper air around 30° latitude, sinking, and causing sub-tropical high pressure. Associated with tropical monsoons and desert climates.
• Ferrel Cell: Extending from 30 to 60° latitude, a thermally indirect cell induced by dynamic forces, with warm air ascending from the polar front and subsidence in the horse latitudes. Completed by Westerly winds on the surface.
• Polar Cell: Extending from 65° to 90° latitudes, a thermally direct cell strongest during winter, with sinking air along the poles moving towards sub-polar lows as the Easterly wind. Interaction of Easterly and Westerly winds in sub-polar lows leads to rising air, completing the polar cell circulation.

Walker Cells:

• In the Southern hemisphere, a horizontal air circulation cell known as the walker cell, responsible for upwelling along the South American Coast and bringing rains to Australia. Resulting from differences in surface pressure and temperature over the Western and Eastern tropical Pacific Ocean, creating a pressure gradient from East to West and a circulation from the Eastern Pacific to the Western Pacific.

Winds

• Wind, or moving air, results from differences in air pressure within the atmosphere. It flows from high-pressure areas to low-pressure areas, and the greater the pressure difference, the faster the air moves.
• Wind is characterized by its direction and speed. The direction is expressed as the direction from which the wind is blowing, such as Easterly winds moving from East to West and Westerly winds moving from West to East.
• Various factors affect wind speed and movement:
  • The Sun: Unequal heating of the Earth's surface by the Sun plays a crucial role in determining wind speed and movement.
  • Pressure Gradient: When there is a pressure differential between two regions, air moves from higher pressure to lower pressure due to the pressure gradient force.
  • Coriolis Effect: Earth's rotation causes objects to deflect to the right in the Northern hemisphere and to the left in the Southern hemisphere. The Coriolis effect influences the direction of movement but not the speed.
  • Friction: The force that affects wind direction, mainly in the lower troposphere, slowing wind movement and reducing the impact of the Coriolis effect.
  • Centripetal Force: This force applies when isobars are curved, as in cyclones, pulling air inward along with pressure gradient and Coriolis forces, creating the gradient wind.

Types of Winds:

• Prevailing or Permanent/Planetary Winds
• Seasonal Winds
• Local Winds
• Monsoon Winds
• Land and Mountain Sea Breezes and Valley Breezes
• Trade Westerlies Winds
• Polar Easterlies
• Hot Winds
• Cold Winds

Planetary Winds:

These winds blow almost consistently in the same direction throughout the year and include:

• Trade Winds or Easterlies: Blow from sub-tropical high-pressure areas toward equatorial low-pressure areas, deflected by the Coriolis effect.
• Doldrums: Calm weather region in the InterTropical Convergence Zone (ITCZ) centered on the Equator, dominated by low pressure and rising air.

Westerlies

• They blow from the horse's latitude (high pressure belt) to the temperate low pressures. In the Northern hemisphere, they are deflected to the right to become South Westerlies and in the Southern hemisphere, they are deflected left to become North Westerlies.
• They blow strongly and more constantly in the Southern hemisphere because uniform water expanses predominates landmasses

• Westerlies, known as the roaring forties, furious fifties, and shrieking sixties in the Southern Hemisphere, also play a role in the formation of extra-tropical cyclones.

Polar Easterlies:

• These winds move from polar high-pressure regions to sub-polar low-pressure regions.
• In the Northern Hemisphere, they blow from North-East to South-West, and in the Southern Hemisphere, from South-East to North-West.
• Characterized by extreme coldness, stability, and low moisture content, causing precipitation mainly in the form of snow.
• Unimpeded by barriers, they cover long distances, influencing the climates of the regions they traverse.

Seasonal Winds:

• Winds changing their direction with the shifting seasons are termed seasonal winds.
• Includes monsoon winds, which alter their direction based on seasonal changes.

Monsoon winds

• Monsoon winds blow from sea to land in summer and from land to sea in winter due to differential heating of land and water.
• Originating in the trade wind belt between the Tropic of Cancer and the Tropic of Capricorn, these winds are prominent in South and Southeast Asia.

Land and Sea Breezes:

• During daytime, land heats up faster than the adjacent sea, creating low pressure over land and high pressure over the sea.
• The pressure gradient induces air movement from the high-pressure sea to the low-pressure land, known as a sea breeze.
• At night, land cools rapidly, becoming cooler than the sea, resulting in a reverse pattern known as land breeze.

Mountain Breeze and Valley Breeze

• In mountaineous regions, during the day the slopes get heated up and as a result the air moves upslope and to fill the resulting gap. The flow of air from the valley blows up the valley is known as the valley breeze
• During the night, the slopes get cooled and the dense air descends into the valley as the mountain wind.

Local Winds

• The local winds blow due to local variation in the
• temperature and pressure influence a very small area.
• These local winds blow in the lower layers of the troposphere.

Hot Winds

•  Hot winds are winds characterised by intense heat and low relative humidity.
• Some important hot winds are explained below.

Cold wind

• Cold winds are winds that induce a cold wave due to their temperature being below the freezing point.
• Mistral: A cold local wind in Spain and France blowing from North-West to South-East, causing a sudden drop in air temperature below freezing point.
• Bora: An extremely cold and dry North-Easterly wind along the shore of the Adriatic sea.
• Blizzard: Affecting polar regions of Canada and the USA, causing temperatures to drop below freezing point.
• Norte: The polar wind in Central America in winter, known as Northern or Northern in the Southern US.
• Gregale: Winter winds in the Central part of the Mediterranean region of Southern Europe.
• Maestro: North-Westerly winds in the central part of the Mediterranean region.
• Pampero: Cold polar winds blowing fast in the Pampas region of South America.
• Levanter: A strong Easterly cold wind in Southern Spain.
• Papagayo: Fast blowing North-Easterly cold and dry winds in the coastal region of Mexico.
• Ponant: Cold Westerly winds in the Mediterranean region, especially the Corsica coast.
• Puna: A cold local wind in the Andes region.
• Virasen: Sea breezes along the Western coasts of Peru and Chile.
• Southern Buster: A fast blowing cold and dry wind in New South Wales (Australia).
• Bise: An extremely cold and dry wind in France and Switzerland.
• Levanter: A strong Easterly cold wind in Southern Spain.
• Joran: A cold wind from Jura mountains (Switzerland) to Lake Geneva (Italy).

Jet Stream:

• The jet stream is a region of long, narrow, high-speed winds that flow in North-Eastward, Eastward, and South-Eastward directions in the middle and upper troposphere or lower stratosphere. 
• Jet streams can have speeds of up to 90 m/second (200 miles/hour) along hundreds of kilometers at their centers, but velocities fall off very sharply both laterally and vertically.
• Jet streams, characterized by high speeds, are confined to narrow bands at heights ranging between 6 and 14 km.
• The Earth's rotation plays a crucial role in the formation of jet streams. Due to the Earth's rotation, with the Equator moving faster than the poles, upper air flows from the Equator to the pole in an easterly direction to conserve momentum, resulting in an increase in speed towards the pole.
• There are various types of jet streams:
  • Polar Frontal Jet Streams: Located between 40° and 60° latitude, where tropical and polar air masses interact. The polar front jet stream exhibits irregularities in its longitudinal section and is discontinuous.
  • Westerly Sub-tropical Jet Stream: Running from West to East, it is more persistent over India and Africa during the summer.
  • Easterly Tropical Jet Stream: Found in the upper troposphere.
  • Polar Night Jet Streams (Stratospheric Sub-polar Jet Stream): Present in both hemispheres, developing above the troposphere during winter nights.

Effects of Jet Streams:

• Heat Balance: Jet streams play a crucial role in maintaining heat balance through the movement of air.
• Temperate Cyclone Paths: Jet streams impact the paths of temperate cyclones.
• Precipitation: The influence of jet streams affects precipitation patterns.
• Air Mass Movement: Jet streams influence the movement of air masses, potentially leading to droughts or floods.

• Aviation: Aviators utilize jet streams for efficient flying by aligning with the direction of the jet stream flow and avoiding them when flying in the opposite direction.

Prelims facts

• In which season the air pressure is lowest?

-Summer Season [UPPSC (Pre) 2014]

• Due to which meteorological factor does the air pressure -The change in air densitychange?

 (MPPSC (Pre) 2012]

• What is the atmospheric pressure at sea level?

-1013.2 millibar (MPPSC (Pre) 2012]

• What is shown with the help of isobars?

-Pressure (JPSC (Pre) 2012]

• Which phenomena can occur when very warm and humid air is rising over a mass of very cold air?

- Calm weather and intense rain and hail (UPPSC (Pre) 2015]

• Where is the Doldrums belt located?

-Near Equator [JPSC (Pre) 2013]

• The directions of wind patterns in the Northern and Southern hemisphere are governed by

-Coriolis effect (IAS (Pre) 2015]

•  Westerlies in the Southern hemisphere are stronger and persistent than in Northern hemisphere because Southern hemisphere has less landmass as

compared to Northern hemisphere [IAS (Pre) 2011]

• Polar Night Jet Streams is also known as stratospheric sub-polar jet stream. These are found in both hemispheres and develops above the troposphere in winter nights..

What are roaring forties, furious fifties and shrieking sixties?

-Westerlies in Southern hemisphere IMPPSC (Pre) 2015]

• The wind flowing from high pressure area towards the -Trade wind (UPPSC (Pre) 1992]

Mediterranean sea are

• The seasonal reversal of winds are the typical characteristic of

-Mansoon climate (IAS (Pre) 2014]

• The general circulation of the atmosphere sets in motion the ocean water circulation which influences

- The Earth's climate [HPSC (Pre) 2022)

• What causes wind to deflect towards left in the Southern hemisphere?

- Rotation of the Earth [IAS (Pre) 2010]

• Which weather condition is indicated by a sudden fall in barometer reading?

-Stormy [IAS (Pre) 2001)

•  Inter-tropical Convergence zone is located

- At the Equator [MPPSC (Pre) 2018] Zone (ITCZ) a low pressure

• Which type of wind can be called a planetary wind?

- Trade wind [IAS (Pre) 2009

Self Check

1. The direction of wind around a low pressure in Northern hemisphere is

(a) clockwise

(b) anti-clockwise

(c) perpendicular to isobars

(d) paralled to isobars

2. Consider the following statements.

IAS (Pre) 2007

1. Either of the two belts over the oceans at about 30° to 35°N and S latitudes is known as 'Horse latitude.
2. Horse latitudes are low pressure belts.

Which of the statements) given above is/are correct?

(a) Only 1

(b) Only 2

(c) Both 1 and 2

(d) Neither 1 nor 2

3. Which one among the following is the idealised global, pattern of surface wind from the Equator to the pole?

1. Doldrums
2. Westerlies
3. Trade winds
4. Polar winds

Codes

(a) 1, 2, 3 and 4

(b) 1, 3, 2 and 4

(c) 2, 1, 4 and 3

(d) 3, 1, 2 and 4

4. Consider the following statements.

WBCS (Pre) 2015

1. Coriolis effect is zero at the Equator.
2. Coriolis effect is more towards the poles.
3. Coriolis effects are related to the decreasing rotational velocity with increasing latitudes.
4. Coriolis effects are related to the increasing rotational velocity with increasing latitudes.

Which of the statements) given above is/are correct?

(a) 1, 2 and 3

(c) 1, 2 and 4

(b) 1 and 3

1. d) 2 and 4

5. Which of the following is a local wind of the USA?

(a) Chinook

(c) Harmattan

(b) Foehn

(d) Mistral

6. Westerlies in the Southern hemisphere are stronger and persistent than in the Northern hemisphere because IAS (Pre) 2011

1. The Southern hemisphere has less landmass as compared to the Northern hemisphere.
2. Coriolis force is higher in the Southern hemisphere as compared to the Northern hemisphere.

Which of the statements) given above is/are correct?

(a) Only 1

(C) Both 1 and 2

(b) Only 2

(d) Neither 1 nor 2

7. Khamsin wind is a local wind of which of the following countries?

(a) Argentina b) Tunisia (c) Egypt (d) Lybia

8. Math Last I with List II and select the correct answer by using the codes given below the lists.

MPPSC

Codes

     A B C D                                         A B C D

(a) 1 2 3 4                                      (b) 4 1 2 3

(c) 3 4 1 2                                      (d) 2 3 4 1

9. Which one of the following statements) is/are correct?

MPSC (Pre) 2014

1. Wind is a moving air which has a speed more than five knots.
2. The minimum speed required to generate wind energy is 25 kms/hr.

Codes

(a) Only 1

(b) Only 2

(c) Neither 1 nor 2

(d) Both 1 and 2

10. Which one of the following pairs) is/are not correctly matched?

UPPSC (Pre) 2016, MPSC (Pre) 2021

(a) Foehn Alps Mountain (Switzerland)

(b) Bora Poland (Southern France)

(c) Mistral Rhine Valley

(d) Khamsin Egypt

11. Consider the following pairs.

Which one of the above pairs is not correctly matched?

(a) Only 1 (b) Only 2 (c) 2 and 3 (d) Only 3

12. Which among the following is not a hot wind?

(a) Sirocco (b) Chinook (c) Mistral (d) Loo

Know Right Answer

1. (b) 
2. (a)
3. (a)
4. (a)
5. (a)
6. (a)
7. (c)
8. (c)
9. (c)
10. (b)
11. (a)
12. (c)