Air (NCERT Class 7 Geography Chapter 4 Notes)

Layers of Atmosphere

Earth is surrounded by a huge blanket of air known as atmosphere. It provides air to breathe and protects from harmful ultraviolet rays.

Composition of the Atmosphere

1. Earth's atmosphere is made up of four major gases: Nitrogen (78%), Oxygen (21%), Argon (0.93%) and Carbon dioxide (0.03%). The rest of other gases make up 0.04 per cent of the atmosphere.
2. Nitrogen is the most abundant gas in the environment. The plants need nitrogen for their survival. Though, they cannot take nitrogen directly from the environment. The bacteria and microorganism take nitrogen from the air and they fix the nitrogen in the soil and from the soil, plants get their nitrogen need.
3. Oxygen is the second most abundant gas in the atmosphere. Humans and animals take oxygen directly from the air whereas plants produce oxygen during the photosynthesis. In this way, oxygen content remains constant.
4. Carbon dioxide is an important gas. Plants use carbon dioxide to produce food and release oxygen. It also helps in keeping the earth warm and temperature to not fall deep. This effect is known as the greenhouse effect.
5. The greenhouse effect is applied in modern agriculture to produce better yield crops. However, due to the mindless destruction of forests, there is an increase in carbon dioxide content, which has led to global warming due to the same greenhouse effect shown by carbon dioxide.

Structure of the Atmosphere
The atmosphere can be divided into 1. Troposphere, 2. Stratosphere, 3. Mesosphere, 4. Thermosphere and 5. Exosphere

1. Troposphere

1. The average height of the troposphere is 13 km. The air we breathe exist here.
2. All weather phenomena such as rain, fog and hailstorm happen in the troposphere.

2. Stratosphere

1. The stratosphere extends up to 50 km. This layer is free from weather-related phenomena, which makes it perfect for the aviation sector.
2. It contains a thin layer of ozone which protects us from harmful UV rays.

3. Mesosphere

1. It lies above the stratosphere and extends up to 80 km.
2. Most of the meteorites burn here.

4. Thermosphere

1. Here temperature rises rapidly with increase in height. It lies above Mesosphere and extends up to 400 km.
2. The ionosphere is part of the thermosphere. It helps in radio signal transmission. The radio signal reflects back to earth from this layer.

5. Exosphere

1. It is the uppermost layer of the atmosphere. It has very thin air.
2. Gases like hydrogen and helium make up most of the air in this layer.

Weather and Climate

1. Weather is the day to day condition of the atmosphere.
2. Climate is the average weather condition of a place for a longer period of time.

Temperature

1. The degree of hotness or coldness is known as the temperature.
2. It changes not only in day and night but through seasons.
3. Temperature depends on various factors such as isolation, habitat and building structure.
4. Isolation is the incoming solar energy intercepted by the earth. It is maximum on equator and minimum on poles. Therefore, temperature decreases as we move from the equator to poles.
5. The temperature is high in urban habitats than rural areas due to the use of concretes and metals.
6. High rising structures also have higher temperature as they tend to trap warm air.
7. The standard unit of measuring temperature is Kelvin. However, in general, we use Celcius (°C) or Fahrenheit (°F) scales.

Air Pressure

1. The pressure exerted by the weight of air on the surface is known as air pressure.
2. As we go up, the air pressure falls. It is highest at sea level.
3. The horizontal distribution of air is affected by temperature.
4. In the high-temperature area, air gets heated and rises. This creates low air pressure zones and leads to the formation of clouds and rain.
5. In the lower temperature area, air gets cold and sinks. This creates a high-pressure area and associated with clear skies.
6. The air always moves from high-pressure areas to low-pressure areas.
7. There is no air on the moon. Henceforth, there is no air pressure.

Winds

1. The movement of air from high-pressure air to low air pressure is known as wind.
2. There are three major types of winds: Permanent winds, Seasonal Winds and Local Winds.
3. Permanent Winds blows constantly throughout the year. These are trade winds, westerlies and easterlies.
4. Seasonal winds change their direction according to the season such as monsoon winds in India.
5. Local winds blow during a particular time in a particular small area. Loo is an example of local winds.

Cyclone

1. Cyclones in India and Southeast Asia are the result of a depression in the Gulf of Thailand near Port Blair.
2. These are devasting in nature and cost heavily in terms of social and economic loss.

Moisture and Rainfall

1. Due to heat, water evaporates from the land and water bodies, it becomes water vapour.
2. The moisture in the air at any time is known as the humidity.
3. The water carrying capacity of air increase with an increase in its temperature.
4. When water vapour rises, it cools down. It condenses to become water droplet and later cloud.
5. When water droplets become too heavy, they fall on earth in the form of rain (precipitation).
6. On the basis of mechanism, there are three types of rainfall: 1. conventional, 2. orographic and 3. cyclonic rainfall.
7. Rainfall is important for the survival of plant and animal life as it is the major source of fresh water to the earth surface.
8. The white trails behind jet planes in the sky are the result of condensed moisture.