Chapter 4 – Air -

Chapter 4 – Air

The atmosphere is a mixture of several gases:

Nitrogen – 78%:
- The most abundant gas in the atmosphere.
- Plants need nitrogen to survive, but they cannot take nitrogen directly from the air.
- Some bacteria living in the soil convert atmospheric nitrogen for plant use.
Oxygen – 21%:
- The second most abundant gas for life.
- Humans and animals take in oxygen for respiration.
- Green plants produce oxygen during photosynthesis.
Carbon Dioxide – 0.03%:
- Used by plants for photosynthesis.
- Humans and animals release carbon dioxide during respiration.
- It is an important greenhouse gas that retains heat on the earth.
- An increase in its volume leads to global warming.
Other gases – about 1%:
- Gases like argon, neon, helium.
Dust particles and water vapour: These are also present in the atmosphere.

The atmosphere is divided into five layers based on altitude:

- It is the lowest and most important layer of the atmosphere.
- It has an average elevation of about 13 km.
- The air we breathe is right here.
- All weather phenomena like rain, fog, hailstorm occur in this layer.

Located above the troposphere, extends to an altitude of about 50 km.
This layer is almost free of clouds and meteorological phenomena, so it is ideal for flying airplanes.
It has an ozone layer, which protects us from the sun’s harmful ultraviolet rays.

iii. Mesosphere:

Located above the mesosphere, altitudes up to 80-400 km.
The temperature in this layer rises rapidly with height.
Ionosphere : is a part of the thermosphere. It contains ionized particles.
This layer is helpful for radio transmission because radio waves transmitted from the earth are reflected back to the earth by this layer.

It is the uppermost layer of the atmosphere.
It contains very thin air, and light gases like helium and hydrogen float into space from here.

Weather :
- It refers to the hour-by-hour or day-to-day state of the atmosphere at any given time.
- This can change very quickly.
- Example: It’s sunny today, it may rain tomorrow.
Climate :
- It is the average weather for a large area for a long period of time (e.g. 25-30 years).
- It is the permanent weather pattern of a place.
- Example: The climate of India is monsoonal.

Temperature is a measure of the degree of warmth and coldness in the air.
Insolation: Energy from the sun that is blocked by the earth.
- It decreases from the equator towards the poles, so the temperature also decreases.
Air pressure: The weight of air that exerts pressure on the earth’s surface.
- High Pressure : Due to cold and heavy air.
- Low Pressure: Due to warm and light air.
- Clouds and precipitation occur in low pressure areas.
- High pressure areas have clear skies.

Wind: The speed of wind from a high pressure area to a low pressure area is called wind.
There are three types of winds:

Permanent Winds: These blow in the same direction continuously throughout the year.

Local Winds : These only blow over a small area during a short particular period of the day or year.

When water evaporates from the earth and various water bodies, it becomes water vapour.
The amount of water vapour in the air is called humidity.
When the air is full of water vapour, it is called a humid day.
Condensation: When water vapour rises and cools, it condenses into water droplets to form clouds.
Precipitation: When these water droplets become so heavy that they cannot float, they fall to the earth as rain.
Types of Rainfall :
- Convectional Rainfall: Warm air rises and cools.
- Orographic Rainfall: When moisture-laden air hits the mountains and rises up.
- Cyclonic Rainfall: When warm and cold air mix.