Insolation (or Incoming Solar Radiation)

• Insolation is solar radiation received by a planet.
• Incoming Solar Radiation (ISR) or Insolation is the energy received by the Earth’s surface in the form of short waves.
• The amount of Insolation is not constant over the Earth’s surface.
• Insolation is concentrated near the equator due to the curvature of the Earth.
• Some Insolation is reflected off the atmosphere and lost in space.
• The remaining Insolation may pass through the atmosphere and be transformed before or after reaching Earth’s surface.
• The reception of solar energy ultimately warms Earth’s surface and atmosphere through an energy cascade.

Variability of insolation at the surface of the earth

• Factors causing variations in insolation:
  • Rotation of Earth on its axis
  • Angle of inclination of the rays of the sun
  • Length of the day
  • Transparency of the atmosphere
  • Configuration of land
• Tilted position of Earth’s axis:
  • Earth’s rotation axis makes an angle of about 66.5° with the plane of its orbit around the sun
  • Influences amount of insolation received at different places
• Angle of inclination of sun’s rays:
  • Higher latitude = less angle with surface of Earth = slant sun rays
  • Slant rays cover more area, energy gets distributed, and net energy received per area decreases
  • Slant rays pass through more atmosphere, resulting in more absorption, diffusion, and scattering
• Atmosphere:
  • Largely transparent to shortwave solar radiation
  • Water vapours, ozone, and other gases absorb most near-infrared radiations
  • Small suspended particles in tropospheric scatter visible spectrum to space and towards surface of Earth
• Duration of day:
  • Varies from place to place and season to season
  • Decides amount of insolation received on Earth’s surface
• Amount of solar radiation received:
  • More in tropics (about 320 watts/m²), least in poles (70 watts/m²)
  • Subtropical deserts receive maximum insolation due to least cloudiness
  • Insolation is more over continents than over oceans at the same latitude.

Terrestrial Radiation, Heating and Cooling of the Atmosphere

• Terrestrial radiation:
  • Solar radiation heats up Earth’s surface in short wave forms
  • Earth radiates energy in long waves to atmosphere
  • Terrestrial radiation heats up atmosphere from below
  • Atmosphere radiates and transmits heat to space
  • Maintains constant temperature at Earth’s surface
• Heating and cooling of atmosphere:
  • Terrestrial radiation heats up lower atmosphere in contact with Earth’s surface
  • Conduction: flow of energy from warmer to cooler body
  • Lower layer of atmosphere rises vertically in currents, called convection
  • Convection restricted to troposphere
  • Transfer of heat through horizontal movement of air called advection
  • Advection more important than convection
  • In middle latitude, most diurnal changes result of advection alone
  • During summer in India, local winds called loo are the outcome of the advection process.

Heat Budget of the Earth

• Earth maintains its temperature due to heat balance between heat received from insolation and heat lost through terrestrial radiation.
• 35% of solar radiation is reflected back to space, known as albedo of the earth.
• Out of remaining 65%, 14% is absorbed by the atmosphere and the rest 51% is absorbed by the earth’s surface.
• 51% of energy received by the earth is radiated back as terrestrial radiation.
• 17% is radiated back to space directly and the remaining 34% is absorbed by the atmosphere.
• Out of 34%, 6% is absorbed directly by the atmosphere, 9% through convection and 19% through latent heat of condensation.
• The total 48% absorbed by the atmosphere is radiated back to space.
• Total radiation returning back from the atmosphere and the earth is 65%, which balances the total of 65% received from the sun.
• This is known as the heat balance or heat budget of the earth.

Variation in the Net Heat Budget at the Surface of the Earth

• Insolation received at the surface of the earth varies from place to place
• Some parts of the earth have surplus radiation balance while others have a deficit
• Between 40°N and 40°S, there is a surplus of net radiation balance
• Regions near the poles are in deficit
• Extra heat energy from the tropics gets redistributed towards the poles
• This redistribution helps to prevent the tropics from getting progressively heated up due to excess heat accumulation
• It also prevents high altitudes from getting permanently frozen due to excess deficit

Factors Controlling Temperature Distribution

• Latitude of the place: Temperature varies according to the solar radiation received, which varies according to the latitude.
• Altitude of the place: Generally, temperature decreases with an increase in height. The vertical decrease in temperature of the troposphere is called the “normal lapse rate” or “vertical temperature gradient”.
• Distance from the sea: The location of a place with respect to the sea influences the temperature. Places near the sea come under the moderating influence of the sea and land breezes which moderate the temperature.
• Air mass and ocean currents: Places under the influence of warm air masses experience higher temperatures while places under the influence of cold air masses experience low temperatures. Places located on the coast where the warm ocean currents flow experience higher temperatures than the places located on the coast where the cold currents flow.