Composition and Structure of Atmosphere - Solutions

 CBSE Class 11 Geography

NCERT Solutions
Chapter 8
Composition and Structure of Atmosphere

  1. Multiple choice questions.
    1. Which one of the following gases constitutes the major portion of the atmosphere?
      1. Oxygen
      2. Nitrogen
      3. Argon
      4. Carbon dioxide.
      Ans. (b) Nitrogen
    2. Atmospheric layer important for human beings is:
      1. Stratosphere
      2. Mesosphere
      3. Troposphere
      4. Ionosphere.
      Ans. (c) Troposphere
    3. Sea salt, pollen, ash, smoke soot, fine soil - these are associated with:
      1. Gases
      2. Dust particles
      3. Water vapour
      4. Meteors.
      Ans. (b) Dust particles
    4. Oxygen gas is in negligible quantity at the height of atmosphere:
      1. 90 km
      2. 120 km
      3. 100 km
      4. 150 km.
      Ans. (b) 120 km
    5. Which one of the following gases is transparent to incoming solar radiation and opaque to outgoing terrestrial radiation?
      1. Oxygen
      2. Nitrogen
      3. Helium
      4. Carbon dioxide.
      Ans. (d) Carbon dioxide.
  1. Answer the following questions in about 30 words.
    1. What do you understand by atmosphere?
      Ans. One of the four components of the Earth's ecosystem (the other three are biosphere , hydrosphere, and lithosphere), it is a band of gases enveloping the Earth's surface. Ninety-nine percent of its mass is concentrated within 20 miles of the earth's surface, and its two largest constituents (in the lowest part) are nitrogen (about 78 percent) and oxygen (about 21 percent). The remaining one percent includes mostly argon and minute amounts of carbon dioxide, helium, krypton, methane, neon, sulfur dioxide, water vapour , and xenon. In meteorology, atmosphere is divided into seven layers called regions (from the lowest to the highest): troposphere, stratosphere, mesosphere, chemosphere, thermosphere, ionosphere, and exosphere. The atmosphere is composed of gases, water vapour and dust particles. The proportion of gases changes in the higher layers of the atmosphere in such a way that oxygen will be almost in negligible quantity at the height of 120 km. Similarly, carbon dioxide and water vapour are found only up to 90 km from the surface of the earth.
    2. What are the elements of weather and climate?
      Ans. Weather is conditions of temperature, humidity, pressure, etc at a given point of time while climate is condition of these elements for a longer period of time. Following are the important elements of weather and climate:
      1. Temperature: A temperature is an objective comparative measurement of hot or cold. It is measured by a thermometer. Several scales and units exist for measuring temperature, the most common being the Celsius scale (with units denoted °C; formerly called degrees centigrade), the Fahrenheit scale (with units denoted °F), and, especially in science, the Kelvin scale (with units denoted K). It affects weather as well as climate.
      2. Pressure: Pressure keeps on decreasing with increase in height.The absolute pressure exerted by the air within the tire, including atmospheric pressure, is 45 pounds per square inch. Pressures less than atmospheric are negative gauge pressures that correspond to partial vacuums.
      3. Wind: Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. The flow of wind also affects weather and climate.
      4. Humidity: Humidity is the amount of water vapor present in the air. Water vapor is the gaseous state of water and is invisible to the human eye. Humidity indicates the likelihood of precipitation, dew, or fog. Clouds and rain are important factors of climate.
    3. Describe the composition of atmosphere.
      Ans. The atmosphere is composed of gases, water vapour and dust particles. Nitrogen constitutes 78.8%, oxygen contributes 20.94% and argon contributes 0.93% in atmosphere. Other gases include carbon dioxide, helium, ozone, methane, hydrogen, krypton, xenon and neon, etc. Nitrogen and oxygen together constitute 99% of the atmosphere. Neon, krypton, xenon are scarce gases. The proportion of gases changes in the higher layers of the atmosphere in such a way that oxygen will be almost in negligible quantity at the height of 120 km. Similarly, carbon dioxide and water vapour are found only up to 90 km from the surface of the earth.
    4. Why is troposphere the most important of all the layers of the atmosphere?
      Ans. The troposphere is the lowermost layer of the atmosphere. Its average height is 13 km and extends roughly to a height of 8 km near the poles and about 18 km at the equator. It is most important layer of the atmosphere because:
      1. Thickness of the troposphere is greatest at the equator because heat is transported to great heights by strong convectional currents.
      2. This layer contains dust particles and water vapour.
      3. All changes in climate and weather take place in this layer.
      4. The temperature in this layer decreases at the rate of 1°C for every 165m of height.
      5. All biological activities take place in this layer. Nearly all of the water vapor and dust particles in the atmosphere are in the troposphere. That is why most clouds are found in this lowest layer, too. The bottom of the troposphere, right next to the surface of Earth, is called the "boundary layer.
  1. Answer the following questions in about 150 words.
    1. Describe the composition of the atmosphere.
      Ans. The composition of the atmosphere can be understood by considering following table.
      Gases of the Atmosphere
      ConstituentFormulae%byVolume
      NitrogenN78.08
      OxygenO220.95
      ArgonAr0.93
      Carbon -dioxideCO0.036
      NeonNe0.002
      HeliumHe0.0005
      KryptonKr0.001
      XenonXe0.00009
      HydrogenH20.00005
      The atmosphere is composed of gases, water vapour and dust particles. Nitrogen constitutes 78.8%, oxygen contributes 20.94% and argon contributes 0.93% in atmosphere. Other gases include carbon dioxide, helium, ozone, methane, hydrogen, krypton, xenon and neon, etc. Nitrogen and oxygen together constitute 99% of the atmosphere. Neon, krypton, xenon are scarce gases. The proportion of gases changes in the higher layers of the atmosphere in such a way that oxygen will be almost in negligible quantity at the height of 120 km. Similarly, carbon dioxide and water vapour are found only up to 90 km from the surface of the earth. Carbon dioxide is meteorologically a very important gas as it is transparent to the incoming solar radiation but opaque to the outgoing terrestrial radiation. It absorbs a part of terrestrial radiation and reflects back some part of it towards the earth's surface. It is largely responsible for the green house effect. Ozone is another important component of the atmosphere. It is found between 10 and 50 km above the earth's surface and acts as a filter. It absorbs the ultra-violet rays radiating from the sun. It prevents them from reaching the surface of the earth.
      Water vapour is such a variable gas in the atmosphere, which decreases with altitude. In the warm and wet tropics, it may account for four per cent of the air by volume, while in the dry and cold areas of desert and polar regions, it may be less than one per cent of the air. Atmosphere has a sufficient capacity to keep small solid particles, which may originate from different sources and include sea salts, fine soil, smoke-soot, ash, pollen, dust and disintegrated particles of meteors.
    2. Draw a suitable diagram for the structure of the atmosphere and label it and describe it.
      Ans. The atmosphere consists of different layers with varying density and temperature. Density is highest near the surface of the earth and decreases with increasing altitude. The atmosphere is divided into five different layers depending upon the temperature condition. They are: troposphere, stratosphere, mesosphere, thermosphere and exosphere.
      1. Troposphere: The troposphere is the lowermost layer of the atmosphere. Its average height is 13 km and extends roughly to a height of 8 km near the poles and about 18 km at the equator. Thickness of the troposphere is greatest at the equator because heat is transported to great heights by strong convectional currents. This layer contains dust particles and water vapour. All changes in climate and weather take place in this layer. The temperature in this layer decreases at the rate of 1 °C for every 165m of height. Nearly all of the water vapor and dust particles in the atmosphere are in the troposphere. That is why most clouds are found in this lowest layer, too. The bottom of the troposphere, right next to the surface of Earth, is called the "boundary layer
      2. Stratosphere: The stratosphere is a layer of Earth's atmosphere. It is the second layer of the atmosphere as you go upward. The troposphere, the lowest layer, is right below the stratosphere. The next higher layer above the stratosphere is the mesosphere.The stratosphere is found above the stropopause and extends up to a height of 50 km. One important feature of the stratosphere is that it contains the ozone layer. This layer absorbs ultra-violet radiation and shields life on the earth from intense, harmful form of energy.
      3. Mesosphere: The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the mesopause. In the mesosphere, temperature decreases as the altitude increases. The mesosphere lies above the stratosphere, which extends up to a height of 80 km. In this layer, temperature starts decreasing with the increase in altitude and reaches up to minus 100°C at the height of 80 km.
      4. Ionosphere: The ionosphere is defined as the layer of the Earth's atmosphere that is ionized by solar and cosmic radiation. It lies 75-1000 km (46-621 miles) above the Earth.The ionosphere is located between 80 and 400 km above the mesopause. It contains electrically charged particles known as ions, and hence, it is known as ionosphere. Radio waves transmitted from the earth are reflected back to the earth by this layer. Temperature here starts increasing with height.
      5. Exosphere: The exosphere is the uppermost region of Earth's atmosphere as it gradually fades into the vacuum of space. Air in the exosphere is extremely thin - in many ways it is almost the same as the airless void of outer space.The uppermost layer of the atmosphere above the thermosphere is known as the exosphere. This is the highest layer but very little is known about it.