Insolation & Heat budget of the Earth | Geography for UPSC CSE PDF Download

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.

• 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 for Insolation & Heat budget of the Earth

  Try yourself:Which of the following statements is true about insolation?

  • A.

    Earth’s surface receives its energy as short wave electromagnetic radiation
  • B.

    On an average earth receives 1.94 calories per sq. cm per minute at the top of the atmosphere
  • C.

    Total insolation received at the equator is 400% more than at poles.
  • D.

    All of the above

  Explanation

  In insolation, Earth’s Surface receives its energy as short wave

  electromagnetic radiation on average earth receives 1.94 calories per Sq

  cm per minute at the top of the atmosphere. Total insolation received at the

  equator is 400% more than at poles.

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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.

Reflection: 

• 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.

Scattering: 

• 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.

Advection: 

• 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.

Compression—Adiabatic 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. In 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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Heat Budget of the Earth

The Earth's heat budget represents a balance between the incoming heat absorbed by the Earth and the outgoing heat radiated back into space. If this balance is disrupted, the Earth will either warm up or cool down over time. This explains why the Earth's temperature remains stable, despite significant heat transfer. In essence, the heat budget of Earth involves the gain and loss of heat, maintaining equilibrium between the heat received and radiated.

What is the Heat Budget of Earth?

The Earth's heat budget is a process that maintains heat balance, with incoming heat being absorbed by the Earth and outgoing heat escaping as radiation. The sun does not heat the Earth evenly; due to its spherical shape, the equatorial regions receive more heat than the polar regions. To address these solar heating imbalances, the atmosphere and oceans continuously work through processes such as water vaporization, convection, rainfall, winds, and ocean circulation.

The combined circulation of the atmosphere and oceans helps maintain Earth's temperature in the following ways:

• The climate's heat engine not only redistributes solar heat from the equator to the poles but also returns heat from Earth's surface and lower atmosphere back to space.
• Earth maintains radiative equilibrium, with a stable global temperature, when the incoming solar energy is balanced by an equal amount of heat radiated into space.
• Areas within the equator and between 40° N and S latitudes receive more sunlight, creating energy surplus regions. In contrast, areas above 40° N and S latitudes lose more heat than they receive, making them energy deficit regions.
• The atmosphere (through planetary winds) and oceans (through ocean currents) transport excess heat from the tropics (energy surplus regions) toward the poles (energy deficit regions), compensating for heat loss at higher latitudes.
• Most heat transfer occurs in the mid-latitudes (30° to 50°), where much of the stormy weather is concentrated.

Therefore, the movement of excess energy from the lower latitudes to the higher latitudes' deficit regions helps maintain an overall energy balance across Earth's surface.

Components of Heat Budget

The components of the Earth's heat budget include the following:
Insolation: Insolation refers to the thermal radiation from the Sun received by Earth's surface per unit area. The processes contributing to thermal balance through insolation include:

• Reflection: This occurs when incoming solar radiation strikes a surface (sky, land, or water) and bounces back without generating heat.
• Absorption: Absorption involves the conversion of electromagnetic radiation into heat energy.
• Scattering: Scattering happens when solar radiation interacts with tiny particles in the Earth's atmosphere, such as air molecules, water droplets, or aerosols, causing the radiation to spread in all directions.
• Terrestrial Radiation: Longwave radiation emitted by Earth's surface or atmosphere is known as terrestrial radiation. The processes that help maintain temperature balance through terrestrial radiation include:
• Latent Heat Transfer: Latent heat is the energy absorbed or released during a substance's phase change (solid to liquid, liquid to gas, etc.). Latent heat transfer refers to the heat transferred during these phase transitions.
• Sensible Heat Transfer: Sensible heat is the energy required to raise the temperature of a substance without causing a phase change. Sensible heat transfer occurs when energy is supplied as heat, causing a temperature shift without altering the state of the substance.
• Emission by Vapour and Clouds: Clouds and water vapour emit substantial amounts of terrestrial radiation, further contributing to the heat budget.

How is the Heat Budget of Earth Analyzed and Calculated?

The Earth's heat budget can be summarized as follows:

• Insolation: The total incoming solar radiation at the top of the atmosphere is considered 100 percent. However, before reaching the Earth's surface, some of this energy is reflected, scattered, and absorbed by the atmosphere. Of the total energy, 35 units are reflected back into space, including:

  • 27 units reflected from cloud tops.
  • 2 units reflected from snow and ice-covered areas (collectively known as Earth's albedo).

• Terrestrial Radiation: The Earth emits 51 units as terrestrial radiation. Of this:

  • 17 units are directly emitted into space.
  • 34 units are absorbed by the atmosphere, including:
    • 6 units absorbed directly.
    • 9 units absorbed through convection and turbulence.
    • 19 units absorbed through latent heat of condensation.

• Absorption of Solar Energy: A total of 65 units of energy are absorbed by the Earth and its atmosphere. This includes:

  • 14 units absorbed within the atmosphere.
  • 51 units absorbed by Earth's surface.

• Radiation Emission: The atmosphere emits 48 units back into space, comprising:

  • 14 units from insolation.
  • 34 units from terrestrial radiation.

Thus, the total radiation returning from both the Earth and the atmosphere is 65 units (17 units from the Earth and 48 units from the atmosphere), which balances the 65 units of solar insolation received. This maintains the Earth's heat budget equilibrium.

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Variation in the Heat Budget of Earth

• Although the Earth maintains a balance between insolation and terrestrial radiation, this is not consistent across all latitudes. In the tropical zone, the insolation and heat budget exceed terrestrial radiation, creating an excess of heat. 
• Conversely, in the polar zone, the heat gain is less than the heat loss, resulting in a heat deficit. This imbalance of heat occurs at different latitudes due to variations in insolation and the heat budget. 
• Winds and ocean currents help mitigate this imbalance by transferring heat from surplus regions to deficit regions. This process of redistributing and balancing heat across latitudes is called latitudinal heat balance.

Effects of the Heat Budget of Earth on the Climate System of Earth

• The Earth’s heat budget plays a crucial role in determining the planet’s climate. When the heat budget is balanced, the Earth’s temperature remains relatively stable, with no significant increase or decrease in average temperature.
• Global weather and climate variations are driven by the uneven heating of the Earth and its atmosphere, which is influenced by latitudinal and seasonal changes in insolation.
• The energy received by the Earth and the energy emitted from it do not perfectly balance. This imbalance is partly due to seasonal changes in solar energy and fluctuations in the Earth's atmospheric composition.
• Changes in the Earth's atmospheric composition affect the amount of energy the atmosphere absorbs and reflects. These variations contribute to a slight but significant energy imbalance on the planet.
• Human activities, particularly increasing carbon dioxide levels in the atmosphere, exacerbate this energy imbalance. As a result, Earth's temperature is expected to rise to compensate for this imbalance.
• With rising concentrations of carbon dioxide and other greenhouse gases, the energy imbalance is expected to grow annually, further contributing to the increase in global temperatures.
• The imbalance in the Earth’s heat budget is a major driver of rising temperatures, which is one of the key impacts of climate change.

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Importance of Heat Budget of Earth

• The Earth’s heat balance is vital for maintaining a livable environment, and the heat budget plays a key role in achieving this.
• It helps keep the planet warm.
• The heat budget is important for improving the efficiency of solar panels that capture and convert solar energy.
• It is responsible for the temperature variation between the equator and the poles.
• It supports photosynthesis, which is essential for plant growth.
• The heat budget also contributes to the variation in rainfall patterns from the equator to the poles.