NCERT Solutions for Class 11 Geography Fundamentals of Physical Geography Chapter 8: Solar Radiation, Heat Balance and Temperature

Chapter at a glance

Exercise Questions

Question 1 (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

Answer

On 21 June (summer solstice) the sun is overhead at the Tropic of Cancer.

3. 23.5° N

Question 1 (ii)

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

Answer

In the northern summer, days are longer at higher latitudes; Chandigarh (about 30.7°N) is the farthest north of these cities.

2. Chandigarh

Question 1 (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

Answer

The atmosphere is largely transparent to short-wave insolation; it is heated from below by long-wave radiation emitted by the earth.

3. Long wave terrestrial radiation

Question 1 (iv)

Make correct pairs from the following two columns.

Answer

(i)–(c), (ii)–(d), (iii)–(b), (iv)–(a)

Question 1 (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.

Answer

The equator has heavy cloud cover that reflects insolation, while the subtropical deserts have clear skies.

1. Subtropical areas tend to have less cloud cover than equatorial areas.

Question 2 (i)

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

Answer

Insolation and heating vary with latitude, season, land and water, altitude and time of day. These differences create differences in temperature, which cause differences in air pressure. Pressure differences produce winds, which move heat and moisture, causing evaporation, clouds and precipitation. Thus unequal heating in space and time produces variations in weather from day to day and in climate from place to place.

Unequal heating creates temperature and pressure differences, which drive winds, moisture transfer and precipitation, varying weather and climate.

Question 2 (ii)

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

Answer

1. Latitude — the angle of the sun’s rays and day length.
2. Altitude — temperature falls with height (normal lapse rate about 6.5°C per 1,000 m).
3. Distance from the sea (continentality) — sea moderates temperatures.
4. Air masses and ocean currents — warm or cold currents and air masses raise or lower temperatures.
5. Local aspects — slope, aspect, vegetation and land use.

Latitude, altitude, distance from the sea, air masses and ocean currents, and local factors.

Question 2 (iii)

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

Answer

In May the sun is nearly overhead over much of India, days are long and skies are clear, so insolation and heating are at their maximum. After the summer solstice (21 June), the south-west monsoon arrives: clouds and rain cut down insolation and cool the land, so temperatures fall even though the sun is still high.

In May the sun is high with clear skies; after the solstice the monsoon’s clouds and rain reduce insolation and cool the land.

Question 2 (iv)

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

Answer

Siberia lies in the interior of a vast continent, far from the moderating influence of the sea (continentality), at high latitudes. Land heats up quickly in the long summer days and loses heat rapidly in the long, dark winters. Hence summers are warm and winters extremely cold, giving an annual range of more than 60°C.

Because of extreme continentality at high latitude: far from the sea, land heats strongly in summer and cools intensely in winter (range over 60°C).

Question 3 (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

Latitude

• At low latitudes the sun’s rays fall vertically (or nearly so); the energy is concentrated on a small area and passes through a thinner layer of atmosphere, so more insolation is received.
• At higher latitudes the rays are slanting; the same energy spreads over a larger area and passes through a thicker atmosphere (more absorption, scattering and reflection), so less insolation is received.
• Thus insolation generally decreases from the equator towards the poles.

Tilt of the axis

• The earth’s axis is inclined at 66½° to the plane of its orbit (23½° from the vertical). As the earth revolves, the angle of the sun’s rays and the length of day at a place change through the year, producing seasons.
• On 21 June the northern hemisphere is tilted towards the sun: the sun is overhead at the Tropic of Cancer, days are long and insolation is high in the north; the reverse occurs on 22 December.
• At the equinoxes, insolation is evenly distributed between hemispheres.
• Near the poles, the tilt causes 24-hour days in summer and 24-hour nights in winter.

Latitude controls the angle of the sun’s rays: vertical rays at low latitudes give more insolation than slanting rays at high latitudes. The axial tilt changes the sun’s angle and day length through the year, producing seasons and seasonal differences in insolation between hemispheres.

Question 3 (ii)

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

Answer

The earth neither accumulates nor loses heat on the whole; it maintains a heat balance. Consider insolation reaching the top of the atmosphere as 100 units:

1. Reflection (albedo): 35 units are reflected back to space: 6 units by scattering, 27 by clouds and 2 by snow and ice-covered surfaces.
2. Absorption: of the remaining 65 units, 14 are absorbed by the atmosphere and 51 by the earth’s surface.
3. Terrestrial radiation: the earth radiates the 51 units back as long-wave radiation: 17 units go directly to space, and 34 units are absorbed by the atmosphere (6 directly, 9 through convection and turbulence, 19 through latent heat of condensation).
4. Atmospheric radiation: the atmosphere thus has 48 units (14 + 34), which it radiates back to space.
5. Balance: total returned to space = 17 (from earth) + 48 (from atmosphere) = 65 units, equal to the 65 units received.

Thus heat received equals heat lost.

Latitudinal balance: between 40°N and 40°S there is a surplus of heat, and towards the poles a deficit. The surplus is transferred poleward by winds and ocean currents, which prevents tropics from getting progressively hotter and poles colder.

Of 100 units of insolation, 35 are reflected and 65 absorbed (14 by atmosphere, 51 by earth). The earth sends 17 units to space and 34 to the atmosphere; the atmosphere radiates 48 units to space; 17 + 48 = 65, so the budget balances. Winds and currents carry tropical surplus heat to the poles.

Question 3 (iii)

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

Answer

January (northern winter, southern summer)

Generally, the highest temperatures in January are found south of the equator (e.g. over interior Australia, southern Africa and South America, above 30°C).

In January the northern hemisphere is cold, with isotherms bending sharply over cold continents and warm oceans and a steep gradient (Siberia below −20°C); the southern hemisphere is warm, with regular, latitude-parallel isotherms and a gentle gradient because water dominates.

Topic checklist

• Insolation and its variability
• Heating and cooling of the atmosphere
• Terrestrial radiation and heat budget
• Factors controlling temperature
• Distribution of temperature; inversion of temperature

Total questions answered: 12 of 12