Atmosphere, Winds and Pressure Belts Notes for UPSC

The atmosphere is the blanket of gases that surrounds the Earth. It gives us air to breathe, protects us from harmful rays of the Sun, keeps the planet warm, carries water as vapour and creates our weather. The movement of air, the winds, is controlled by pressure differences and the rotation of the Earth. For UPSC, this chapter is important in Prelims (layers, pressure belts, winds, jet streams, cyclones) and in Mains (climate, monsoon, cyclones and climate change). This note explains the key ideas in simple words with tables.

Composition of the Atmosphere

Dry air is mainly made up of nitrogen (about 78 percent), oxygen (about 21 percent), argon (about 0.9 percent) and a small amount of carbon dioxide (about 0.04 percent), along with traces of neon, helium and other gases. The atmosphere also has water vapour (up to 4 percent, mostly in the lower layer) and dust particles. Though carbon dioxide and water vapour are small in quantity, they trap heat and keep the Earth warm through the greenhouse effect.

Layers of the Atmosphere

The ozone layer is damaged by chemicals like CFCs, which is why the Montreal Protocol was adopted to phase them out.

Heat Budget and Temperature

The Sun's energy reaching the Earth is called insolation. Out of 100 units received, about 35 units are reflected back to space (this is the Earth's albedo), about 14 units are absorbed by the atmosphere and about 51 units are absorbed by the Earth's surface. The Earth later returns this heat to space as terrestrial radiation, so the heat gained equals the heat lost, which keeps the average temperature stable. Temperature differs from place to place because of latitude, altitude, distance from the sea, ocean currents, winds and cloud cover. Normally, temperature falls with height, but sometimes a layer of warm air lies above cold air near the ground, which is called temperature inversion. It traps pollutants and causes fog.

Pressure Belts of the World

Air pressure is the weight of air on a surface. It is not the same everywhere, and there are seven pressure belts on the Earth:

• Equatorial low-pressure belt (doldrums): hot air rises, with calm winds and heavy rain.
• Sub-tropical high-pressure belts (about 30 degrees N and S, the "horse latitudes"): sinking dry air, dry weather and large deserts.
• Sub-polar low-pressure belts (about 60 degrees N and S): meeting of warm and cold air and stormy weather.
• Polar high-pressure belts: very cold, heavy air.

Planetary Winds

Wind blows from a high-pressure area to a low-pressure area, and the Earth's rotation (the Coriolis force) deflects it to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The three main permanent wind systems are:

Seasonal and Local Winds

Seasonal winds change direction with the season, as in the monsoon. Daily local winds include sea and land breezes (sea breeze by day, land breeze by night) and mountain and valley breezes. Some famous local winds are:

• Loo: hot, dry wind of the northern Indian plains in summer.
• Chinook: warm, dry wind on the eastern slopes of the Rockies; called the "snow eater". The Foehn is similar in the Alps.
• Mistral: cold wind flowing down the Rhone valley in France.
• Sirocco: hot wind from the Sahara to the Mediterranean region.
• Harmattan: dry, dusty wind from the Sahara to West Africa, also called the "doctor" for its healthy dryness.

Jet Streams

Jet streams are narrow, fast-moving, westerly air currents in the upper troposphere (9 to 13 km above the surface), moving at 150 to 300 km per hour or more. The main ones are the Polar Front Jet and the Sub-tropical Jet. A separate Tropical Easterly Jet appears over India in summer and helps the monsoon. Jet streams influence storms, rain and even the time taken by flights.

Air Masses, Fronts and Cyclones

An air mass is a huge body of air with uniform temperature and humidity. When a warm and a cold air mass meet, the boundary is a front, such as a cold front or a warm front, and this often brings rain. Mid-latitude (temperate) cyclones form along fronts in the westerlies and bring rain and snow to Europe and North America, and the western disturbances reach north-west India in winter. Tropical cyclones form over warm oceans (above about 26.5 degrees Celsius) and have a calm centre called the eye surrounded by very strong winds. They are called hurricanes in the Atlantic, typhoons in the western Pacific and cyclones in the Indian Ocean. Their landfall can bring heavy rain, storm surge and floods, so early warning systems are very important.

Types of Rainfall

• Convectional rain: warm air rises, cools and falls as rain, common near the equator in the afternoon.
• Orographic (relief) rain: moist wind is forced up a mountain, as on the Western Ghats; the leeward side is a dry rain shadow.
• Cyclonic (frontal) rain: caused by meeting of warm and cold air masses.

Humidity, Clouds and Condensation

Air always has some water vapour. Absolute humidity is the actual amount of water vapour in the air, while relative humidity is the amount of vapour compared with the maximum the air can hold at that temperature, shown in percentage. When air cools to its dew point, vapour turns into water droplets by condensation, forming dew, fog, mist and clouds. Clouds are of four main kinds: cirrus (thin, high and feathery), cumulus (puffy, fair-weather), stratus (layered, low and grey) and nimbus (rain-bearing). Learning these basic terms helps in understanding rainfall and weather forecasts, and questions on them are often direct.

Anticyclones and Tornadoes

An anticyclone is the opposite of a cyclone: a high-pressure system in which winds blow outward from the centre, clockwise in the Northern Hemisphere. It generally brings clear skies, dry weather and cold or hot spells. A tornado is a small but extremely violent, rotating funnel of air that extends from a thunderstorm to the ground. It is most common in the central United States ("Tornado Alley") and is far more localised than a cyclone. Remember that a tornado is not the same as a tropical cyclone, even though both are rotating storms.

How to Study This Chapter

1. Draw a diagram of the layers and mark one key feature for each.
2. Sketch the globe with the pressure belts and the direction of the winds, so that the logic of the trade winds and westerlies becomes clear.
3. Prepare a short table of local winds with region and nature (hot or cold).
4. Link it with Indian geography: monsoon, western disturbances, loo and cyclones.

Mains Angle: Atmosphere and Climate Change

Burning fossil fuels increases greenhouse gases and traps more heat, which raises the temperature of the lower atmosphere. Ozone depletion weakens the stratosphere's protective shield. Changes in the pressure belts and jet streams can shift rainfall belts and strengthen cyclones. A good answer explains the process (greenhouse effect), gives examples (rising sea level, extreme weather) and suggests solutions such as cutting emissions, renewable energy and international agreements.

Practice Statements and Common Mistakes

• "All weather occurs in the stratosphere." Wrong. It occurs in the troposphere.
• "The ozone layer is in the stratosphere." Correct.
• "Trade winds blow from the equator to the sub-tropical highs." Wrong. They blow towards the equator.
• Do not mix up the Chinook (warm) with the Mistral (cold).

Quick Revision

• Layers: troposphere, stratosphere, mesosphere, thermosphere, exosphere.
• Winds: trade winds, westerlies, polar easterlies; Coriolis deflects right in the north.
• Rain types: convectional, orographic, cyclonic.

The atmosphere ties together climate, weather and life. Draw the layers and the pressure belts, learn the winds with a simple table and practise explaining cyclones in your own words, and this topic will become easy to score.

Related Notes to Read Next

Continue your preparation with these related notes from the same subject:

• Solar System and Motions of the Earth Notes for UPSC
• Climatic Regions of the World Notes for UPSC
• Major Mountains, Rivers, Lakes and Deserts of the World Notes
• All World Geography notes

Frequently Asked Questions (FAQ)

What are the gases present in the atmosphere?

The atmosphere is made mainly of nitrogen (about 78 percent), oxygen (about 21 percent), argon (about 0.9 percent) and carbon dioxide (about 0.04 percent), with some water vapour and dust.

What are the five layers of the atmosphere?

They are the troposphere, stratosphere, mesosphere, thermosphere and exosphere, from the surface upward.

In which layer does weather occur?

All weather phenomena such as clouds, rain and storms occur in the troposphere, the lowest layer of the atmosphere.

Where is the ozone layer located?

The ozone layer is in the stratosphere, at roughly 15 to 35 km above the Earth, and absorbs harmful ultraviolet radiation from the Sun.

What is albedo?

Albedo is the share of solar radiation reflected back to space by the Earth’s surface and clouds. The Earth’s average albedo is about 35 percent.

What are the major pressure belts of the world?

They are the equatorial low, the two sub-tropical highs, the two sub-polar lows and the two polar highs.

What are trade winds?

Trade winds are steady winds that blow from the sub-tropical high-pressure belts towards the equatorial low, from the north-east in the Northern Hemisphere and from the south-east in the Southern Hemisphere.

What is the Coriolis force?

The Coriolis force is an apparent force caused by the Earth’s rotation that deflects winds and ocean currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

What are jet streams?

Jet streams are narrow, fast westerly air currents in the upper troposphere that influence weather. Important ones are the polar front jet, the sub-tropical jet and the tropical easterly jet.

What is the difference between a tropical and a temperate cyclone?

Tropical cyclones form over warm oceans, have an eye and bring violent winds, while temperate cyclones form along fronts in mid-latitudes and bring rain and snow over a wide area.

What is temperature inversion?

Temperature inversion is a situation in which a layer of warm air lies above cooler air near the ground. It traps pollution and often causes fog.

What are the main types of rainfall?

Convectional rain from rising warm air, orographic rain when moist wind rises over mountains and cyclonic or frontal rain when warm and cold air masses meet.