NCERT Solution CBSE Class 7 Social Science Geography 2025
Chapter 4 Air Summary with NCERT Solution
Summary of Chapter -
Air
The earth is surrounded by a huge layer of air called the atmosphere, which is essential for the survival of all living beings. It provides the air we breathe and protects us from the harmful rays of the sun. Without the atmosphere, the earth would be too hot during the day and too cold at night, making life impossible.
Composition of the Atmosphere
The air around us is a mixture of many gases. The two main
gases are Nitrogen (78%) and Oxygen (21%). Other gases like Carbon Dioxide,
Argon, Helium, and Ozone are present in small amounts. There are also dust
particles in the air.
- Nitrogen: The most abundant gas, but humans and animals do not use it directly. Plants need nitrogen for growth, which they get with the help of bacteria in the soil.
- Oxygen: Essential for breathing and burning. Green plants
release oxygen during photosynthesis.
- Carbon Dioxide (CO₂): Used by plants to make food and is
also responsible for the greenhouse effect. Excess CO₂ from pollution leads to global
warming.
Structure of the Atmosphere
The atmosphere is divided into five layers:
1. Troposphere: The lowest and most important layer where all weather changes happen. It extends up to 13 km from the earth’s surface.
2. Stratosphere: Extends up to 50 km. It contains the ozone
layer, which protects us from harmful UV rays. This layer is ideal for flying
airplanes.
3. Mesosphere: Reaches up to 80 km. Meteorites burn up in
this layer when they enter the earth’s atmosphere.
4. Thermosphere: Extends up to 400 km. It contains the ionosphere,
which helps in radio communication.
5. Exosphere: The outermost layer with very thin air, where
gases like hydrogen and helium escape into space.
Weather and Climate
- Weather refers to the day-to-day conditions of the
atmosphere, such as temperature, rainfall, and wind. It can change within a few
hours.
- Climate is the average weather pattern of a place over a
long period. For example, deserts have a hot and dry climate, while polar
regions are cold.
Temperature
The degree of hotness or coldness of the air is called temperature.
It varies between day and night and changes with the seasons. The sun is the
main source of heat. The earth’s temperature is higher near the equator and
lower at the poles because sunlight is more direct at the equator. Cities are
usually hotter than villages due to concrete buildings and pollution.
Air Pressure
Air pressure is the force exerted by air on the earth’s
surface. It is highest at sea level and decreases with height. Areas with high
temperature have low pressure (hot air rises), while cold areas have high
pressure (cold air sinks).
Wind
The movement of air from a high-pressure area to a
low-pressure area is called wind. There are three types of winds:
1. Permanent Winds – These blow throughout the year in the
same direction (Trade Winds, Westerlies, and Easterlies).
2. Seasonal Winds – Change direction in different seasons
(like monsoons in India).
3. Local Winds – Blow in a small area for a short period
(e.g., Loo – a hot wind in northern India).
Cyclones
Cyclones are strong storms that bring heavy rain and strong
winds. In 1999, a super cyclone hit Odisha, causing massive destruction due to
high wind speed, rainfall, and tidal waves.
Moisture and Rainfall
Moisture in the air is called humidity. When the air cools,
water vapor condenses to form clouds. When the clouds become heavy, they
release water as precipitation (rain, snow, or hail).
There are three types of rainfall:
1. Convectional Rainfall – Due to heat, warm air rises and
cools to form rain.
2. Orographic (Relief) Rainfall – When moist air hits
mountains, it rises, cools, and causes rain.
3. Cyclonic Rainfall – When warm and cold air meet, clouds
form and bring heavy rain.
Importance of Rainfall
Rainfall provides fresh water for drinking, irrigation, and
plant growth. Too little rain can cause droughts, while too much rain can lead
to floods.
~~The END~~
Chapter 4 Air
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