CBSE Class 7 Social Science Geography Revision Notes Chapter 4

CBSE Class 7 Geography Chapter 4 Notes – Air

Class 7 Geography Chapter 4 Notes are prepared by experienced subject matter experts at Extramarks to help the students in their exams. The notes are updated as per the latest CBSE Syllabus. These well-structured and comprehensive notes explain each topic very clearly. Students will not have any doubts regarding the chapter after going through the notes. Students will be able to answer Important Questions of the chapter from the pointers included in the notes. By going through these notes, students will understand the concepts better and fetch more marks in the exam. 

Air Class 7 Notes Geography Chapter 4 Notes

Access Class 7 Geography Chapter 4 Notes – Air

The Atmosphere And Its Composition

• Air

Air is the mixture of several gases which make up the Earth’s atmosphere. The Earth’s atmosphere primarily consists of nitrogen and oxygen, which are present in large quantities in the atmosphere. Other gases like carbon dioxide, hydrogen, ozone, helium, argon, etc., are also present in a smaller amount. Except for these gases, dust particles are also present in the air.

• Nitrogen

The atmosphere comprises 78%  nitrogen gas. Therefore, it is the most abundant gas found in the atmosphere. This gas is crucial for plant growth. But plants are not capable of absorbing it directly from the atmosphere. Several microbes living in the soil and the roots of the plants absorb nitrogen and convert it into a form that plants can use easily.

• Oxygen

After nitrogen, oxygen is the most abundant gas in the atmosphere. All living beings need oxygen for respiration. Green plants produce oxygen through photosynthesis and maintain the balance of oxygen in the atmosphere.  The balance is disturbed when humans destroy forests or cut trees in localities.

• Carbon Dioxide

Carbon dioxide is another vital gas in the atmosphere that is present in small quantities. Through photosynthesis, plants are able to capture the carbon dioxide that is released into the air by humans and other animals during respiration. As a result, the balance is maintained. However, burning fossil fuels releases large quantities of carbon dioxide, which disturbs this balance.

The Structure Of The Atmosphere

The atmosphere has five layers, namely, Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere.

• Troposphere

This is the lowest layer of the atmosphere. It extends up to a height of thirteen kilometres from the Earth’s surface. This is the layer where life flourishes. All of the gases, including the oxygen required for breathing, are present in this layer. Several phenomena of weather, like rainfall, thunderstorms, fog, etc., take place in the troposphere.

• Stratosphere

The average height of this layer is fifty kilometres. No weather phenomena can be seen here. Hence, it is ideal for aircraft to fly in the stratosphere. This atmospheric layer has a sub-layer of ozone which protects the Earth from the harmful effects of  UV rays.

• Mesosphere

This layer extends up to eighty kilometres. When meteoroids enter from space into the Earth’s atmosphere, they burn up in this layer.

• Thermosphere

This sphere is marked by a rapid increase in temperature. The thermosphere extends up to four hundred kilometres. It contains a sub-layer called the ionosphere, which is responsible for transmitting radio waves from one place to another on the Earth.

• Exosphere

This is the topmost layer of the atmosphere. Low-density gases like hydrogen and helium flow into space from the exosphere.

Weather And Climate

Weather is the condition of the atmosphere of a given place at a certain time. The weather of a place may change within a day. Even if the sun shines brightly in the day, there is no certainty that it will not rain in the evening. The climate is the average weather condition of a place as observed for a long period of time.

There are several factors that influence the weather and climate of an area. They are:

• Temperature
• Air Pressure
• Wind
• Moisture
• Cyclone

Temperature

• The temperature of a place refers to the intensity of the hotness or coldness of the atmosphere of the given area.
• The Sun is the direct source of the temperature on Earth. The temperature of a place depends on the amount of sunlight it receives. Therefore, the temperature decreases in winter and increases in summer.
• Insolation is another reason for the rise in temperature. Insolation is the solar energy that gets trapped in the Earth’s atmosphere and causes an overall temperature increase.
• The largest amount of heat is intercepted by the Earth in the equatorial region. The polar regions, on the other hand, do not get enough exposure to sunlight. The amount of insolation is less there. For this reason, ice continues to cover the polar regions all year round.
• The construction materials of the multistory buildings in the cities, like concrete and steel frames, heat up quickly during the day and release the heat at night. This heat cannot escape the atmosphere and gets trapped there, thus, increasing the overall temperature of the region. That is why cities are usually warmer than rural areas.

Air Pressure

• Air pressure is the weight of an air column exerted on the Earth’s surface. 

• It is influenced by the temperature of the place.

• Air pressure decreases as the height of a place increases. The places at sea level have higher air pressure than those at higher altitudes.

• The air becomes light and warm when the temperatures are high. As the light air moves upward, it creates a low-pressure belt. Low-pressure zones usually witness cloudy skies and wet weather.

• On the contrary, in places where the temperature is relatively low, the air is cold and heavy, creating a high-pressure belt. These places usually have clear skies and bright weather.

• The air tends to blow from a high-pressure area to a low-pressure area.

Wind

• The movement of air from the high-pressure zone to the low-pressure zone is known as wind.
• There are three types of wind, namely, permanent wind, seasonal wind, and local wind.
• Permanent winds blow regularly in one direction throughout the year. For example, Trade Winds, Easterlies, and Westerlies.
• Seasonal winds blow in a particular season in a given place. For example, monsoon winds blow in India only during monsoons.
• Local winds blow in a limited geographical locale during a particular time of the day or year. For example, land breezes and sea breezes are the local winds of coastal areas. Loo, which blows in northern India, is also an instance of local wind.

Moisture

• The humidity of a place refers to the amount of moisture in the air.
• Water heated up with sunlight evaporates into the atmosphere. The humidity of a place increases with the increasing amount of water vapour in the air.
• As the temperature rises, so does the capacity of the atmosphere to hold moisture.
• On humid days, clothes take a long time to dry.  People are bothered because sweat does not easily evaporate from their bodies. The white trail that becomes visible in the sky after a jet plane passes, is the condensed water vapour released by the engine. It dissolves with the movement of the air.
• The water that diffuses in the air in the form of vapour forms clouds in the high sky. Clouds are nothing but an accumulation of moisture. Rain occurs when clouds reach their maximum level of moisture saturation.
• This rainwater is further stored on the Earth in the form of groundwater. Trees and plants hold the water droplets in the spoil with their roots. If trees in a place are cut down, the place becomes devoid of any natural mechanism to restrict the rainwater in that area. As a result, the water flows down and floods other regions.
• Rainwater is essential for survival as it is a major source of freshwater on Earth.
• Convectional rainfall, orographic rainfall, and cyclonic rainfall are the three different forms of rainfall.
• If there is excessive rainfall, it causes a flood; but if there is not enough rain, it leads to drought.

Cyclone

• A cyclone is a natural phenomenon and is considered a weather disturbance that causes extreme damage to life and property.
• A cyclone rotating around a low-pressure centre is known as a hurricane.
• The centre of low pressure is called the “eye of the storm.”
• Odisha, on the eastern coast of India, is a cyclone-prone zone.
• This state frequently becomes the victim of storms formed in the Bay of Bengal.

Important Questions And Answers

Q1. What are the different types of rainfall? Name at least one place for each type of rainfall.

A1. There are three types of rainfall.

• Convectional Rainfall: This type of rainfall happens where the temperature is exceptionally high, for example, in the equatorial regions. Water evaporates from the water bodies to form clouds in the upper sky and comes back in the form of rainfall. This type of rainfall usually does not happen in India.
• Orographic Rainfall: The Western Ghat hills experience this type of rainfall. The air saturated with moisture from the Arabian Sea cannot cross the height of the Western Ghats and causes rain on its windward side.
• Cyclonic Rainfall: This type of rainfall is usually seen on the eastern coast of India. The cyclone formed in the Bay of Bengal rushes towards India and hits the coastal region first, causing rain. Odisha is a place that experiences rain due to cyclones.

Q2. What is the atmosphere? Why is it crucial for life?

A2. An atmosphere is a layer of air surrounding the Earth like a blanket.

The atmosphere contains many types of gases that are essential for life, for example, nitrogen, oxygen, and carbon dioxide. The layer of ozone in the stratosphere protects us from harmful cosmic rays. Furthermore, the atmosphere negates the meteors’ ability to cause devastation on Earth.

Q3. What is meant by the greenhouse effect? Name a few gases that cause the greenhouse effect.

A3. When solar energy is trapped for an extended period, it raises the global temperature on Earth. This phenomenon is called the greenhouse effect.

The primary greenhouse gases include carbon dioxide, methane, chlorofluorocarbons, etc.

Q4. What is meant by weather?

A4. The term weather refers to the atmospheric conditions of a given place at a particular time. Temperature, air pressure, moisture, wind, etc., are the determining factors of the weather of a place.

Q5. What is insolation?

A5. Insolation is the amount of solar energy trapped by the Earth. It has two units: kilowatt hours per square metre per day (kWh/sq/day), and W/sq.

Q6. What is the barometric pressure?

A6. Barometric pressure, or air pressure, is one of the several factors that influence the weather of a place. It is the pressure exerted by the air column on the Earth’s surface. At sea level, air pressure is at its maximum.

Q7. Why do clothes dry more slowly on rainy days?

A7. The humidity of the atmosphere remains very high on rainy days. So, it cannot absorb more moisture. Therefore, the moisture does not evaporate easily from the clothes, leaving them with a damp effect.