# RBSE Class 11 Geography Notes Chapter 13 Air Pressure Belts and Winds

Go through these RBSE Class 11 Geography Notes Chapter 13 Air Pressure Belts and Winds contain important concepts so that students can score better marks in the exam.

## Rajasthan Board RBSE Class 11 Geography Notes Chapter 13 Air Pressure Belts and Winds

→ The pressure that is exerted on the surface by the gaseous envelope of the atmosphere around the Earth is called air pressure.

→ Air pressure is the highest near the Earth’s surface, while with increase in altitude, the air pressure decreases.

→ The air pressure was first discovered by Gurich.

→ Air pressure is controlled by air heat. The air spreads by getting more heat, which reduces the air pressure.

→ Due to the force of gravity, the air near the land is denser than the air in the upper atmosphere.

→ The vertical distribution of air pressure is more important than its horizontal distribution.

→ Air pressure has special significance in weather forecasts.

→ The more prevalent unit of measuring air pressure is millibar.

→ Factors influencing air pressure include temperature, water vapour content, elevation from sea level, dynamic factors, etc.

Air Pressure and Wind

• There is a very close relationship between air pressure and wind circulation.
• Wind arises from the difference in air pressure and rainfall and temperature is also affected by this.
• The winds provide humidity to the continents from the oceans, which results in precipitation.

Air Pressure Belts

• The determination of air pressure belts is done on the basis of the temperature and assuming the earth as a single type of land.
• Due to the variation of the factors of air pressure on the earth, an unequal distribution of air pressure is natural.
• The air pressure belts are divided mainly into the Equatorial low-pressure belt, Subtropical high-pressure belt, Subpolar low-pressure belt and Polar high-pressure belt.
• The equatorial low-pressure belt extends from 5° north to 5° south latitudes of the equator. It is also called belt of calm or doldrum belt. This is a thermally induced belt.
• The sub-tropical high-pressure belt is found between 30° -35 ° latitudes in both the hemispheres. It is also called as horse latitudes. This is a dynamic belt.
• The sub-polar low-pressure belt is found between 60° – 65 ° latitude in both the hemispheres. This is also a dynamic induced belt.
• The polar high-pressure belt is found near the poles in a spread. This is a thermally induced belt.

Distribution of Air Pressure

• Air pressure on the map is represented by the isobars. For this, January and July months are considered the most suitable.
• In January, Sun shines vertically on the Tropic of Capricorn. For this reason, the temperature in the southern hemisphere is higher and air pressure is also less.
• In January, in the northern hemisphere, the fully developed subtropical high pressure belt area is found in the continents.
• In July, the sun shines vertically on the Tropic of Cancer. Due to this, due to high heat in the Northern Hemisphere, the center of low pressure develops.
• In July, the high air pressure belt develops in the southern hemisphere.

Seasonal Changes of Air Pressure Belts

• Due to the difference in the nature of the conditions of the Sun rays and the density of Sun rays, the location of the land and water, there are daily and annual changes in the air pressure.
• During summer time, as the sun is in the Northern Hemisphere, the air pressure belts shift 5° towards the North, while during winter time, when the sun is in the Southern Hemisphere, the belts shift 5° towards the South.
• The ideal condition of air pressure belts is only on 21st March and 23rd September, when the Sun is vertical to the equator.

Vertical Distribution of Atmospheric Pressure

• Pascal first described the decrease in air pressure along with increase in altitude in the atmosphere.
• Generally, in the troposphere, the average decreasing/lapsing air pressure rate is approximately 34 millibars per 300 meters.
• At high altitude, the gases become increasingly rare and light. As a result, air pressure is greatly reduced.

Winds

• Dynamic air in horizontal pattern is called wind. Wind flows from the high air pressure zone to the lower air pressure zone.
• If the earth was stationary, then the winds would blow straight by making right angles on the isobars. But that does not really happen.
• The direction and speed of the winds is influenced by the barometric gradient, the rotation of the earth and the landforms found on earth.

Nomenclature of Winds

• The direction in which the winds move, they are named according to the same direction.
• The winds coming from the west direction are Westerlies/Westerly Winds and the winds coming from east direction are called Easterly Winds.

Classification of Winds

• Winds have been classified into permanent winds, seasonal winds and local winds, depending on their area of influence and duration.

Permanent Winds

• Permanent winds blow in a fixed direction and a fixed order throughout the year. These are also known as the prevailing winds, planetary winds, global winds and permanent winds.
• Permanent winds have been divided into trade winds, westerlies and polar winds.
• In both the hemispheres, those winds are called trade winds, which blow from the subtropical high pressure belts towards equatorial low pressure belts.
• In both the hemispheres, the winds blowing from subtropical high pressure belts towards subpolar low pressure belts are called the westerlies.
• In the southern hemisphere, the westerlies winds blowing between 40-50° latitudes are called the roaring forties, those blowing near 50° latitude are called Furious fifties and those blowing near the 60° Southern latitude are called Shrieking/Screaming Sixties.
• Polar winds move from polar high pressure towards. Sub-polar low air pressure exists in both the hemispheres. These polar winds in the Northern Hemisphere are called easterlies Noreaster.

Periodic Winds

• The winds which change their direction according to the weather or time are called periodic winds.
• The periodic winds have been divided into monsoon winds, land and sea winds and mountain and valley winds.
• The winds that change their direction according to the weather are called monsoon winds.
• With regard to the origin of monsoon winds, thermal theory, dynamic theory and modern theories have been developed.
• Convergence is formed by the conjunction of trade winds near the equator, and it is called tropical convergence.
• Winds which blow from land towards the sea during the night are called terrestrial winds and winds which blow from the sea towards the surface during the day are called sea winds.
• Winds th&t blow from the bottom of a valley towards mountainous slopes during the day are called valley winds, while the winds that blow from mountainous slopes towards the valley during the night are called mountain winds.

Local Winds

• The winds that blow due to the difference in temperature and air pressure of a particular location are called local winds.
• Among the features of local winds, the major ones are that they are cold or hot, dust-fed or snow-fed, etc.
• In such local winds, Chinook, Bora, Fohn, Sirocco, Harmattan, Khamsin, Mistral, Blizzard, Brick Fielder and Willy-Willy are included.

### Air Pressure Belts and Winds Notes Important Terms

→ Atmospheric Pressure: The pressure exerted by all the air layers in the atmosphere on any regional unit surface is called Atmospheric Pressure. It is measured by an instrument called Barometer.

→ Storm/Squall: Fierce atmospheric storms with strong winds of high velocity.

→ Millibar: The unit of measure of Air pressure which is equal to one-thousandth part of 1 bar or equal to 1000 Dyne. Millibars are often used to display the isobars on weather maps.

→ Water Vapour: Water present in the state of vapour in the atmosphere.

→ Wind Circulation: The process of movement of the wind due to pressure and speed related factors in a certain direction.

→ Lower Latitude: The area between 30° North and 30° South latitudes from the equator is called the lower latitude.

→ Higher Latitude: Latitudes situated between Arctic circle (66$$\frac{1}{2}^{\circ}$$ northen latitude) to North Pole and between Antarctic circle (66$$\frac{1}{2}^{\circ}$$ Southern latitude) to south pole. Generally, latitudes above 60 0 are called high latitudes.

→ Latitude: The angular distance of a place north or south of equator, usually expressed in degrees and minutes.

→ Pressure Belts: Belts of air pressure, showing the precise conditions of air pressure.

→ Equator: The imaginary circle passing horizontally across the earth is surface midway between the two poles on the globe, which divides the globe into two equal hemispheres.

→ Equatorial low pressure belt: The thermal low-pressure belt, which can be found generally at 5° latitude, on both side of the equator.

→ Subtropical high pressure belt: High pressure belts found between 30° to 35° latitude in the north and south hemispheres.

→ Subpolar low pressure belt: The lower pressure belt found between 60 – 65° latitudes in both the northern and southern hemisphere. As a result of the rotating motion of the earth, the air of this belt spreads and moves else where and thus low air pressure develops here.

→ Polar high pressure belt: Due to extremely low temperature in the adjacent parts of the North and South Pole (from 70° Northern Latitude to the North Pole and from 70° Southern latitude to the South Pole), high air pressure is always found to exist.

→ Centrifugal force: The force acting from the center to periphery of a revolving body. The tendency of transferring of objects and substances from the centre of the earth to its surface is found due to the centrifugal force of the Earth.

→ Convection Current: Transmission of energy from one part of a liquid substance or gas into another part due to the transfer of its own molecules.

→ Doldrum Belt: Low-pressure equatorial belt where the convergence of the north¬eastern and south-eastern trade winds takes place, due to which the air is almost calm on the surface or it blows at very low speed.

→ Horse Latitude: The sub-tropical high pressure belt found in the middle of the northern and southern hemisphere between the westerlies and trade winds, usually located between 30° to 35° latitudes.

→ Pressure/Barometric gradient: The difference of the air pressure between two points on the land, is called Pressure/Barometric gradient. It is displayed on the weather chart or on the map through the distance between the isobars.

→ Rotation: The movement (spinning) of the Earth around on its axis.

→ Coriolis force: Due to the rotation of the earth, there is a disturbance in the direction of the winds. This transformational force is called the Coriolis force. It has been named on the basis of a scholar named Coriolis.

→ Westerlies or West Winds: Permanent or regular winds blowing towards sub-polar low pressure belt from subtropical high pressure belt in both the north and south hemispheres, which blow from the south-west to north-east direction in the northern hemisphere and from north-west to south-east direction in the southern hemisphere.

→ Easterly Winds: Seasonal winds which blow from east direction. In Northern India, blowing of easterly winds during summer time provides relief from heat. As they proceed from the Bay of Bengal, the humidity is relatively high due to this.

→ Permanent winds: Those winds which blow in a fixed direction and a fixed order through out the year, are called permanent winds. These are also called prevailing winds.

→ Periodic Winds: The winds which change their direction according to the weather or time are called Periodic Winds.

→ Local Winds: Winds that blow due to the difference in temperature and air pressure in a particular location are called local winds. These winds are contrary to the permanent winds blowing there.

→ Trade Winds: Winds blowing towards the equatorial low pressure belt from subtropical high pressure belt in both the hemispheres. These winds do not blow straight and deflect towards their left side in the northern hemisphere and towards their right side in the southern hemisphere.

→ Polar Winds: Extremely cold winds blowing from the polar high pressure belt towards the temperate low pressure belt. Their direction is towards the North-east in the northern hemisphere, and towards the South-east in the southern hemisphere.

→ Roaring Forties: In the southern hemisphere between 40° and 50° latitudes, in the absence of terrestrial blockade, the prevailing west winds continuously blow at high speeds upon the open oceans. Due to rapid velocity, these winds move in the adjacent sea parts near 40° latitude. Hence, they are called Roaring Forties.

→ Furious Fifties: In the southern hemisphere, westerlies or west winds in the oceans blowing furiously near 50° south latitudes.

→ Shrieking/Screaming Sixties: In the southern hemisphere, the westerlies winds blowing near 60° latitude are known as Screaming Sixties.

→ Noreaster: Hot and dry wind descending from the mountains in New Zealand Island. In the plains of Northern India, it refers to the stormy winds, ‘Mausim’, often accompanied by devastating storm and heavy rains.

→ Monsoon Winds: Winds blowing in the Earth’s lower atmosphere (above the surface level) whose direction changes along with seasonal change. This word has originated from the Arabic word ‘Mausim’, which generally means the seasonal wind or monsoon wind.

→ Valley Wind or Tal Wind: Those winds that blow in the day, up from the valley along with the mountain slope, are called Valley Wind.

→ Thermal Concept: The concept of the origin of monsoon in which the origin of monsoon is believed to be based on the relation between heat and pressure.

→ Insolation: The solar energy that is received from the sun is called insolation. This energy reaches the earth’s surface in the form of short waves.

→ Convergence: Convergence refers to the conjunction of the winds that come blowing from two opposite directions.

→ Inter-Tropical Convergence Zone: The low air pressure belt found on the equator and its adjacent areas, where north-east and south-eastern trade winds converge. Here the wind movement is very slow and the air remains calm.

→ Jet Stream: The strong western wind blowing below the tropopause at the highest altitude in the atmosphere.The jet stream is mainly found in two domains:

• Jet stream blowing between 20 to 30° latitude in the Northern and Southern Hemisphere.

→ Terrestrial Radiation: Long-wave electromagnetic radiation originating from Earth and its atmosphere due to which the lower part of the atmosphere becomes heated up.

→ Inversion of Temperature: The condition temperature distribution in the atmosphere which indicates heat depletion along with elevation is normal. But when there is a tendency of increase in temperature in place of decrease, with rise in altitude this process is called inversion of temperature.

→ Chinook: In North America, the southern-western hot and dry wind which blows through the eastern slope of the Rocky Mountains is called Chinook.

→ Fohn/Fon: The German word for the temporary hot local wind blowing in the north of the Alps Mountain in Europe.

→ Bora: A gentle and dry wind that blows in the eastern coast of the Adriatic Sea in southern Europe and northern Italy, which often runs from the north-east direction in winter.

→ Sirocco: The hot, dry and sandy-wind which blows from the Sahara Desert towards the north direction, which crosses the Mediterranean Sea and enters the coastal countries of Italy, Spain, etc.

→ Harmattan: Hot dry wind that blows in West Africa, from the north-east and east towards the South-west and west direction.

→ Khamsin: The hot dry southern wind that blows in Egypt, which is related to the Sirocco wind of North Africa. Khamsin blows in the period of approximately 50 days from April to June.

→ Mistral: Dry cold winds which are often driven by strong and fast air currents in the north or porth-west of southern France. This wind blows when high pressure is formed on France and low pressure is formed on the Mediterranean Sea.

→ Blizzard: Snowflake storms along with snowflakes that blow in high latitudes in the form of tremendous cold waves in the winter season.

→ Brick Fielder: Hot-winds in South-Eastern Australia mainly blowing during summer, which are often filled with clouds of dust.

→ Willy-Willy: The tropical cyclone that blows during the summer season in North-Western Australia, whose time and route is often uncertain.

→ Prairies: Flat and large mid-latitudinal grasslands in North America, extending from the Rocky Mountains in the west to Lake Michigan in the east.

→ Blood Rain: Flowing with Sirrocco air, red soil descends along with rain in southern Italy. This is called blood rain.

→ Gibli: Local name of Sirrocco wind in Libya and Tunisia.

→ Doctor Winds: Winds called by various names locally in different parts of the world, on whose arrival, the harsh seasonal conditions are remarkably changed to comfortable ones. It provides relief to people. These relieving winds are known as Doctor Winds.

→ Ice Point: The temperature at which water (at standard pressure) is converted into ice.