NCERT Solutions for Class 11 Geography Chapter 7 - Composition and Structure of Atmosphere

91. Multiple choice questions.

(i) Which one of the following gases constitutes the major portion of atmosphere?

(a) Oxygen

(b) Nitrogen

(c) Argon

(d) Carbon dioxide

Ans. (b) Nitrogen

(ii) Atmospheric layer important for human beings is:

(a) Stratosphere

(b) Mesosphere

(c) Troposphere

(d) Ionosphere

Ans. (c) Troposphere

(iii) Sea salt, pollen, ash, smoke soot, fine soil — these are associated with:

(a) Gases

(b) Dust particles

(c) Water vapour

(d) Meteors

Ans. (b) Dust particles

(iv) Oxygen gas is in negligible quantity at the height of atmosphere:

(a) 90 km

(b) 120 km

(c) 100 km

(d) 150 km

Ans. (b) 120 km

(v) Which one of the following gases is transparent to incoming solar radiation and opaque to outgoing terrestrial radiation?

(a) Oxygen

(b) Nitrogen

(c) Helium

(d) Carbon dioxide

Ans. (d) Carbon dioxide

92. Answer the following questions in about 30 words.

(i) What do you understand by atmosphere?

Ans. The atmosphere, a concoction of several gases, surrounds the globe on all sides. It has gases that are essential for life, like as carbon dioxide for plants and oxygen for people and other creatures. The atmosphere makes up a significant portion of the earth’s mass, and 99 percent of it is contained within the first 32 kilometres of space above the planet’s surface. Only when it blows as wind can one feel the colourless, odourless air.

(ii) What are the elements of weather and climate?

Ans. The main elements of atmosphere which are subject to change and which influence human life on earth are temperature, pressure, winds, humidity, clouds and precipitation.

(iii) Describe the composition of atmosphere.

Ans. There are gases, water vapour, and dust particles in the atmosphere. In the atmosphere, nitrogen makes up 78.8% of the total, oxygen makes  up 20.94%, and argon makes up 0.93%. Other gases include krypton, xenon, neon, methane, hydrogen, helium, ozone, and carbon dioxide.

(iv) Why is troposphere the most important of all the layers of the atmosphere?

Ans. The troposphere the most important of all the layers of the atmosphere because of the following reasons:

(a) This layer is where all climatic and meteorological changes occur.

(b) This layer is crucial for all cellular function.

(c) At the equator, the troposphere is at its thickest because heat is carried to vast altitudes by powerful convectional currents.

(d) This layer contains water vapour and dust particles.

93. Answer the following questions in about 150 words.

(i) Describe the composition of the atmosphere.

Ans. The atmosphere is composed of gases, water vapour and dust particles.

(a) Gases: In terms of meteorology, carbon dioxide is a crucial gas. It filters off some of the terrestrial radiation and reflects back some of it toward the surface of the planet. The greenhouse effect is largely its fault. Other gases have a constant volume, however during the past few decades, carbon dioxide has increased more than other gases. Between 10 and 50 kilometres above the surface of the earth, ozone serves as a filter by absorbing the sun’s ultraviolet rays and blocking their path to the earth’s surface.

(b) Water Vapour: Another changeable gas in the atmosphere is water vapour. With altitude, it gets smaller. It may make up 4% of the air in the humid and warm tropics, whereas in the chilly and dry desert and polar regions, it may make up less than 1% of the air. Along with temperature, water  vapour falls from the equator to the poles. Additionally, it absorbs some of the sun’s insolation, maintains the heat that the earth radiates, and functions as a blanket to keep the planet’s temperature balanced.

(c) Dust Particles: The lower parts of the atmosphere often have the highest concentration of dust particles. Compared to equatorial and polar locations, subtropical
and temperate regions have a higher concentration of dust particles because of dry winds. In order to create clouds, water vapour condenses around dust and salt
particles, which serve as hygroscopic nuclei.

(ii) 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.

These are troposphere, stratosphere, mesosphere, ionosphere and exosphere.

The troposphere is the lowermost layer of the atmosphere. Its average height is 13 km. It 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 165 m of height. This is the most important layer for all biological activities. The zone separating the troposphere from stratosphere is known as the tropopause.

The stratosphere is the second layer. It is found above the tropopause and extends up to a height of 50 km. The stratosphere contains the ozone layer. Ozone layer absorbs ultra-violet radiation and shields life on the earth from intense, harmful form of energy.

The mesosphere lies above the stratosphere. It 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. The upper limit of mesosphere  is known as the mesopause.

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.

The exosphere is the uppermost layer of the atmosphere. This is the highest layer but very little is known about it. Whatever contents are there, these are extremely rarefied in this layer, and it gradually merges with the outer space. All layers of the atmosphere must be exercising influence on us, geographers are concerned with the first two layers of the atmosphere.

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