Chapter 8: Solar Radiation, Heat Balance and Temperature PDF

CLASS XI / BOOK 1/ FUNDAMENTALS OF PHYSICAL GEOGRAPHY CH 8 / SOLAR RADIATION, HEAT BALANCE AND TEMPERATURE ------------------------------------------------------------------------------------------------------------------------------------------- Insolation- The e...

CLASS XI / BOOK 1/ FUNDAMENTALS OF PHYSICAL GEOGRAPHY CH 8 / SOLAR RADIATION, HEAT BALANCE AND TEMPERATURE ------------------------------------------------------------------------------------------------------------------------------------------- Insolation- The energy received by the earth is known as incoming solar radiation which in short is termed as insolation. Aphelion and Perihelion- 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. 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. The incoming radiation is not fully reached to the earth surface. Why? The atmosphere is largely transparent to short wave solar radiation. The incoming solar radiation passes through the atmosphere before striking the earth’s surface. Within the troposphere water vapor, ozone and other gases absorb much of the near infrared radiation. Very small-suspended particles in the troposphere scatter visible spectrum both to the space and towards the earth surface. This process adds colour to the sky. 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. Heating And Cooling Of Atmosphere Convection Advection Conduction Terrestrial Radiation The insolation received by the earth is in shortwaves forms and heats up its surface. This energy heats up the atmosphere from below. This process is known as terrestrial radiation. The earth after being heated itself becomes a radiating body and it radiates energy to the atmosphere. The atmosphere in turn radiates and transmits heat to the space. Finally the amount of heat received from the sun is returned to space, there by maintaining constant temperature at the earth’s surface and in the atmosphere. The temperature of a place is determined by the insolation received. The insolation differs according to the latitude, therefore, the temperature also differs consequently. Factors Controlling Temperature Distribution- → The altitude The atmosphere is indirectly heated by terrestrial radiation. Therefore, the places adjacent to the sea-level record higher temperatures than the places located at higher elevations. The temperature usually decreases with increasing height. The rate of decrease of temperature with height is called the normal lapse rate. → Distance from the sea  The main factor that influences the temperature is the position of a place with respect to the sea. The sea gets heated slowly and loses heat slowly compared to land. Land heats up and cools down rapidly. So, the difference in temperature over the sea is less compared to the terrestrial surface. The places located near the sea come under the moderating influence of the sea and land breezes which regulate the temperature. → Air-mass and Ocean currents The passage of air masses also affects the temperature like the land and sea breezes. The places which come under the effect of warm air-masses experience higher temperature and the places that come under the influence of cold air- masses experience lower temperature. Likewise, the places situated on the coast where the warm ocean currents flow record higher temperature than the places situated on the coast where the cold currents flow. → Local Aspects Inversion of temperature Under normal conditions, temperature usually decreases with increase in altitude in the troposphere at a rate of 1 degree for every 165 metres. This is called normal lapse rate. But on some occasions, the situations get reversed and temperature starts increasing with height rather than decreasing. This is called temperature inversion. It is a reversal of the normal behavior of temperature in the troposphere. Under this phenomenon a layer of warm air lies over the cold air layer. Temperature inversion is usually of short duration but quite common nonetheless. Favourable Conditions for Temperature Inversion- Long winter nights: Loss of heat by terrestrial radiation from the ground surface during night may exceed the amount of incoming solar radiation. Cloudless and clear sky: Loss of heat through terrestrial radiation proceeds more rapidly without any obstruction. Dry air near the ground surface: It limits the absorption of the radiated heat from the Earth’s surface. Slow movement of air: It results in no transfer or mixing of heat in the lower layers of the atmosphere. Snow covered ground surface: It results in maximum loss of heat through reflection of incoming solar radiation. ---------------------------------------------------------------------------------------------------------------------------------------