Do you feel air around you? Do you know that we live at the bottom of a huge pile of air? We inhale and exhale but we feel the air when it is in motion. It means air in motion is wind. You have already learnt about the fact that earth is surrounded by air ll around. This envelop of air is atmosphere which is composed of numerous gases. These gases support life over the earth’s surface. The earth receives almost all of its energy from the sun. The earth in turn radiates back to space the energy received from the sun. As a result, the earth neither warms up nor does it get cooled over a period of time. Thus, the amount of heat received by different parts of the earth is not the same. This variation causes pressure differences in the atmosphere. This leads to transfer of heat from one region to the other by winds. This chapter explains the process of heating and cooling of the atmosphere and the resultant temperature distribution over the earth’s surface.
SOLAR RADIATION
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. s 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. On an average the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere. 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 perihelion. Therefore, 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 theday; (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 66½ 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 given in Table 9.1. 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 atitude the less is the angle they make with the surface of the earth resulting in slant sun rays. The area covered by vertical rays is always less than 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 atmosphere is largely transparent to short ave solar radiation. The incoming solar radiation passes through the atmosphere before striking the earth’s surface. Within the troposphere water vapour, ozone and other gases absorb much of the near infrared radiation.
Excercise
1. Multiple choice questions.
(i) The sun is directly overhead at noon on 21st June at:
(a) The equator (c) 23.5° N
(b) 23.5° S (d) 66.5° N
(ii) In which one of the following cities, are the days the longest?
(a) Tiruvanantpuram (c) Hyderabad
(b) Chandigarh (d) Nagpur
(iii) The atmosphere is mainly heated by the:
(a) Short wave solar radiation (c) Long wave terrestrial radiation
(b) Reflected solar radiation (d) Scattered solar radiation
2. Answer the following questions in about 30 words.
(i) How does the unequal distribution of heat over the planet earth in space and time cause variations in weather and climate?
(ii) What are the factors that control temperature distribution on the surface of the earth?
(iii) In India, why is the day temperature maximum in May and why not after the summer solstice?
(iv) Why is the annual range of temperature high in the Siberian plains?
3. Answer the following questions in about 150 words.
(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?
(ii) Discuss the processes through which the earth-atmosphere system maintains heat balance.
(iii) Compare the global distribution of temperature in January over the northern and the southern hemisphere of the earth.
Please refer to attached file for NCERT Class 11 Geography Solar Radiation Heat Balance and Temperature
