Class 7 Geography Chapter 4 Notes / Air Class 7 Notes Pdf
Class 7 Geography Chapter 4 Notes notes pdf provide you with a basic understanding of Air, Atmosphere, Structure of Atmosphere, Weather and Climate. Class 7th Geography Ch 4 Air notes are based on NCERT Books. Students can also download class 7 geography chapter 4 notes pdf for offline study. We provide NCERT Notes for all Subjects you can also check them.
Air Class 7 Notes Overview
Air Notes Class 7 pdf is a very interesting chapter in class 7 geography notes. If you searching for Class 7 Geography Ch 4 notes we provide that in this post.
Our earth is surrounded by a vast blanket of air called the atmosphere.
The atmosphere provides us with Air that we breathe and protects us from the harmful effects of the sun’s rays. Without this blanket of protection, we would be baked alive by the heat of the sun during the day and get frozen during the night. It is the mass of air that has made the temperature on the earth liveable.
Composition of the Atmosphere
There are a number of gases present in the atmosphere.
Air is a mixture of many gases. Nitrogen and Oxygen are the most abundant and Carbon dioxide, helium,
ozone, argon and hydrogen are found in lesser quantities. Apart from these gases, tiny dust particles are also present in the air.
Percentage of Different in the Atmosphere
- Nitrogen = 78%
- Oxygen = 21%
- Carbon Dioxide = 0.03%
- Argon = 0.93%
- All Other = 0.04%
When air is heated, it expands, becomes lighter and goes up. Cold air is denser and heavy. That is why it tends to sink down. When hot air rises, cold air from the surrounding area rushes there to fill in the gap. That is how air circulation takes place.
- Nitrogen is the most plentiful gas in the air.
- When we inhale, we take some amount of nitrogen into our lungs and exhale it.
- Plants need nitrogen for their survival.
- The plant can not take nitrogen directly from the air. Bacteria, that live in the soil and roots of some plants, take nitrogen from the air and change its form so that plants can use it.
- Oxygen is the second most plentiful gas in the air.
- Humans and animals take oxygen from the air as they breathe.
- Green plants produce oxygen during photosynthesis. In this way, oxygen content in the air remains constant.
- Carbon dioxide is another important gas.
- Green plants use carbon dioxide to make their food and release oxygen.
- Humans or animals release carbon dioxide.
- The amount of carbon dioxide released by humans or animals seems to be equal to the amount used by the plants which makes a perfect balance.
- The burning of fuels, such as coal and oil realise CO2. They add billions of tons of carbon dioxide into the atmosphere each year. that disturb the balance.
- The increased volume of carbon dioxide is affecting the earth’s weather and climate.
Carbon dioxide released in the atmosphere creates a greenhouse effect by trapping the heat radiated from the earth. It is therefore called a greenhouse gas and without it, the earth would have been too cold to live in. However, when its level in the atmosphere increases due to factory smoke or car fumes, the heat retained increases the temperature of the earth. This is called global warming. This rise in temperature causes the snow in the coldest parts of the world to melt. As a result, the sea level rises, causing floods in the coastal areas. There may be drastic changes in the climate of a place leading to the extinction of some plants and animals in the long run.
Structure of the Atmosphere
Our atmosphere is divided into five layers starting from the earth’s surface. These are Troposphere,
Stratosphere, Mesosphere, Thermosphere and Exosphere.
- This layer is the most important layer of the atmosphere.
- Its average height is 13 km.
- The air we breathe exists here.
- Almost all the weather phenomena like rainfall, fog and hailstorm occur in this layer.
- It lies the above the stratosphere.
- It extends up to a height of 50 km.
- This layer is almost free from clouds and associated weather phenomena, making conditions
- Most ideal for flying aeroplanes.
- One important feature of the stratosphere is that it contains an Ozone layer.
- This is the third layer of the atmosphere.
- It lies above the stratosphere.
- It extends up to the height of 80 km.
- Meteorites burn up in this layer on entering from space.
- In the thermosphere, the temperature rises very rapidly with increasing height.
- The ionosphere is a part of this layer.
- It extends between 80-400 km.
- This layer helps in radio transmission. In fact, radio waves transmitted from the earth are reflected back to the earth by this layer.
- The uppermost layer of the atmosphere is known as the exosphere.
- This layer has very thin air.
- Light gases like helium and hydrogen float into space from here.
Weather And Climate
Weather is the hour-to-hour, day-to-day condition of the atmosphere. Weather can change dramatically from day to day.
The average weather condition of a place for a longer period of time represents the climate of the place.
- The degree of hotness and coldness of the air is known as temperature.
- The temperature of the atmosphere changes not only between day and night but also from season to season.
- An important factor that influences the distribution of temperature is insolation.
- Insolation is the incoming solar energy intercepted by the earth.
- The amount of insolation decreases from the equator towards the poles. Therefore, the temperature decreases in the same manner.
The standard unit of measuring temperature is degree Celsius. It was invented by Anders Celsius. On the Celsius scale, the water freezes at 0°C and boils at 100°C.
The air above us presses us with a great force on our bodies. However, we don’t even feel it. This is because the air presses us from all directions and our body exerts a counter pressure.
Air pressure is defined as the pressure exerted by the weight of air on the earth’s surface.
- As we go up the layers of the atmosphere, the pressure falls rapidly. The air pressure is highest at sea level and decreases with height.
- Horizontally the distribution of air pressure is influenced by the temperature of the air at a given place.
- In areas where the temperature is high the air gets heated and rises. This creates a low-pressure area. Low pressure is associated with cloudy skies and wet weather.
- In areas having lower temperatures, the air is cold. It is therefore heavy. Heavy air sinks and creates a high-pressure area. High pressure is associated with clear and sunny skies.
- The air always moves from high-pressure areas to low-pressure areas.
On the moon there is no air and hence no air pressure. Astronauts have to wear special protective space suits filled with air when they go to the moon. If they did not wear these space suits, the counter pressure exerted by the body of the astronauts would make the blood vessels burst. The astronauts would bleed.
The movement of air from high-pressure areas to low-pressure areas is called wind.
Winds can be broadly divided into three types.
- Permanent winds – The trade winds, westerlies and easterlies are the permanent winds. These blow constantly throughout the year in a particular direction.
- Seasonal winds – These winds change their direction in different seasons. For example monsoons in India.
- Local winds – These blow only during a particular period of the day or year in a small area. For example, land and sea breeze. Do you recall the hot and dry local wind of northern planes of India? It is called a loo.
A wind is named after the direction from which it blows, e.g. the wind blowing from the west is called westerly.
When water evaporates from land and different water bodies, it becomes water vapour. Moisture
in the air at any time, is known as humidity.
- When the air is full of water vapour we call it a humid day.
- As the air gets warmer, its capacity to hold the water vapour increases and so it becomes more and more humid.
- On a humid day, clothes take longer to dry and sweat from our body does not evaporate easily, making us feel very uncomfortable.
- When the water vapour rises, it starts cooling.
- The water vapour condenses causing formation of droplets of water.
- Clouds are just masses of such water droplets.
- When these droplets of water become too heavy to float in the air, then they come down as precipitation.
Jet planes flying in the sky leave a white trail behind them. The moisture from their engines condenses. We see trails of this condensed moisture for some time when there is no air movement to disturb it.
- Precipitation that comes down to the earth in liquid form is called rain.
- Most of the groundwater comes from rainwater.
- Plants help preserve water. When trees on hillsides are cut, rainwater flows down the bare mountains and can cause flooding of low-lying areas.
- On the basis of mechanism, there are three types of rainfall:
- The convectional rainfall,
- The orographic rainfall and
- The cyclonic rainfall
Rainfall is very important for the survival of plants and animals. It brings fresh water to the earth’s surface. If
rainfall is less – water scarcity and drought occur. On the other hand, if it is more, floods take place.
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