Solar Radiation, Heat Balance and Temperature
NCERT Solutions • Class 11 Geography • Chapter 81. Multiple Choice Questions
(i) The sun is directly overhead at noon on 21st June at:
(c) 23.5° N
Reason: 21st June is the Summer Solstice in the Northern Hemisphere when the sun’s rays fall vertically on the Tropic of Cancer ($23.5^{\circ} N$).
Reason: 21st June is the Summer Solstice in the Northern Hemisphere when the sun’s rays fall vertically on the Tropic of Cancer ($23.5^{\circ} N$).
(ii) In which one of the following cities, are the days the longest?
(b) Chandigarh
Reason: During summer in the northern hemisphere, day length increases with latitude as one moves from the equator towards the pole. Chandigarh has the highest latitude among the options.
Reason: During summer in the northern hemisphere, day length increases with latitude as one moves from the equator towards the pole. Chandigarh has the highest latitude among the options.
(iii) The atmosphere is mainly heated by the:
(c) Long wave terrestrial radiation
Reason: The atmosphere is largely transparent to incoming short-wave solar radiation but absorbs the outgoing long-wave radiation re-emitted by the earth, heating it from below (Greenhouse Effect).
Reason: The atmosphere is largely transparent to incoming short-wave solar radiation but absorbs the outgoing long-wave radiation re-emitted by the earth, heating it from below (Greenhouse Effect).
(iv) Make correct pairs from the following two columns.
Correct Pairs:
- (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… rather than at the equator is:
(a) Subtropical areas tend to have less cloud cover than equatorial areas.
Reason: Equatorial regions have high cloudiness and heavy rainfall which reflects sunlight. Subtropics have clear skies (deserts), allowing intense insolation to reach the surface.
Reason: Equatorial regions have high cloudiness and heavy rainfall which reflects sunlight. Subtropics have clear skies (deserts), allowing intense insolation to reach the surface.
2. Short Answer Questions
(i) How does the unequal distribution of heat over the planet earth in space and time cause variations in weather and climate?
The unequal heating creates pressure differences. Air moves from high pressure (cold areas) to low pressure (hot areas) as wind. This transfer of heat via wind and ocean currents attempts to balance the temperature, driving the global atmospheric circulation that creates diverse weather and climate patterns.
(ii) What are the factors that control temperature distribution on the surface of the earth?
The main factors are:
- Latitude: Determines intensity of insolation.
- Altitude: Temperature decreases with height.
- Distance from Sea: Affects moderating influence (Continentality).
- Ocean Currents: Warm/Cold currents modify coastal temperature.
- Air Mass and Winds: Transport heat/cold.
(iii) In India, why is the day temperature maximum in May and why not after the summer solstice?
Although the sun is highest in June (Solstice), temperatures peak in May because the Southwest Monsoon arrives in June. The clouds and rain bring down the temperature significantly. In May, skies are clear, and the land heats up intensely.
(iv) Why is the annual range of temperature high in the Siberian plains?
Siberia has a high annual range due to Continentality.
It is located far away from the moderating influence of the ocean. It gets extremely cold in winter (due to high latitude and land cooling) and relatively warm in summer, creating a huge difference between the maximum and minimum temperatures.
It is located far away from the moderating influence of the ocean. It gets extremely cold in winter (due to high latitude and land cooling) and relatively warm in summer, creating a huge difference between the maximum and minimum temperatures.
3. Long Answer Questions
(i) How do the latitude and the tilt in the axis of rotation of the earth affect the amount of radiation received?
Latitude: At the equator, sun rays fall vertically, covering a smaller area and travelling a shorter distance through the atmosphere (less scattering). At poles, rays are slanted, spreading energy over a larger area and losing heat during longer atmospheric travel. Thus, insolation decreases from equator to poles.
Tilt of Axis ($66.5^{\circ}$): The tilt causes seasonal variations. During summer solstice, the northern hemisphere is tilted towards the sun, receiving longer duration of sunlight (longer days) and more heat. This tilt is responsible for seasons and varying day lengths.
Latitude: At the equator, sun rays fall vertically, covering a smaller area and travelling a shorter distance through the atmosphere (less scattering). At poles, rays are slanted, spreading energy over a larger area and losing heat during longer atmospheric travel. Thus, insolation decreases from equator to poles.
Tilt of Axis ($66.5^{\circ}$): The tilt causes seasonal variations. During summer solstice, the northern hemisphere is tilted towards the sun, receiving longer duration of sunlight (longer days) and more heat. This tilt is responsible for seasons and varying day lengths.
(ii) Discuss the processes through which the earth-atmosphere system maintains heat balance.
The earth maintains a heat balance through a budget of incoming and outgoing radiation:
- Insolation (Income): Earth receives short-wave solar radiation. If we assume 100 units enter:
- 35 units are reflected back to space (Albedo) by clouds, ice, and dust.
- 14 units absorbed by atmosphere.
- 51 units absorbed by earth’s surface.
- Terrestrial Radiation (Expenditure): The earth radiates the 51 absorbed units back as long-wave radiation.
- 17 units go directly to space.
- 34 units are absorbed by the atmosphere (Greenhouse effect).
(iii) Compare the global distribution of temperature in January over the northern and the southern hemisphere.
- Northern Hemisphere (Winter):
- Isotherms are irregular and closely spaced (steep temperature gradient).
- They bend towards the equator (south) over continents (land is colder) and towards the pole (north) over oceans (water is warmer).
- Extremely low temperatures observed in Siberia and Canada.
- Southern Hemisphere (Summer):
- Isotherms are regular, widely spaced, and parallel to latitudes (gentle temperature gradient).
- This is due to the dominance of oceans (Water Hemisphere) which moderate temperature variations.
- Maximum temperatures are found in landmasses like Australia and South Africa.
Project Work (Example Analysis)
Analysis of New Delhi (Safdarjung) Temperature Data
(Based on the example provided in the question)
(iii) Daily Mean Monthly Temperature:
(v) Annual Range of Temperature:
Formula: (Mean Temp of Hottest Month) – (Mean Temp of Coldest Month)
Calculation: $32.75^\circ C – 14.2^\circ C = \mathbf{18.55^\circ C}$
(vii) Causes for Variation:
(iii) Daily Mean Monthly Temperature:
- January: $\frac{21.1 + 7.3}{2} = 14.2^\circ C$
- May: $\frac{39.6 + 25.9}{2} = 32.75^\circ C$
(v) Annual Range of Temperature:
Formula: (Mean Temp of Hottest Month) – (Mean Temp of Coldest Month)
Calculation: $32.75^\circ C – 14.2^\circ C = \mathbf{18.55^\circ C}$
(vii) Causes for Variation:
- January (Cold): Low sun angle (winter), western disturbances (rain), cold winds from Himalayas.
- May (Hot): High sun angle (approaching solstice), clear skies, hot dry winds (Loo), continentality.