Solar Insolation & Heat Budget

Solar Insolation and Heat Budget – UPSC World Geography Notes

The Earth maintains a constant temperature, neither gaining nor losing heat as a whole. This equilibrium is achieved when the amount of heat absorbed from insolation equals the heat lost through terrestrial radiation. The Heat Budget, or balance, of the Earth is determined by the relationship between incoming solar radiation (insolation) and terrestrial radiation. Insolation refers to the energy received by the Earth’s surface in the form of short waves. This article aims to elucidate the concepts of Insolation and Heat Budget, offering valuable insights for Geography preparation in the context of the UPSC Civil Service exam.


  • Insolation refers to incoming solar radiation intercepted by the Earth.
  • The Earth absorbs a portion of this solar radiation (insolation), subsequently radiating it back into space through terrestrial radiation.
  • The Earth’s heat budget is the mechanism by which the planet sustains a consistent temperature by managing the intake and outflow of heat.

Factors Affecting Insolation

  • The distribution of insolation on the Earth’s surface is not uniform, varying by location and time.
  • Tropical regions receive the highest yearly insolation, decreasing steadily towards the poles.
  • Seasons influence insolation, with greater amounts in summers and reduced levels in winters. Key factors include:
    • Rotation of the Earth on its axis.
    • Earth’s revolution.
    • Angle of incidence of the sun’s rays.
    • Duration of the day.
    • Transparency of the atmosphere.

Rotation of the Earth on its axis

Due to the Earth’s rotation, one hemisphere receives sunlight while the other remains in darkness, influencing the quantity of solar insolation in that particular half of the world.

The Earth’s Revolution

  • The Earth rotates on its axis at a 66.5-degree angle to the plane of its orbit around the sun.
  • The distribution of insolation at various latitudes is primarily influenced by the Earth’s rotation on its inclined axis.
  • Due to the curvature of the Earth’s surface, insolation is concentrated toward the equator.
  • The Earth’s spin axis is tilted by 23.4 degrees relative to a line perpendicular to the Earth’s orbital plane.
  • As the Earth orbits the Sun, insolation is concentrated in the northern hemisphere during summer and then shifts to the southern hemisphere during winter (simultaneously, it is winter in the northern hemisphere).
  • The orbit of revolution around the Sun is elliptical.

The angle of incidence of the sun’s rays

  • The Earth’s geoid shape, resembling a sphere, leads to varying angles at which the sun’s rays reach its surface.
  • The angle difference is dependent on the latitude of the location.
  • Lower latitudes experience smaller angles between the sun’s rays and the Earth’s surface.
  • Vertical rays cover a smaller area compared to slanting rays.
  • Energy is spread over a larger space, resulting in a decline in the net energy received per unit area.

Varying lengths of day and night

  • This factor varies across locations and seasons, determining the amount of insolation received on the Earth’s surface.
  • The duration of the day influences the level of insolation, with longer days resulting in higher insolation and vice versa.
  • Day length is also influenced by the Earth’s revolution around the sun and its tilted axis in both the northern and southern hemispheres.
  • The Earth’s inclined axis, at an angle of 66 1/2 degrees, leads to seasonal variations in day and night lengths.
  • In the northern hemisphere’s winter (December), the hours of darkness increase as one moves northward.
  • On December 22nd, at the Arctic Circle (66.12 degrees North), the sun never ‘rises,’ resulting in a full day of darkness.
  • Beyond the Arctic Circle, the number of days with total darkness increases, with half the year spent in darkness at the North Pole (90 degrees North).
  • In summer (June), the situation is reversed, with increasing daylight as one moves closer to the poles.
  • At the Arctic Circle, there is no sunset on mid-summer (June 21st), providing a continuous 24-hour period of daylight.
  • The region north of the Arctic Circle is popularly known as the ‘Land of the Midnight Sun’ during summer.
  • At the North Pole, there are six months of continuous daylight, making it the ultimate example of the ‘Land of the Midnight Sun.’

Transparency of the Atmosphere

  • Transparency in the atmosphere is influenced by factors such as cloud cover, thickness, dust particles, and water vapor.
  • Clouds, dust, and water vapor can reflect, absorb, or transmit insolation.
  • Thick clouds hinder the ability of solar energy to reach the Earth’s surface.
  • Water vapor, on the contrary, absorbs solar radiation, leading to a reduction in the amount of insolation reaching the surface.

Insolation – Mechanism

This process occurs through the combined influence of atmospheric and ocean circulation, working together to regulate the Earth’s temperature in the following manner.

  • The climate’s heat engine redistributes solar heat from the equator to the poles and from the Earth’s surface and lower atmosphere back to space.
  • Radiative equilibrium occurs when the influx of incoming solar energy is balanced by an equivalent flow of heat to space, resulting in relatively stable global temperatures.
  • Equator and 40° N and S latitudes receive surplus energy from sunlight, while areas beyond 40° N and S latitudes experience energy deficits.
  • Most significant heat transfer happens across mid-latitudes (30° to 50°), associated with much of the stormy weather.
  • The flow of surplus energy from lower latitudes to higher latitudes’ deficit energy zones maintains an overall equilibrium across the Earth’s surface.
  • This process contributes to the Earth’s Heat Budget.

Heat Budget

A heat budget refers to the precise equilibrium between the incoming heat absorbed by the planet and the outgoing heat emitted as radiation. Any disturbance to this balance can result in the Earth either becoming progressively warmer or cooler over time.

Heat Budget – Mechanism

  • At the top of the atmosphere, the insolation received is 100%.
  • Energy is reflected, scattered, and absorbed as it passes through the atmosphere.
  • Only a portion of the insolation reaches the Earth’s surface.
  • Before reaching the surface, about 35 units are reflected back to space.
    • 27 units are reflected from the tops of clouds.
    • 2 units are reflected from snow and ice-covered areas.
  • The albedo of the Earth indicates the quantity of reflected radiation.
  • Out of the initial 100 units, 65 units are absorbed by the Earth’s surface.
    • 14 units are absorbed by the atmosphere.
    • 21 units are absorbed by clouds.
  • In the form of terrestrial radiation, the Earth emits 51 units back into space.
  • Seventeen units are directly radiated into space, with the remaining 34 units absorbed by the atmosphere. This absorption includes 6 units directly absorbed, 9 units through convection and turbulence, and 19 units through the latent heat of condensation.
  • The atmosphere absorbs a total of 48 units, subsequently radiating them back into space. This includes 14 units from insolation and 34 units from terrestrial radiation.
  • Consequently, the combined radiation returning from the Earth and atmosphere amounts to 17 + 48 = 65 units, balancing the total of 65 units received from the sun.


  • The Earth maintains a constant temperature, neither gaining nor losing heat as a whole.
  • This equilibrium is achieved when the amount of heat absorbed as insolation equals the amount lost through terrestrial radiation.
  • Albedo, a reflection coefficient with a value of less than one, measures how much light is reflected back without absorption when it strikes a surface.
  • Solar radiation undergoes reflection, scattering, and absorption as it passes through the atmosphere.
  • The Earth’s albedo represents the quantity of radiation reflected, and different surfaces have varying albedo values.

The phenomenon known as the “Urban Heat Island Effect” occurs when densely developed areas, such as cities, exhibit higher average temperatures compared to the surrounding suburban or rural areas, primarily influenced by albedo effects. Factors contributing to the elevated temperatures include reduced foliage, increased population density, and a prevalence of dark surfaces in infrastructure such as asphalt roads and brick buildings.

Variation in the Net Heat Budget of the Earth

  • The Earth maintains overall balance between insolation and terrestrial radiation.
  • Latitudinal variations exist, with the tropical zone having greater insolation than terrestrial radiation, resulting in a surplus of heat.
  • The polar zone experiences smaller heat gain than loss, creating a heat-deficit region.
  • Insolation induces heat imbalances across latitudes.
  • Winds and ocean currents aid in mitigating the imbalance by transporting heat from surplus to deficit regions.
  • This process of redistributing and balancing heat at different latitudes is known as Latitudinal Heat Balance.


  • The Earth’s heat balance, governed by its Heat Budget, is essential for creating a habitable environment.
  • This balance ensures the Earth stays warm and is crucial for enhancing the efficiency of solar panels that capture and convert solar energy.
  • It plays a pivotal role in determining diverse rain patterns, varying from the equator to the poles.
  • The heat balance is accountable for temperature fluctuations observed from the equator to the poles.
  • It contributes significantly to the process of photosynthesis, supporting the growth of plants.


The sun stands as the primary and most potent heat source, distributing its heat unequally across the globe. This uneven distribution serves as the fundamental driver of various climatic characteristics. Consequently, grasping the temperature distribution patterns across seasons becomes crucial for a comprehensive understanding of other climatic factors, including wind systems, pressure systems, precipitation, and more.

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