Quick Revision Notes & Formula Sheet

Class: 7th
Subject: Social Science (Geography)
Chapter: Air (वायु)

The earth is surrounded by a huge blanket of air called the atmosphere.

Nitrogen ($N_2$ - 78%): Most abundant gas in the air. It helps in the survival of plants.
Oxygen ($O_2$ - 21%): Second most plentiful gas. Essential for the survival of humans and animals (respiration).
Argon ($Ar$ - 0.93%): Present in small quantities.
Carbon Dioxide ($CO_2$ - 0.03%): Essential for plants to make food (photosynthesis). It creates a greenhouse effect by trapping heat radiated from the earth.

The atmosphere is divided into five distinct layers starting from the earth's surface:

Troposphere (क्षोभमंडल):
- Height: Average 13 km from the surface.
- Importance: Most important layer; air we breathe exists here. Weather phenomena like rainfall, fog, and hailstorm occur in this layer.
Stratosphere (समतापमंडल):
- Height: Up to 50 km height.
- Importance: Free from clouds and weather disturbances, making it ideal for flying aeroplanes. Contains the Ozone layer which protects us from harmful ultraviolet (UV) rays.
Mesosphere (मध्यमंडल):
- Height: Up to 80 km.
- Importance: Meteorites burn up in this layer on entering from space.
Thermosphere / Ionosphere (बाह्य वायुमंडल / आयनमंडल):
- Height: 80 to 400 km.
- Importance: Temperature rises very rapidly with height. Radio waves transmitted from the earth are reflected back to the earth by this layer.
Exosphere (बहिर्मंडल):
- Height: Uppermost layer.
- Importance: Very thin air. Light gases like helium and hydrogen float into space from here.

Weather (मौसम): Hour-to-hour, day-to-day condition of the atmosphere. (e.g., sunny, rainy, cloudy day).
Climate (जलवायु): The average weather condition of a place for a longer period of time (e.g., tropical, desert climate).

Definition: The degree of hotness and coldness of the air.
Insolation (आतップ): Incoming solar energy intercepted by the earth. Temperature decreases from the equator towards the poles.
Standard Unit: Degree Celsius ($^\circ C$).

Definition: Pressure exerted by the weight of air on the earth's surface.
Key Rule: Air pressure is highest at sea level and decreases with height.
High-Pressure Area: Associated with clear and sunny skies (cold areas).
Low-Pressure Area: Associated with cloudy skies and wet weather (warm areas). Air always moves from high-pressure areas to low-pressure areas.

Definition: The movement of air from high-pressure area to low-pressure area.
Types of Winds:
1. Permanent Winds (स्थाई पवनें): Trade winds, Westerlies, and Easterlies. Blow constantly throughout the year in a particular direction.
2. Seasonal Winds (मौसमी पवनें): Winds that change their direction in different seasons (e.g., Monsoons in India).
3. Local Winds (स्थानीय पवनें): Blow only during a particular period of the day or year in a small area (e.g., Loo in the hot summer of northern India).

Definition: Moisture in the air at any time. When air is full of water vapour, it is called a humid day.
Precipitation (वर्षण): When water vapour rises, it cools and condenses to form clouds. Falling down to the earth in liquid or solid form is called precipitation (Rain, Snow, Sleet, Hail).
Types of Rainfall:
1. Convectional Rainfall
2. Orographic (Relief) Rainfall
3. Cyclonic Rainfall

Atmosphere (वायुमंडल): Blanket of air surrounding the earth.
Greenhouse Effect (ग्रीनहाउस प्रभाव): Trapping of heat by carbon dioxide in the atmosphere, preventing the earth from becoming too cold.
Barometer (बैरोमीटर): Instrument used to measure air pressure.
Thermometer (थर्मामीटर): Instrument used to measure temperature.
Rain Gauge (वर्षा माпи): Instrument used to measure the amount of rainfall.
Wind Vane (वाताग्र सूचक): Instrument used to show the direction of the wind.

Air