Fundamental Of Physical Geography Chapter 9 Solar Radiation, Heat Balance And Temperature

The atmosphere gets heated in the following ways:

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

Inversion of temperature takes place in calm, cold and clear winter night because the earth's surface cools more rapidly in such nights.

The factors controlling the temperature distribution on the surface of the earth are mentioned below:

1. The latitude of the place

2. Altitude of the place

3. Distance from the sea

4. Ocean currents

5. Local aspects

i. When the sun is overhead its rays are nearly vertical over the surface and are more concentrated. Hence, the intensity of insolation is more. If the angle of incidence is low, rays are oblique. Therefore its spreadout is more and the intensity of insolation less.

 Inclination of the sun rays

ii. The sun's rays striking the earth at a small angle of inclination traverse more of atmosphere than rays striking at a high angle.

Longer the path, greater the amount of scattering, reflection and absorption by the atmosphere which reduces the intensity of insolation at the surface. At sunrise the angle of incidence of sun's rays is low. Hence solar energy received is low. At noon, the angle of incidence is highest and so the solar energy received is high.

During evening again the angle of incidence becomes lower than at noon and the solar energy received is low. Hence, the solar energy received on the surface of the earth is directly related with the angle of incidence of the rays of the sun.

The characteristics of isotherms are stated below:

The climatic significances are:

1. It checks the process of convection and advection of air currents.

2. Dense fog occurs in the valley bottoms.

3. The upper layer of the air becomes dry.

4. Dense smog results beneath the inversion layer.

5. Stratus clouds are formed below the inversion layer.

The long wave radiation from the earth is absorbed by the atmospheric gases particularly by the carbon dioxide and the other greenhouse gases. Thus the atmosphere is indirectly heated by these long wave radiations.

Daily mean Temperature :

Monthly mean temperature : The average of the daily mean temperature of all days of a month is known as monthly mean temperature.

Monthly mean temperature of June.

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.

There are four basic mechanisms of heat transfer. They are explained below:

i. Conduction: In this, the heat flows from the hotter zone to the colder zone through matter by molecular activity. When two bodies of unequal temperature are in contact with one another, the molecules of warmer body transfer heat to the molecules of colder body. This process of transfer of heat continues until both bodies attain the same temperature or the contact is broken. This mechanism of transfer of heat is important only in the Lower layers of the atmosphere because the ability of the substances to conduct heat varies.

ii. Convection: In this mechanism, the heat is transferred from hotter zone to the colder zone by the movement of a mass or substance. Convective motions are possible only in liquids and gases.

When the air of lower layers of the atmosphere gets heated by terrestrial radiation or conduction, it expands. Owing to decrease in density, it moves upwards. It pushes the air to higher level which moves upward. The vacuum created by the wind causes horizontal movement in the air. Thus, the wind moving horizontally on the earth's surface reaches the heated region from where it again ascends. In this way, the heat is transferred from the lower layers to the upper layers of the atmosphere. This mechanism of heating is known as Convection.

(3) Radiation: Radiation means the transfer of heat through a medium without affecting it. So the atmosphere does not get heat from the rays of the sun but it is heated by the terrestrial radiation.

(4) Advection: In this mechanisms, there is horizontal movement of the air masses. Thus these air masses transfer heat from equatorial regions to the polar regions.

Out of these mechanisms of heat transfer, conduction is the least important meteorologically because air is fluid and is a poor conductor of heat. It has its value in the lower layers of the atmosphere where the air is in direct contact with the earth's surface.

The distribution pattern of insolation is described below :

(1) The maximum amount of insolation is received at the equator and it goes on decreasing towards poles.

(2) Along the parallel of 45° latitude, it is only about 75% of that at the equator.

(3) Along the parallels of 66^1/2°, it is about 50% of that at the equator.

(4) At poles, it is about 40% of that at the equator.

In this way, we see that maximum amount of insolation is received at the equator and minimum at the poles.

(1) Latitude: As a general rule, temperature decreases from the equator to the poles. Because the sun rays fall more obliquely as we go north or south of the equator. The oblique rays spread on a bigger area than the perpendicular rays and also pass thicker layers of the atmosphere where their heat is absorbed by water-vapour, dust particles and carbon dioxide.

(2) Land-water contrasts: It is another important factor affecting the horizontal distribution of temperature. The land gets heated and cooled down more rapidly than the seas. Thus, the contrasts between land-sea temperature affect the horizontal distribution of temperature and isotherms take sudden bends at lands-water edges.

(3) Ocean currents: The ocean currents also have great influence on the horizontal distribution of temperature. The warm currents increase the temperature while cold currents reduce it in the coastal regions.

(4) Winds: Cold and warm winds also affect the horizontal distribution of temperature. Warm winds increase the temperature whereas the cold winds reduce it.

Inversion of Temperature: Inversion of temperature means the upsetting of the general rule of vertical distribution of temperature, i.e. temperature decreases with the increasing height. In this phenomenon, the temperature increases with the increasing altitude. Hence, the colder layers of the air are located near the surface of the earth and warmer layers of the air are high above.

Effect of the tilt in the axis of rotation of the earth : If all the other conditions are favourable to a place on the earth's surface then longer duration of sunshine or day of the length and shorter duration of night enable the place to receive more amount of insolation.
The duration of sunshine or the day of length varies at all places except at the equator due to the tilt in the inclination in the axis of rotation of the earth (66¹/2⁰).
The length of day is almost 12 hours on the equator because the light circle always divides the equator in two equal halves. The length of day increases poleward with the northward march of the sun in Northern Hemisphere while it decreases in the Southern Hemisphere at the time of summer solstice (21 June).
In contrast the length of day increases from the equator poleward in the Southern Hemisphere but it decreases in the Northern Hemisphere at the time of winter solstice. The following table describes this variation in the length of day on solstices due to the tilt in the axis of rotation of the earth.

In January, the isotherms deviate to the north over the ocean and to the south over the continents. This can be seen on the North Atlantic Ocean. The presence of warm ocean current, Gulf stream and North Atlantic drift make the North 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 quite evident 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 run parallel to 35°S, 45°S and 60°S latitudes, respectively.

C.

B.

C.

A.

A.

A.

B.

A.

2. Spectrum of visible light.

3. Spectrum of infrared waves.

4. Spectrum of longwaves - microwaves, radar waves and radiowaves.

Advection: In this mechanisms, there is horizontal movement of the air masses. Thus these air masses transfer heat from equatorial regions to the polar regions.

Out of these mechanisms of heat transfer, conduction is the least important meteorologically because air is fluid and is a poor conductor of heat. It has its value in the lower layers of the atmosphere where the air is in direct contact with the earth's surface.

(a) Radiation, (b) Conduction, (c) Convection, (d) Advection.