NCERT Solutions for Class 11 Geography - Solar Radiation, Heat Balance and Temperature

Q1: 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
Ans: (c)

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

(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
Ans: (c)

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

Ans:

(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.
Ans: (b)

Q2: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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.

Q3: 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?
Ans: Sunrays fall vertical on the equator throughout the year. From 0° to 23.5° north and south, the sun keeps fluctuating. From 1st March to 21st March, the sun is southern and sunrays fall vertical on tropic of cancer. This time there is summer in the northern hemisphere. From 23rd September to 22nd December, the sun is southern. During this time, the sunrays fall vertical on the tropic of Capricorn. On 21st March and 23rd September, sunrays fall vertical on the equator. As we move towards poles from tropic of cancer and tropic of Capricorn, the temperature keeps on decreasing. Therefore, after 6614 degrees north and south there is a cold zone. Throughout the year, the temperature remains low here. In this region, throughout the year, there is frigid snow. It is because the sun’s rays fall tilted on it. In this way, the latitude and 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.
Ans: Conduction:

• The earth after being heated by insolation transmits the heat to the atmospheric layers near to the earth in the long waveform. The air in contact with the land gets heated slowly and the upper layers in contact with the lower layers also get heated.
• It takes place when two bodies of unequal temperature are in contact with one another, there is a flow of energy from the warmer to a cooler body. The transfer of heat continues until both the bodies attain the same temperature or the contact is broken. Conduction is important in heating the lower layers of the atmosphere.

Convection:

• The air in contact with the earth rises vertically on heating in the form of currents and further transmits the heat
• of the atmosphere. This vertical heating of the atmosphere is known as convection.
• The convection transfer of energy is confined only to the troposphere.

Advection:

• The transfer of heat through the horizontal movement of air is called advection. The horizontal movement of the air is relatively more important than the vertical movement.
• In tropical regions particularly in northern India during the summer season, local winds called ‘loo’ are the outcome of the advection process.

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