NCERT Solutions for Class 11th: Ch 9 Solar Radiation, Heat Balance and Temperature

NCERT Solutions for Class 11th: Ch 9 Solar Radiation, Heat Balance and Temperature 

Exercises

Page No: 78

1. Multiple choice questions.

(i) The sun is directly overhead at noon on 21st June at:
(a) The equator
(b) 23.5° S
(c) 23.5° N
(d) 66.5° N
►(c) 23.5° N

(ii) In which one of the following cities, are the days the longest?
(a) Tiruvanantpuram
(b) Chandigarh 
(c) Hyderabad
(d) Nagpur
► (a) Tiruvanantpuram

(iii) The atmosphere is mainly heated by the:
(a) Short wave solar radiation
(b) Reflected solar radiation 
(c) Long wave terrestrial radiation
(d) Scattered solar radiation
► (c) Long wave terrestrial radiation

(iv) Make correct pairs from the following two columns.

(i) Insolation (a) The difference between the mean temperature of the warmest and the coldest months
(ii) Albedo (b) The lines joining the places of equal temperature
(iii) Isotherm (c) The incoming solar radiation
(iv) Annual range (d) The percentage of visible light reflected by an object

Answer

(i) Insolation (c) The incoming solar radiation
(ii) Albedo (d) The percentage of visible light reflected by an object
(iii) Isotherm (b) The lines joining the places of equal temperature
(iv) Annual range (a) The difference between the mean temperature of the warmest and the coldest months

v) The main reason that the earth experiences highest temperatures in the subtropics in the northern hemisphere rather than at the equator is:
(a) Subtropical areas tend to have less cloud cover than equatorial areas.
(b) Subtropical areas have longer day hours in the summer than the equatorial.
(c) Subtropical areas have an enhanced “green house effect” compared to equatorial areas.
(d) Subtropical areas are nearer to the oceanic areas than the equatorial locations.
► (b) Subtropical areas have longer day hours in the summer than the equatorial.

2. Answer the following questions in about 30 words.

(i) How does the unequal distribution of heat over the planet earth in space and time cause variations in weather and climate?

Answer

The earth receives almost all of its energy from the sun. The earth in turn radiates back to space the energy received from the sun. As a result, the earth neither warms up nor does it get cooled over a period of time. Thus, the amount of heat received by different parts of the earth is not the same. This variation causes pressure differences in the atmosphere. This leads to transfer of heat from one region to the other by winds. Thus, the unequal distribution of heat over the planet earth in space and time cause variations in weather and climate.

(ii) What are the factors that control temperature distribution on the surface of the earth?

Answer

The factors that control temperature distribution on the surface of the earth are:
• The latitude of the place
• The altitude of the place
• Distance from the sea, the airmass circulation
• The presence of warm and cold ocean currents
• Local aspects

(iii) In India, why is the day temperature maximum in May and why not after the summer solstice?

Answer

The day temperature maximum in May because of the summer solstice. At that time, sun's rays are overhead the tropic of cancer (23.5°N). Tropic of Cancer passes through the middle of India. It remains till the end of May in India. Before summer solstice i.e., 21st June, monsoon starts in India which brings a cooling effect to the climate of India. This is why, India experiences high temperature before summer solstice.

(iv) Why is the annual range of temperature high in the Siberian plains?

Answer

The mean January temperature between 80°N and 50°N is minus 20°C and the temperature in July is more than 10°C. That is why annual range of temperature is very high.

3. Answer the following questions in about 150 words.

(i) How do the latitude and the tilt in the axis of rotation of the earth affect the amount of radiation received at the earth’s surface?

Answer

The amount of insolation received is the angle of inclination of the rays. This depends on the latitude of a place. The higher the latitude the less is the angle they make with the surface of the earth resulting in slant sun rays. The area covered by vertical rays is always less than the slant rays. If more area is covered, the energy gets distributed and the net energy received per unit area decreases. Moreover, the slant rays are required to pass through greater depth of the atmosphere resulting in more absorption, scattering and diffusion. Thus, the high latitudinal areas get less isolation an vice versa.
Sunrays fall vertically on equator throughout the year. The sun rays keep changing from 0° to 23.5° north and south. The sun is in the southern hemisphere and its rays fall vertically on tropic of cancer from 1st March to 21st March. The sun is in the northern hemisphere and its rays fall vertically on tropic of Capricorn from 23rd September to 22nd September. As we towards the poles, temperature keeps on decreasing. After 66 1⁄2 ° north and south there is cold zone. Here, the temperature remains low throughout the year because the sun's rays fall tilted on it. Thus, the tilt in the axis of rotation of the earth affect the amount of radiation received at the earth’s surface.

(ii) Discuss the processes through which the earth-atmosphere system maintains heat balance.

Answer

The energy of sun reaches earth through radiation and circulates through various processes. The earth as a whole does not accumulate or loose heat. It maintains its temperature. This can happen only if the amount of heat received in the form of insolation equals the amount lost by the earth through terrestrial radiation. 

• Of the 100% energy radiated by Sun. While passing through the atmosphere some amount of energy is reflected, scattered and absorbed. 

• Only the remaining part reaches the earth surface. Roughly 35 units are reflected back to space even before reaching the earth’s surface. 

• Of these, 27 units are reflected back from the top of the clouds and 2 units from the snow and ice-covered areas of the earth. 

• The remaining 65 units are absorbed, 14 units within the atmosphere and 51 units by the earth’s surface. The earth radiates back 51 units in the form of terrestrial radiation.
• Of these, 17 units are radiated to space directly and the remaining 34 units are absorbed by the atmosphere. 48 units absorbed by the atmosphere are also radiated back into space. 

• Thus, the total radiation returning from the earth and the atmosphere respectively is 17+48=65 units which balance the total of 65 units received from the sun. 

This is termed the heat budget or heat balance of the earth which explains the earth neither warms
up nor cools down despite the huge transfer of heat that takes place.

(iii) Compare the global distribution of temperature in January over the northern and the southern hemisphere of the earth.

Answer

The isotherms are generally parallel to the latitude. In the northern hemisphere the land surface area is much larger than in the southern hemisphere. Hence, the effects of land mass and the ocean currents are well pronounced. In January the isotherms deviate to the north over the ocean and to the south over the continent. This can be seen on the North Atlantic Ocean. The presence of warm ocean currents, Gulf Stream and North Atlantic drift, make the Northern Atlantic Ocean warmer and the isotherms bend towards the north. Over the land the temperature decreases sharply and the isotherms bend towards south in Europe. The effect of the ocean is well pronounced in the southern hemisphere. Here the isotherms are more or less parallel to the latitudes and the variation in temperature is more gradual than in the northern hemisphere. The isotherm of 20° C, 10° C, and 0° C runs parallel to 35° S, 45° S and 60° S latitudes respectively.


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