Insolation and Heat budget of the Earth Notes | EduRev

Geography for UPSC CSE

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Insolation (or Incoming Solar Radiation)

The insolation entrance into the upper atmosphere is just the beginning of a complex series of events in the atmosphere and at Earth’s surface.

Insolation and Heat budget of the Earth Notes | EduRev

  • 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 solar energy reception and the resulting energy cascade ultimately warms Earth’s surface and the atmosphere.
  • The average value of incoming solar radiation (Insolation) received at the thermopause, i.e. 480km above the earth’s surface when the earth is at an average distance from the sun is called solar constant. 
  • The average value of the solar constant is estimated to be 1.968 calories per cm2 per minute.
  • The earth receives the energy emitted by the sun in the form of electromagnetic radiations. 
  • The quantity of radiations is about 1.968 calories/cm2/ minute. A calorie is that amount of energy required to raise the temperature of one gram of water by one degree Celsius.
  • The Sun gives off energy in the form of electromagnetic radiation— sometimes referred to as radiant energy. (The Sun also gives off energy as streams of ionized particles called the solar wind, but we can ignore that kind of energy in our discussion here because its effect on weather is minimal.)
  • We experience different kinds of electromagnetic radiation every day: visible light, microwaves, X-rays, and radio waves are all forms of electromagnetic radiation.
  • Electromagnetic radiation varies enormously in wavelength—ranging from the exceedingly short wavelengths of gamma rays and X-rays (with some wavelengths less than one-billionth of a meter) to the exceedingly long wavelengths of television and radio waves (with some wavelengths measured in kilometres. 
    Question 1:Which of the following statements is true about insolation?

Several processes deplete the solar radiation as it passes through the earth’s atmosphere like:

  • Radiation or emission is the process by which electromagnetic energy is emitted from an object. So the term “radiation” refers to both the emission and the flow of electromagnetic energy. 
  • All objects emit electromagnetic energy, but hotter objects are more intense radiators than cooler objects. In general, the hotter the object, the more intense its radiation.

(Radiation intensity is commonly described in W/m2 — the amount of energy emitted or received in a given period of time in a given area.)

  • Because the Sun is much hotter than Earth, it emits about two billion times more energy than Earth. Also, the hotter the object, the shorter the wavelengths of that radiation. 
  • Hot bodies radiate mostly short wavelengths of radiation, whereas cooler bodies radiate mostly long wavelengths.


  • The radiations are reflected in the space by the surface and atmosphere of the earth.
  • The total reflection of the incoming solar radiation is called albedo and is expressed in terms of the percentage of insolation. Clouds are the most important reflectors by far. 
  • Their reflectivity ranges from 40 to 90% depending upon the thickness and type of cloud.
  • The term albedo refers to the overall reflectivity of an object or surface, usually described as a percentage, the higher the albedo, the greater the amount of radiation reflected. 
  • For example, Snow has a very high albedo (as much as 95 percent), whereas a dark surface, such as dense forest cover, can have an albedo as low as 14 percent.

Absorption: Electromagnetic waves striking an object may be assimilated by that object—this process is called absorption. Different materials have different absorptive capabilities, with the variations depending on the wavelength of radiation involved.


  • It is the process by which small particles, with a size comparable to the radiations' wavelength, deflect the radiations in a different direction. 
  • The direction of radiation changes as it keeps on scattered by the particles.
  • The amount of scattering that takes place depends on the wavelength of the light and the size, shape, and composition of the molecule or particulate.
  • In general, shorter wavelengths are more readily scattered than longer wavelengths by the gases in the atmosphere.

Transmission: Some radiation passes through the atmosphere without reflection, refraction, absorption, or scattering. This is called transmission.

Conduction: The transfer of heat from one molecule to another without changes in their relative positions is called conduction. This process enables energy to be transferred from one part of a stationary body to another or from one object to a second object when the two are in contact.

Convection: In the process of convection, energy is transferred from one point to another by the predominately vertical circulation of a fluid, such as air or water. Convection involves movement of the warmed molecules from one place to another.


  • When the dominant direction of energy transfer in a moving fluid is horizontal (sideways), the term advection is applied. 
  • In the atmosphere, wind may transfer warm or cool air horizontally from one place to another through advection. 
  • Some wind systems develop as part of large atmospheric convection cells: the horizontal component of air movement within such a convection cell is properly called advection.

Expansion Adiabatic Cooling

The expansion in rising air is a cooling process even though no energy is lost. As air rises and expands, the molecules spread through a greater volume of space—the “work” done by the molecules during expansion reduces their average kinetic energy, so the temperature decreases. This is called adiabatic cooling—cooling by expansion (adiabatic means without the gain or loss of energy). In the atmosphere, any time air rises, it cools adiabatically.

CompressionAdiabatic Warming: 

Conversely, when air descends, it becomes warmer. The descent causes compression as the air comes under increasing pressure the work done on the molecules by compression increases their average kinetic energy. 

The temperature increases even though no energy was added from external sources. This is called adiabatic warming—warming by compression. In the atmosphere, any time air descends, it warms adiabatically. Adiabatic cooling of rising air is one of the most important processes involved in cloud development and precipitation, whereas the adiabatic warming of descending air has just the opposite effect.

Latent Heat: The physical state of water in the atmosphere frequently changes—ice changes to liquid water, liquid water changes to water vapour, and so forth. Any phase change involves an exchange of energy known as latent heat (latent is from the Latin, “lying hidden”).

The two most common phase changes are evaporation, in which liquid water is converted to gaseous water vapour, and condensation, in which water vapour is converted to liquid water.

During the process of evaporation, latent heat energy is “stored” and so evaporation is, in effect, a cooling process. On the other hand, during condensation, latent heat energy is released and so condensation is, in effect, a warming process.

Concept of twilight (dawn and dusk)

Twilight is the time between day and night when there is light outside, but the Sun is below the horizon.

The diffused light that occurs before the sunrise and sunset gives valuable working hours for humans. The light scattered by the gas molecules and reflected by water vapour and dust particles cause illumination of the atmosphere. Such effects can be enhanced due to pollution and other suspended particles as those in volcanic eruptions and forest fires.

In the morning, twilight begins with the dawn, while in the evening it ends with dusk. Several atmospheric phenomena and colours can be seen during twilight. Astronomers define the three stages of twilight – civil, nautical, and astronomical – based on the Sun’s elevation, which is the angle that the geometric center of the Sun makes with the horizon.

➤ Civil Twilight

  • Civil twilight occurs when the Sun is less than 6 degrees below the horizon. In the morning, civil twilight begins when the Sun is 6 degrees below the horizon and ends at sunrise. In the evening, it begins at sunset and ends when the Sun reaches 6 degrees below the horizon.
  • Civil dawn is the moment when the geometric center of the Sun is 6 degrees below the horizon in the morning.
  • Civil Dusk is when the geometrical center of the Sun is 6 degrees below the horizon in the evening.
  • Civil twilight is the brightest form of twilight. There is enough natural sunlight that artificial light may not be required to carry out outdoor activities during this period. The naked eye can observe only the brightest celestial objects during this time.
  • Several countries use this definition of civil twilight to make laws related to aviation, hunting, and the usage of headlights and street lamps.

➤ Nautical Twilight, Dawn and Dusk

  • Nautical twilight occurs when the geometrical center of the Sun is between 6 degrees and 12 degrees below the horizon. This twilight period is less bright than civil twilight, and artificial light is generally required for outdoor activities.
  • Nautical dawn occurs when the Sun is 12 degrees below the horizon during the morning.
  • Nautical dusk occurs when the Sun goes 12 degrees below the horizon in the evening.

The term, nautical twilight, dates back to when sailors used the stars to navigate the seas. During this time, most stars can be easily seen with naked eyes.mIn addition to being important to navigation on the seas, nautical twilight also has military implications. For example, the United States’ military uses nautical twilight, called begin morning nautical twilight (BMNT) and end of evening nautical twilight (EENT), to plan tactical operations.

➤ Astronomical Twilight, Dawn, and Dusk

  • Astronomical twilight occurs when the Sun is between 12 degrees and 18 degrees below the horizon.
  • Astronomical dawn is the time when the geometric center of the Sun is at 18 degrees below the horizon. Before this time, the sky is absolutely dark.
  • Astronomical dusk is the instant when the geographical center of the Sun is at 18 degrees below the horizon. After this point, the sky is no longer illuminated.

The duration of dawn and twilight is a function of latitude because the sun's angle above horizon determines the distance travelled by the light in the atmosphere. Lower angle produces longer dawn and twilight periods. At the equator, the light is almost perpendicular hence the dawn and twilight are 30-45 min long while at poles there are about 7 weeks of dawn and 7 weeks of twilight leaving only 2.5 months of near darkness.

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