CBSE NCERT Class VII (7th) | Social Studies | Geography
Chapter : Air
CBSE NCERT Solved Question Answer
Q1. Define the following:
a) Atmosphere: A huge blanket of air surrounding the earth is called Atmosphere.
b) Temperature: The degree of hotness and coldness of the air is known as temperature.
c) Insolation: Insolation is the incoming solar energy intercepted by the earth.
d) Air Pressure: Air pressure is defined as the pressure exerted by the weight of air on the earth’s surface.
e) Wind: The movement of air from high pressure area to low pressure areas is called wind.
f) Humidity: Moisture in the air at any time is known as humidity.
g) Rain: Precipitation that comes down to the earth in liquid form is called rain.
Q2. What are the uses of Atmosphere?
1. All living beings on this earth depend on the atmosphere for their survival. It provides us the air we breathe and protects us from the harmful effects of the sun’s rays.
2. Without this blanket of protection, we would be baked alive by the heat of the sun during day and get frozen during night. So it is this mass of air that has made the temperature on the earth liveable.
Q3. Explain the composition of the Atmosphere.
Nitrogen and oxygen are two gases which make up the bulk of the atmosphere. 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.
Q4. State the uses of Nitrogen, Oxygen and carbon di oxide.
Nitrogen is the most plentiful gas in the air. When we inhale, we take some amount of nitrogen into our lungs and exhale it. But plants need nitrogen for their survival. They cannot 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.
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 make a perfect balance.
Q5. Describe the 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
Troposphere: 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.
Stratosphere: Above the troposphere lies the stratosphere. It extends up to a height of 50 km. This layer is almost free from clouds and associated weather phenomenon, making conditions most ideal for flying aeroplanes. One important feature of stratosphere is that it contains a layer of ozone gas.
Mesosphere: 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 the space.
Thermosphere: In thermosphere temperature rises very rapidly with increasing height. Ionosphere is a part of this layer. It extends between80-400 km. This layer helps in radio transmission. In fact, radio waves transmitted from the earth are reflected back to the earth by this layer.
Exosphere: The upper most layer of the atmosphere is known as exosphere. This layer has very thin air. Light gases like helium and hydrogen float into the space from here.
Weather is this hour-to-hour, day to day condition of the atmosphere. Weather can change dramatically from day to day. However, the average weather condition of a place for a longer period of time represents the climate of a place.
Q7. Why is temperature in cities much higher than that in the villages?
Temperature in cities is much higher than that of villages as the concrete and metals in buildings and the asphalt of roads get heated up during the day. This heat is released during the night. Also, the crowded high rise buildings of the cities trap the warm air and thus raise the temperature of the cities.
Q8. Explain the phenomenon of air pressure.
As we go up the layers of 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 temperature of air at a given place. In areas where 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 temperature, 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.
Q9. Why do astronauts have to wear special suits in the space?
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.
Q10. Explain the different types of winds. Winds can be broadly divided into three types.
1. Permanent winds – The trade winds, westerlies and easterlies are the permanent winds. These blow constantly throughout the year in a particular direction.
2. Seasonal winds – These winds change their direction in different seasons. For example, monsoons in India.
3. Local winds – These blow only during a particular period of the day or year in a small
area. For example, land and sea breeze.
Q11. What is precipitation?
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 air, then they come down as precipitation.
Q12. Name the different types of rainfall.
On the basis of mechanism, there are three types of rainfall:
The convectional rainfall, The orographic rainfall and The cyclonic rainfall
Q13. State the uses of the following instruments:
Thermometer: Measures the temperature
Barometer: Measures atmospheric pressure
Rain Gauge: Measures the amount of rainfall
Wind Vane: Shows the direction of the wind
Q14. What is Global Warming? How does it affect our Earth?
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 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 extinction of some plants and animals in the long run.