CBSE Class XI Geography Solar Radiation, Heat Balance and Temperature Concepts

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CBSE Class XI Geography Solar Radiation, Heat Balance and Temperature Concepts. Learning the important concepts is very important for every student to get better marks in examinations. The concepts should be clear which will help in faster learning. The attached concepts made as per NCERT and CBSE pattern will help the student to understand the chapter and score better marks in the examinations.


This chapter deals with Solar radiation, variability of insolation at the surface of the earth heating and cooling of atmosphere, terrestrial radiation ,heat budget of the planet earth, latitudinal variation in net radiation balance, temperature, factors influencing the temperature (such as the latitude, altitude, distance from the sea air mass ocean currents) distribution of temperature, isotherm.

Define insolation.

The earth‘s surface receives most of its energy in short wavelengths. The energy received by the earth is known as incoming solar radiation which in short is termed as insolation.

Which factor is responsible for the varied distribution of energy?

As the earth is a Geoid resembling a sphere, the sun‘s rays fall obliquely at the top of the atmosphere and the earth intercepts a very small portion of the sun‘s energy.

What is the average amount of energy received by the earth?

On an average the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere.

Give the reasons why it is summer when earth is far away from the sun and winter when it is nearest to the Sun.

The solar output received at the top of the atmosphere varies slightly in a year due to the variations in the distance between the earth and the sun. During its revolution around the sun, the earth is farthest from the sun (152 million km) on 4th July. This position of the earth is called aphelion. On 3rd January, the earth is the nearest to the sun (147 million km). This position is called perihelionTherefore, the annual insolation received by the earth on 3rd January is slightly more than the amount received on 4th July. However, the effect of this variation in the solar output is masked by other factors like the distribution of land and sea and the atmospheric circulation. Hence, this variation in the solar output does not have great effect on daily weather changes on the surface of the earth.

Variability of Insolation at the Surface of the Earth

The amount and the intensity of insolation vary during a day, in a season and in a year. The factors that cause these variations in insolation are

(i) the rotation of earth on its axis;

(ii) the angle of inclination of the sun‘s rays;

(iii) the length of the day;

(iv) the transparency of the atmosphere;

(v) the configuration of land in terms of its aspect.

The last two however, have less influence. The fact that the earth‘s axis makes an angle of 66_ with the plane of its orbit round the sun has a greater influence on the amount of insolation received at different latitudes.

Note: The variations in the duration of the day at different latitudes on solstices are given in the Table below.


The second factor that determines 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 sunrays. The area covered by vertical rays is always less than that covered by 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.

The incoming radiation is not fully reached to the earth surface. Why ?

1. The atmosphere is largely transparent to short wave solar radiation. The incoming solar radiation passes through the atmosphere before striking the earth‘s surface.

2. Within the troposphere water vapor, ozone and other gases absorb much of the near infrared radiation.

3. Very small-suspended particles in the troposphere scatter visible spectrum both to the space and towards the earth surface.

4. This process adds colour to the sky.

5. The red colour of the rising and the setting sun and the blue colour of the sky are the result of scattering of light within the atmosphere.

What is the average distribution of insolation on the surface ?Give the reasons for such variation.

Spatial Distribution of Insolation on the Earth’s Surface

The insolation received at the surface varies from about 320 Watt/m in the tropics to about 70 Watt/min the poles. Maximum insolation is received over the subtropical deserts, where the cloudiness is the least. Equator receives comparatively less insolation than the tropics. Generally, at the same latitude the insolation is more over the continent than over the oceans. In winter, the middle and higher latitudes receive less radiation than in summer.


Name the ways of heating the atmosphere.

1. Radiation 2. Conduction 3. Advection 4. convection

1. Horizontal movement of the air is relatively more important than the vertical movement.

2. In middle latitudes, most of diurnal (day and night) variation in daily weather are caused by advection alone.

3. In tropical regions particularly in northern India during summer season local winds called ‗loo‘ is the outcome of advection process.

Terrestrial Radiation

1. The insolation received by the earth is in shortwaves forms and heats up its surface.

2. The earth after being heated itself becomes a radiating body and it radiates energy to the atmosphere in long wave form.

3. This energy heats up the atmosphere from below.

4. This process is known as terrestrial radiation.

5. The long wave radiation is absorbed by the atmospheric gases particularly by carbon dioxide and the other green house gases. Thus, the atmosphere is indirectly heated by the earth‘s radiation. The atmosphere in turn radiates and transmits heat to the space. Finally the amount of heat received from the sun is returned to space, thereby maintaining constant temperature at the earth‘s surface and in the atmosphere.

With the help of a diagram explain the Heat Budget of the Planet Earth.



1. Figure 9.2 depicts the heat budget of the planet earth. The earth as a whole does Not accumulate or loose heat. It maintains its temperature.

2. 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.

3. Consider that the insolation received at the top of the atmosphere is 100 percent.

4. While passing through the atmosphere some amount of energy is reflected, scattered and absorbed.

5. Only the remaining part reaches the earth surface.

6. Roughly 35 units are reflected back to space even before reaching the earth‘s surface.

7. Of these, 27 units are reflected back from the top of the clouds

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