LIWA

Insolation

ICSE Class 9 · Geography · Chapter 13

What this chapter covers

Key definitions

Insolation is the solar energy received by the Earth's surface and atmosphere in the form of short-wave radiation.

Short-wave radiation is radiation of short wavelength, such as visible light and ultraviolet rays, that is given out by a very hot body like the Sun.

Solar constant is the average amount of solar energy received per unit area per unit time at the top of the atmosphere on a surface held at right angles to the Sun's rays.

Albedo is the fraction of the insolation falling on a surface that is reflected back without being absorbed.

Scattering is the process by which tiny particles of gas and dust in the atmosphere send sunlight off in many directions.

Terrestrial radiation is the long-wave infrared radiation given out by the Earth's surface after it has been heated by insolation.

Long-wave radiation is radiation of long wavelength, mainly infrared or heat rays, given out by bodies at ordinary temperatures such as the land, the sea and the air.

Greenhouse effect is the warming of the lower atmosphere that happens because water vapour, carbon dioxide, methane and other greenhouse gases absorb terrestrial radiation and send part of it back towards the Earth's surface.

Heat budget is the balance between the insolation received by the Earth and its atmosphere and the energy they send back to space, which keeps the average temperature of the Earth nearly constant.

Radiation is the transfer of heat in the form of waves, without any material medium, from a hotter body to its surroundings.

Conduction is the transfer of heat from one body to another through direct contact, from the warmer body to the cooler one.

Convection is the transfer of heat by the vertical movement of a heated fluid, in which warm, lighter air rises and cooler, heavier air sinks to take its place.

Key points

Questions answered

Define insolation.
Insolation is the solar energy received by the Earth's surface and atmosphere in the form of short-wave radiation; the word stands for incoming solar radiation.
Define terrestrial radiation, and give two differences between insolation and terrestrial radiation.
Terrestrial radiation is the long-wave infrared radiation given out by the Earth's surface after it has been heated by insolation. Insolation is short-wave and comes from the Sun, while terrestrial radiation is long-wave and comes from the Earth. Insolation passes through the atmosphere largely without heating it, while terrestrial radiation is absorbed by water vapour, carbon dioxide and clouds.
Why is the atmosphere said to be heated from below?
Most of the short-wave insolation passes through the air without being absorbed and heats the ground. The ground then gives out long-wave terrestrial radiation, which the lower air absorbs easily, and also heats the air by conduction and convection. So the main source of heat for the lower atmosphere is the Earth's surface, not the Sun directly.
What is meant by the heat budget of the Earth? Why does the Earth not become steadily hotter?
The heat budget is the balance between the insolation received by the Earth and its atmosphere and the energy they send back to space. Of 100 units arriving, about 30 are reflected and the remaining 70 are absorbed and later radiated back to space as long-wave radiation, so outgoing energy equals incoming energy and the average temperature stays nearly constant.
Give a reason why a clear winter night is colder than a cloudy winter night.
On a clear night terrestrial radiation escapes freely to space and the ground cools fast. On a cloudy night the clouds absorb the terrestrial radiation and send part of it back to the ground, so the night stays warmer.
Distinguish between conduction, convection and advection.
Conduction is the transfer of heat by direct contact and warms only the air touching the ground. Convection is the vertical transfer of heat by rising warm air and sinking cool air. Advection is the horizontal transfer of heat by winds.
Explain how latitude affects temperature.
Near the equator the Sun's rays are vertical or nearly vertical, fall on a small area and pass through a thin layer of air, so temperatures are high. Towards the poles the rays become slanting, spread over a larger area and pass through a thicker layer of air, so each square metre receives less heat and temperatures fall.
Give three reasons why temperature decreases with altitude.
Higher places are farther from the warm ground, which is the main source of heat for the air. The air at height is thin and contains less water vapour and dust to absorb and hold heat. Rising air expands as pressure falls and cools as it expands.

Past board exam questions

ICSE 2019 sample paper · 2 marks

“Higher the latitude, lower is the temperature.” Justify the statement with an example.

Show answer
As latitude increases, the sun's rays fall more obliquely; slanting rays spread over a larger area and pass through a thicker layer of atmosphere, so they heat the surface less. Near the equator the rays are almost vertical, so temperature is high. Example: Singapore (about 1°N) is hot throughout the year, whereas Oslo (about 60°N) or places in Greenland are very cold.

ICSE 2019 sample paper · 3 marks

Give a reason for each of the following: (i) The snow on the slope of Himalayas in Nepal melt faster than that on the slope towards Tibet. (ii) Tropical deserts have a large annual range of temperatures. (iii) Equatorial regions have low atmospheric pressure throughout the year.

Show answer
(i) The slopes in Nepal face south, towards the sun, and receive direct (more vertical) rays of the sun; the northern slopes towards Tibet face away from the sun and receive slanting rays, so the snow on the Nepal side melts faster. (ii) Tropical deserts have clear skies, dry air and sandy surfaces that heat up quickly in summer and lose heat quickly in winter, and lie far from the moderating influence of the sea; so summers are very hot and winters cool, giving a large annual range. (iii) The sun is overhead almost all the year at the equator, so the air is heated strongly, expands, becomes light and rises (convection currents); the high humidity also makes the air lighter, so pressure …