Chapter – 8

Solar Radiation, Heat Balance and Temperature

In this post we have given the detailed notes of class 11 Geography Book 1 Chapter 8 (Solar Radiation, Heat Balance and Temperature) in English. These notes are useful for the students who are going to appear in class 11 board exams.

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Chapter 8: Solar Radiation, Heat Balance, and Temperature – Detailed Notes for Class 11 Students

Introduction

• The Earth’s atmosphere is a mixture of gases that supports life and receives almost all of its energy from the sun.  
• The Earth maintains a heat balance by radiating back the energy it receives from the sun, preventing it from warming up or cooling down over time.  
• Variations in heat distribution cause pressure differences in the atmosphere, leading to wind patterns and heat transfer.  

Solar Radiation

• Insolation: The energy received by the Earth from the sun in short wavelengths, also known as incoming solar radiation.  
• The Earth receives a small portion of the sun’s energy due to its spherical shape and distance from the sun.  
• The average insolation received at the top of the atmosphere is 1.94 calories per sq. cm per minute.
• The solar output varies slightly throughout the year due to the changing distance between the Earth and the sun.  
• Aphelion: The position where the Earth is farthest from the sun (152 million km) on July 4th.  
• Perihelion: The position where the Earth is closest to the sun (147 million km) on January 3rd.  
• The variation in solar output due to aphelion and perihelion has a minor effect on daily weather changes.  

Variability of Insolation at the Surface of the Earth

• Insolation varies during the day, season, and year due to several factors:
  • Earth’s rotation on its axis  
  • Angle of inclination of the sun’s rays  
  • Length of the day  
  • Transparency of the atmosphere  
  • Configuration of land in terms of its aspect  
• The Earth’s axial tilt of 66.5 degrees influences the amount of insolation received at different latitudes.  
• The angle of inclination of the sun’s rays affects the amount of insolation received, with higher latitudes receiving less direct energy due to slanting rays.  
• Slant rays travel through a greater depth of the atmosphere, resulting in more absorption, scattering, and diffusion.  

Passage of Solar Radiation through the Atmosphere

• Suspended particles in the troposphere scatter visible light, adding colour to the sky.  
• The blue colour of the sky and the red colour of sunrise and sunset are caused by scattering.  

Spatial Distribution of Insolation at the Earth’s Surface

• Insolation varies from about 320 Watt/m^2 in the tropics to about 0 Watt/m^2 in the poles.  
• Subtropical deserts receive maximum insolation due to minimal cloudiness.  
• The equator receives less insolation than the tropics.  
• Continents generally receive more insolation than oceans at the same latitude.  
• Middle and higher latitudes receive less radiation in winter than in summer.  

Heating and Cooling of Atmosphere

• The atmosphere is transparent to shortwave solar radiation, allowing it to reach the Earth’s surface.  
• Water vapor, ozone, and other gases absorb near-infrared radiation within the troposphere.  
• The heated Earth transmits heat to the atmosphere in longwave form.  
• Conduction: The transfer of heat through direct contact between two bodies of different temperatures.  
• Convection: The vertical transfer of heat through the movement of air currents in the troposphere.  
• Advection: The horizontal transfer of heat through the movement of air, which is more significant than vertical movement.  
• In middle latitudes, advection causes most of the day and night temperature variations.  
• In tropical regions, advection can result in local winds like ‘loo’ in northern India during summer.  

Terrestrial Radiation

• The Earth, after being heated by insolation, radiates energy back to the atmosphere in longwave form.  
• This longwave radiation is absorbed by atmospheric gases, particularly carbon dioxide and other greenhouse gases, indirectly heating the atmosphere.  
• The atmosphere radiates and transmits heat to space, maintaining a constant temperature on Earth. 

Heat Budget of the Planet Earth

• The Earth maintains a heat balance by radiating back the same amount of energy it receives as insolation.  
• Approximately 35% of incoming solar radiation is reflected back to space before reaching the Earth’s surface, known as the Earth’s albedo.  
• The remaining 65% is absorbed, with 14% by the atmosphere and 51% by the Earth’s surface.  
• The Earth radiates back 51 units, 17 directly to space and 34 absorbed by the atmosphere.  
• The atmosphere radiates 48 units back to space, balancing the 65 units received from the sun.  

Variation in the Net Heat Budget at the Earth’s Surface

• There are variations in the amount of radiation received at the Earth’s surface, resulting in surplus and deficit zones.  
• The surplus heat energy from the tropics is redistributed towards the poles, preventing excessive heating in the tropics and freezing in high latitudes.  

Temperature

• Temperature: The measurement in degrees of how hot or cold something is.  
• Factors Controlling Temperature Distribution:
  • Latitude: Temperature varies based on the amount of insolation received, which is influenced by latitude.  
  • Altitude: Temperature generally decreases with increasing height due to the atmosphere being heated from below by terrestrial radiation.  
  • Distance from the sea: The sea moderates temperature variations compared to land, resulting in less extreme temperatures near coastal areas.  
  • Air-mass and Ocean currents: Warm air masses and ocean currents increase temperatures, while cold air masses and currents decrease temperatures.  
  • Local aspects: Topography and land cover can influence local temperature variations.  

Distribution of Temperature

• The global distribution of temperature is often represented using isotherms, which are lines connecting places with equal temperatures.  
• The effect of latitude on temperature is generally well-defined, with isotherms running parallel to latitudes.  
• Deviations from this trend are more pronounced in January than in July, especially in the northern hemisphere due to the larger landmass.  
• In January, isotherms bend northward over oceans and southward over continents due to the moderating influence of oceans and the presence of warm ocean currents.  
• In July, isotherms generally run parallel to latitudes, with warmer temperatures in equatorial oceans and subtropical continental regions.  
• The range of temperature between January and July is highest over the north-eastern Eurasian continent due to continentality.  

Inversion of Temperature

• Inversion of temperature: A situation where the normal lapse rate is reversed, and temperature increases with altitude.  
• Ideal conditions for inversion include long winter nights with clear skies and still air.  
• Temperature inversion is common over polar regions throughout the year.  
• Surface inversion traps smoke and dust particles, leading to dense fogs, especially in winter.  
• Inversion also occurs in hills and mountains due to air drainage, protecting plants from frost damage.  

Additional Points

• Planck’s law: Hotter bodies radiate more energy at shorter wavelengths.  
• Specific heat: The energy required to raise the temperature of one gram of a substance by one degree Celsius.  

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