Air — Class 7 Social Science

The atmosphere is the thin blanket of air that surrounds the Earth. It is held in place by the gravitational pull of the Earth and extends up to about 1,600 km from the surface, though most of the air is within 30 km.

Gases in the Atmosphere:

• Nitrogen (78%): The most abundant gas; essential for plant growth and is a part of proteins in living organisms
• Oxygen (21%): Essential for breathing and survival of all living organisms; also needed for burning (combustion)
• Other gases (1%): Includes carbon dioxide, argon, neon, helium, hydrogen, ozone, and water vapour

Important Minor Gases:

• Carbon dioxide (CO₂): Absorbs heat radiated by the Earth and keeps the atmosphere warm (greenhouse effect); used by plants for photosynthesis
• Ozone: Found in the upper atmosphere; protects us from the Sun's harmful ultraviolet (UV) rays
• Water vapour: Varies from place to place; creates clouds, rain, and other forms of precipitation; plays a key role in the water cycle

Other Components:

• Dust particles: Present in the atmosphere; help in the formation of clouds by acting as condensation nuclei
• Smoke and pollutants: Released by vehicles, factories, and burning; cause air pollution

The atmosphere is divided into five layers based on temperature and other properties. Each layer has distinct characteristics:

1. Troposphere (0–12 km):

• The lowest layer and the most important for us
• Contains almost all the water vapour and dust particles
• All weather phenomena (clouds, rain, storms) occur here
• Temperature decreases with height (about 6.5°C per km)
• Average height: 13 km at the equator, 8 km at the poles

2. Stratosphere (12–50 km):

• Contains the ozone layer which absorbs harmful UV rays from the Sun
• Temperature increases with height (due to absorption of UV rays by ozone)
• Almost no weather activity; ideal for flying aeroplanes (jet aircraft fly here)
• Very little water vapour or dust

3. Mesosphere (50–80 km):

• Temperature decreases with height; the coldest layer (can reach –100°C)
• Meteorites burn up in this layer (shooting stars)
• Protects Earth from falling meteorites

4. Thermosphere (80–400 km):

• Temperature increases rapidly with height (can exceed 1,000°C)
• Contains the ionosphere — a layer of electrically charged particles (ions)
• The ionosphere helps in transmission of radio waves
• Auroras (Northern and Southern Lights) occur in this layer

5. Exosphere (above 400 km):

• The outermost layer of the atmosphere
• Very thin air; gradually merges into outer space
• Satellites orbit in this layer

Though often used interchangeably, weather and climate are two very different concepts:

Weather:

• The day-to-day condition of the atmosphere at a particular place and time
• Changes frequently — it can change within hours or even minutes
• Includes temperature, humidity, rainfall, wind speed, cloudiness, and sunshine
• Example: "Today it is sunny" or "It is raining this afternoon"
• Measured at a specific time and place

Climate:

• The average weather pattern of a place over a long period of time (usually 25–30 years or more)
• Does not change frequently; remains relatively stable over decades
• Helps us understand the general conditions of a place
• Example: "India has a tropical monsoon climate" or "Antarctica has a polar climate"

Key Difference:

• Weather = short-term, changes daily, specific time and place
• Climate = long-term average, relatively stable, describes a region

Temperature is the degree of hotness or coldness of the air. It is one of the most important elements of weather and climate.

How Temperature is Affected:

• Insolation: The incoming solar radiation that heats the Earth's surface. The atmosphere is heated indirectly — the Sun heats the Earth, and the Earth heats the air above it
• The atmosphere is heated from below, not from above

Temperature and Altitude:

• As we go higher, the temperature decreases
• The rate of decrease is about 6.5°C per 1,000 metres (called the normal lapse rate)
• This is why hills and mountains are cooler than plains
• Example: If temperature at sea level is 30°C, at 2,000 m height it would be about 17°C

Temperature and Latitude:

• Temperature is highest near the equator (where the Sun's rays fall directly/vertically)
• Temperature decreases as we move towards the poles (where Sun's rays fall at an angle/obliquely)
• This is because vertical rays heat a smaller area more intensely, while oblique rays spread over a larger area

Air pressure is the weight of air pressing down on the Earth's surface. The air has weight, and this weight creates pressure on everything below it.

Key Facts about Air Pressure:

• Air pressure is highest at sea level because there is more air above pressing down
• Air pressure decreases with altitude — the higher we go, the less air there is above us
• This is why at high altitudes (like mountains), people experience nose bleeding and difficulty in breathing
• Air pressure is measured with a barometer

Air Pressure and Wind:

• Air always moves from areas of high pressure to areas of low pressure
• This horizontal movement of air is called wind
• The greater the difference in pressure between two areas, the faster the wind blows
• When the air above a place is heated, it rises (because warm air is lighter), creating low pressure
• Cooler air from surrounding high-pressure areas rushes in to fill the gap — this is wind

Air Pressure and Temperature:

• Hot areas (like the equator) have low air pressure because warm air rises
• Cold areas (like the poles) have high air pressure because cold air sinks
• This difference in pressure between the equator and poles drives global wind patterns

Wind is the horizontal movement of air from high-pressure areas to low-pressure areas. Winds are classified into three main types:

1. Permanent Winds (Planetary Winds):

• Blow constantly throughout the year in the same direction
• Caused by permanent pressure belts on Earth
• Trade Winds: Blow from the subtropical high-pressure areas towards the equatorial low-pressure belt. They blow from NE to SW in the Northern Hemisphere and SE to NW in the Southern Hemisphere
• Westerlies: Blow from the subtropical high-pressure belt towards the sub-polar low-pressure belt (from west to east); these affect weather in temperate regions
• Polar Easterlies: Blow from the polar high-pressure areas towards the sub-polar low-pressure belt (from east to west)

2. Seasonal Winds:

• Change direction with the change of season
• The best example is the monsoon winds of India
• Summer monsoon: Winds blow from the sea to the land (SW direction), bringing heavy rainfall
• Winter monsoon: Winds blow from the land to the sea (NE direction), bringing dry weather
• The word "monsoon" comes from the Arabic word mausim, meaning "season"

3. Local Winds:

• Blow in small areas and are caused by local differences in heating
• Land Breeze: Blows from land to sea at night. At night, land cools faster than sea, so air pressure is higher over land and wind blows towards the sea
• Sea Breeze: Blows from sea to land during the day. During the day, land heats up faster than sea, so air pressure is lower over land and wind blows from sea to land
• Loo: A hot and dry local wind that blows in northern India during summer

The atmosphere contains water in the form of water vapour. The amount of water vapour in the air is called humidity. Moisture in the air plays a vital role in weather and precipitation.

Key Concepts:

• Humidity: The amount of water vapour present in the air. It varies from place to place and time to time
• Evaporation: The process by which water from oceans, rivers, lakes, and other water bodies turns into water vapour due to heat from the Sun
• Condensation: The process by which water vapour cools down and changes back into tiny water droplets. This forms clouds
• Precipitation: When water droplets in clouds become too heavy, they fall back to Earth as rain, snow, sleet, or hail

How Clouds Form:

• The Sun heats water on Earth's surface, causing evaporation
• Warm moist air rises upward (because warm air is lighter)
• As the air rises, it cools down
• Water vapour in the rising air condenses around dust particles to form tiny water droplets
• Millions of these tiny water droplets together form a cloud
• When the droplets grow too heavy, they fall as precipitation

Forms of Precipitation:

• Rain: Liquid water drops falling from clouds (most common form)
• Snow: Frozen water crystals falling in very cold conditions
• Sleet: Raindrops that freeze while falling through cold air
• Hail: Lumps of ice that form when raindrops are carried upward by strong air currents and freeze repeatedly

Rainfall occurs when moist air rises, cools, and the water vapour condenses. Based on how the air is forced to rise, rainfall is classified into three types:

1. Convectional Rainfall:

• Occurs when the Sun heats the ground intensely during the day
• The heated air becomes lighter and rises rapidly (convection)
• As it rises, it cools and condenses, forming cumulonimbus clouds
• Produces heavy rainfall with thunder and lightning
• Very common in equatorial regions and during hot summer afternoons in India
• Usually occurs in the afternoon

2. Orographic or Relief Rainfall:

• Occurs when moist air from the sea is forced to rise over a mountain or hill
• As the air rises along the mountain slope (windward side), it cools and condenses, causing heavy rainfall
• The other side of the mountain (leeward side) receives very little or no rain — this is called the rain shadow area
• Example: The Western Ghats in India receive heavy orographic rainfall on the windward side, while the Deccan Plateau on the leeward side is relatively dry

3. Cyclonic or Frontal Rainfall:

• Occurs when warm moist air meets cold air
• The warm air is lighter and rises over the cold air
• As it rises, it cools and condenses, causing rainfall
• The boundary between warm and cold air masses is called a front
• Common in temperate regions (mid-latitudes)
• Can cause continuous rainfall for several days