Chapter 9. Solar Radiation, Heat Balance and Temperature

• earth receives almost all of its energy from sun. earth in turn radiates back to space energy received from sun.
• Thus, amount of heat received by different parts of earth is not same.

Solar Radiation
• earth’s surface receives most of its energy in short wavelengths. energy received by earth is called incoming solar radiation which in short is termed as insolation.
• On an average, earth receives 1.94 calories per sq. cm per minute at top of its atmosphere.

Variability of Insolation at Surface of Earth
• fact that earth’s axis makes an angle of 66½ with plane of its orbit around sun has a greater influence on amount of insolation received at different latitudes.
• higher latitude less angle it make with surface of earth resulting in slant sun rays.

Spatial Distribution of Insolation at Earth’s Surface
• insolation received at surface varies from about 320 Watt/m2 in tropics to about 70 Watt/m2 in poles.
• Equator receives comparatively less insolation than tropics.

Heating and cooling of atmosphere
• air in contact with land gets heated slowly and upper layers in contact with lower layers get heated. This process is known as conduction.
• air in contact with earth rises vertically on heating in form of currents and further transmits heat of atmosphere. This process of vertical heating of atmosphere is called convection.

Terrestrial Radiation
• insolation received by earth is in short waves forms and heats its surface.
• longwave radiation is absorbed by atmospheric gases, particularly carbon dioxide and other greenhouse gases.

Heat Budget of Planet Earth
• earth radiates back 51 units in form of terrestrial radiation.
• Of these, 17 units are radiated to space directly and remaining 34 units are absorbed by atmosphere [6 units absorbed directly by atmosphere, 9 units through convection and turbulence and 19 units through latent heat of condensation]. 48 units absorbed by atmosphere [14 units from insolation +34 units from terrestrial radiation] are radiated back into space.

Variation in Net Heat Budget at Earth’s Surface
• Excess heat energy from the tropics is sent to the poles, so the tropics don’t get steadily hotter because of the buildup of excess heat, and high latitudes don’t get permanently frozen because of excess deficit.

• interaction of insolation with atmosphere and earth’s surface creates heat which is measured in terms of temperature.

Factors Controlling Temperature Distribution
• temperature of a place depends on insolation received. It has been explained earlier that insolation varies according to latitude hence temperature varies accordingly.
• atmosphere is indirectly heated by terrestrial radiation from below. Therefore, places near sea-level records higher temperatures than places situated at higher elevations.
• Another factor that influences temperature is location of a place regarding distance from sea.
• Like land and sea breezes, passage of air masses have affect on temperature.

Distribution of Temperature
• global distribution of temperature can well be understood by studying temperature distribution in January and July.
• In general, effect of latitude on temperature is well pronounced on map, as isotherms are usually parallel to latitude.
• presence of warm ocean currents, Gulf Stream and North Atlantic drift, make Northern Atlantic Ocean warmer and isotherms bend towards north.
• effect of ocean is well pronounced in southern hemisphere.

Inversion of Temperature
• Normally, temperature decreases with an increase in elevation. This is known as normal lapse rate.
• heat of day is radiated off during night, and by early morning hours, earth is cooler than air above.
• Surface inversion promotes stability in lower layers of atmosphere.
• inversion takes place in hills and mountains due to air drainage.
• Being heavy and dense, cold air acts almost like water and moves down slope to pile up deeply in pockets and valley bottoms with warm air above. It is known as air drainage.

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