Insolation (or Incoming Solar Radiation)
- Insolation is the amount of solar radiation that is received by a planet. The energy received by the earth’s surface in the form of short waves is termed as Incoming Solar Radiation or Insolation.
- The insolation is not constant over the surface of the Earth — it is concentrated near the equator because of the curvature of the Earth.
- Some of the insolation is reflected off the atmosphere back out into space, where it is lost. The remaining insolation may pass through the atmosphere, where it can be transformed either before or after reaching Earth’s surface.
- This reception of solar energy and the resulting energy cascade that ultimately warms Earth’s surface and the atmosphere.
Variability of insolation at the surface of the earth
The amount and intensity of solar radiation received by the earth (insolation) vary during a day, in a season and in a year. The following are the factors that cause these variations:
- The rotation of the earth on its axis.
- The angle of inclination of the rays of the sun
- The length of the day.
- The transparency of the atmosphere, and
- The configuration of the land in terms of its aspect.
(The insolation depends more on the first three factors)
The tilted position of the earth’s axis is known as the inclination of the earth’s axis. The earth’s rotation axis makes an angle of about 66.5° with the plane of its orbit around the sun and this greatly influences the amount of insolation received at different places.
The amount of insolation also depends on the angle of inclination of the sun’s rays. The higher the latitude the less is the angle they make with the surface of the earth which results in slant sun rays. The slant rays cover more area than the vertical rays. When more area is covered, the energy gets distributed and the net energy received per area decreases. Also, the slant rays have to pass through a greater depth of the atmosphere which results in more absorption, diffusion and scattering.
Before striking the earth’s surface, the incoming solar radiation passes through the atmosphere. The atmosphere is largely transparent to shortwave solar radiation. Water vapours, ozone and other gases present in the atmosphere absorb most of the near-infrared radiations. Small suspended particles in the troposphere scatter the visible spectrum both to space and towards the surface of the earth. The blue colour of the sky and the red colour of the rising and setting sun are the results of the scattering of light within the atmosphere.
Duration of the day varies from place to place and season to season. It decides the amount of insolation received on the earth’s surface.
The amount of solar radiation received at the surface of the earth is more in the tropics (about 320 watts/m²) and least in the poles (70 watts/m²). The subtropical deserts receive maximum insolation as the atmosphere is more transparent (least cloudiness). At the same latitude, the insolation is more over the continents than over the oceans.
Terrestrial Radiation, Heating and Cooling of the Atmosphere
- Terrestrial Radiation – The solar radiation received by the earth is in short wave forms and it heats up its surface. The earth acts as a radiating body and radiates energy in the form of long waves to the atmosphere. This process is called terrestrial radiation and these long wave radiations heat up the atmosphere from below. The atmosphere in turn radiates and transmits heat to space. This maintains the constant temperature at the earth’s surface, as the amount of heat received from the sun is transmitted to space.
- Heating and cooling of the atmosphere (conduction, convection and advection):
- The terrestrial radiation heats up the lower atmosphere which is directly in contact with the surface of the earth. This process is called conduction in which there is a flow of energy from the warmer to the cooler body and the transfer continues till both the bodies attain the same temperature.
- As the lower layer of the atmosphere heats up, it rises vertically in the form of currents and transmits the heat of the atmosphere. This vertical heating of the atmosphere is called convection and is restricted only to the troposphere.
- The transfer of heat through the horizontal movement of air is called advection. During summer in India, the local winds called loo is the outcome of the advection process. Advection is relatively more important than convection. In middle latitude, most of the diurnal (day and night) changes are the result of advection alone.
Heat Budget of the Earth
The surface of the earth maintains its temperature, this is because the amount of heat received by the earth in the form of insolation equals the amount of heat lost by the earth through terrestrial radiation.
When 100% of solar radiation reaches the earth’s atmosphere, about 35% is reflected back to space even before reaching the surface of the earth. The reflected amount is called the albedo of the earth. This amount of energy does not heat either the earth or the atmosphere.
- Out of the remaining 65% of the energy, 14% are absorbed by the atmosphere and the rest, 51% by the surface of the earth (34% through direct solar radiation and 17% from scattered radiation).
- 51% of the 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 (6% is absorbed directly by the atmosphere, 9% through convection and 19% through latent heat of condensation).
- The total 48% absorbed by the atmosphere (14% from insolation and 34% from terrestrial radiation) are also radiated back to space.
- Thus, the total radiation returning back from the atmosphere and the earth is respectively 48+17=65% which balances the total of 65% received from the sun.
- This is termed the heat balance or heat budget of the earth, and explains how the earth maintains its temperature despite the huge transfer of heat.
Variation in the Net Heat Budget at the Surface of the Earth
- The insolation received at the surface varies from place to place, some part of the earth has surplus radiation balance while the other part is deficit.
- There is a surplus of net radiation balance between 40°N and 40°S and the regions near the poles are in deficit. The extra heat energy from the tropics gets redistributed towards the poles, and as a result, the tropics don’t get progressively heated up due to the accumulation of excess heat nor do the high altitudes get permanently frozen due to excess deficit.
Factors Controlling Temperature Distribution
The temperature at any place is influenced by the following factors:
- Latitude of the place – The temperature of a place depends upon the solar radiation received. The insolation varies according to the latitude, so the temperature also varies accordingly. The solar radiations pass vertically along the equator. The angle of incidence decreases from the equator towards the poles. The area heated by the solar radiation increases towards the poles, therefore temperature decreases from the equator to the poles.
- Altitude of the place – The terrestrial radiation heats up the atmosphere from below. Hence, the places near the sea level have a higher temperature than the places at higher altitudes. Generally, temperature decreases with the increase in height. The vertical decrease in temperature of the troposphere is called the “normal lapse rate” or “vertical temperature gradient”. The temperature reduces at the rate of 6.5°C per km of ascent.
- Distance from the sea – The location of a place with respect to the sea also influences the temperature of a place. The variation of temperature over the sea is less compared to the land because the land heats up and cools down quickly, while the sea gets heated up slowly and also loses heat slowly. The places near the sea come under the moderating influence of the sea and land breezes which moderate the temperature.
- Air mass and ocean currents – The places which come under the influence of warm air masses experience higher temperatures and the places that come under the influence of cold air masses experience low temperatures. The 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.