Insolation and Heat Budget

“Insolation and heat budget” is the topic of Climatology. It deals with, how Earth receives solar Radiation and the mechanism through which it protects Earth from overheating and maintains Earth’s Temperature.  This article is relevant for various examinations including SSC and Banking. Questions about this topic are common these days.

Insolation

The amount of insolation received at the earth’s surface decreases from the equator to the poles on average, but there is temporal variation in the amount of solar irradiance received at different latitudes at various times during the year.

• Insolation is defined as the Solar energy (radiation) received by Earth. 
  
• It is the solar radiation reaching any particular area.
  
• It is the source of energy for the entire Earth.

At different latitudes, the level of solar radiation reaching the outer edge of our atmosphere is considerably greater than the quantity of insolation received at the ground surface. This trend demonstrates that a significant portion of the incoming solar radiation is lost as it passes through the atmosphere due to cloud cover, earth’s atmosphere sediment (scattering), ability to reflect, and solubility (through ozone). At the time of the summer solstice, maximum insolation reaches the outer limit of the atmosphere at the north pole, whereas maximum insolation is received at the ground surface between latitudes 30°- 40° N on June 21st due to the presence of a subtropical high-pressure belt and anticyclonic conditions.

Based on the amount of insolation received over the course of a year, the globe is split into 3 zones.

1. Low Latitude Zone: Between the Tropics of Cancer and Capricorn, there is a low latitude or tropical zone. Due to the sun’s northward and southerly motions, every location experiences overhead sun (vertical solar rays) twice a year. As a result, every location experiences its highest and lowest insolation levels twice a year. The region experiences the maximum insolation of any other zone, and seasonal change is minimal.
2. Middle Latitude Zone:  In both hemispheres, this zone is extended between 23.5⁰ and 66⁰ latitudes.
3. Polar Zone: In both hemispheres, the polar zone covers the area between latitudes 66° and 900 (poles). Every place experiences its maximum and minimum insolation once per year, however occasionally insolation reaches zero due to the lack of direct sunlight.

Factors Affecting The Distribution of Insolation

The amount and intensity of insolation vary during the day and night and with seasons as well. The factors which cause these variations are:

The angle of inclinations of the sun’s rays

• The area which receives direct sun rays or makes less angle receives more radiation than the areas which make a greater angle. 
• The amount of insolation that will be received at a given location is mostly determined by the angle formed by the sun’s rays and the tangent to the earth’s surface there.
• At the equator, the sun’s beams are roughly vertical (the maximum angle is 90° between the sun’s rays as well as the tangent towards the earth’s surface) and get increasingly oblique as you move poleward. In other terms, poleward is where the sun’s rays are at a smaller angle.
• The amount of energy received per unit area rises when vertical rays are dispersed over the smallest feasible portion of the earth’s surface and heat it. The quantity of energy received per unit area drops, however, when oblique rays cover a bigger portion of the earth’s surface.
• In comparison to vertical rays, oblique rays must travel through a thicker section of the atmosphere. The oblique rays must therefore go farther than the vertical rays.

Length of the day

• The longer the duration of sunshine more will be the quality of solar radiation received. Days in the Equatorial region are longer than in the polar region so the Equatorial region receives more insolation than the polar region. 
• The circle of illumination, also known as the light circle, constantly divides the equator into two equal half, resulting in a 12-hour day at the equator. However, the length of the day lengthens poleward with the northward march of the sun in the northern hemisphere whereas it shortens around the summer solstice in the southern hemisphere (21 June). On the other hand, during the time of the winter solstice (22 December), the length of the day grows from the poleward in the southern hemisphere but decreases in the northern hemisphere (southward march of the sun).
• It’s crucial to note that during the sun’s northward movement from 21 March to 23 September, the length of the day at the north pole increases to 6 months, while the length of the night at the south pole increases to 6 months.) In contrast, during the southward migration of the sun, the length of the day becomes 6 months at the south pole (23 September to 21 March), while the duration of the night becomes 6 months at the north pole.
• Even though the days are the longest at the poles, insolation is at its lowest because 

            (i) the sun’s beams are more or less parallel to the ground 

            (ii) the majority of the solar radiation is reflected by the ice cover.

• It is clear that the length of the day does not have as much of an impact on the amount of insolation received as the angle of the sun’s beams.
  

Transparency of atmosphere

The atmosphere is not transparent to the radiation coming from Sun. Different gases present in the atmosphere respond in different ways with Sun’s Radiation. Some absorb it and cause the greenhouse effect.

Distance from Sun 

• The earth should receive the most insolation at the period of perihelion when it is closest to the sun, and the least at the time of aphelion, when it is farthest from the sun. In fact, because of the low amount of insolation received in January, when the earth is closest to the sun, the northern hemisphere experiences winter rather than summer. On the other hand, due to the great quantity of insolation received in July, when the earth is furthest from the sun, summer instead of winter occurs in the northern hemisphere.
• Insolation is distributed differently depending on the angle of the sun’s rays and the duration of the day than depending on how far the earth is from the sun.
• Insolation received varies in a year due to variations in its distance from the sun. Insolation received on perihelion (3rd January) is more than the amount of insolation on Aphelion (4th July).   

Earth’s rotation on its axis and revolution of the article around the sun.

• Perihelion – When the earth is nearest to the sun(147 million km), usually on 3rd January.

  Aphelion – When the earth is farthest from the sun(152 million km), usually on 4th July.  
  

• On average, the Earth receives 1.94 calories per sq cm per min.

SUNSPOT

• Periodic disruptions and explosions cause sunspots to form on the solar surface. Every year has a different number of sunspots. Studies have revealed that the variation in sunspot number is cyclical in nature. In other words, a cycle of 11 years completes the increase and decrease in the number of sunspots. The highest number of sunspots occurs every eleventh year.
• The amount of insolation received at the earth’s surface likewise increases as the energy radiated from the sun increases along with the number of sunspots. On the other hand, as the number of sunspots declines, less insolation reaches the surface of the planet.

Heat Budget

• Earth receives solar insolation in the form of short waves and radiates them back into space in the form of long waves.
  
• The long wave radiation is absorbed by atmospheric gases, like carbon dioxide and greenhouse gases. These gases indirectly heat Earth’s atmosphere. The atmosphere in turn radiates and transmits heat to space.
• Finally, the amount of heat received from the sun is returned to space, thereby maintaining a constant temperature at the earth’s surface and in the atmosphere.
  
• The earth gets energy(heat) from the insolation received this happens through various processes like Conduction, Convection, and advection.

Conduction – Conduction is defined as the flow of energy from a warm body to a cold body when both are in contact. 

Convection – It is the process of transfer of heat usually between fluids such as gases and Liquids. 

Convection is the movement of a mass of material from one location to another in order to transmit heat energy.

• Only in fluids or gases can the convection process work because the internal mass motion of these substances causes the convection of heat energy. After receiving heat energy (insolation) from the sun, the earth’s surface warms up. As a result, the air that comes into touch with the warmer earth’s surface also becomes heated and expands. Vertical air circulation is established as a result of warmer air being lighter and rising upward. On the other hand, as a result of volume contraction, the considerably cooler air in the atmosphere gets heavier and descends to the earth’s surface. Due to the dry adiabatic rate and the warm ground surface, the air that is descending is warmed. Due to an increase in volume and a decrease in density, this heated air ascends once more.

Advection – Transfer of heat especially horizontally in the atmosphere 

• This heat budget keeps the Earth’s temperature in check and ensures life thrives.
  .
• Earth maintains its temperature as it radiates back heat (radiation) in space which is called Albedo.
  
• Albedo is defined as the fraction of light reflected by any Body.
  
• If we take the entire insolation received as 100 units then this insolation undergoes various activities in space. It may get absorbed, reflected, or scattered. Nearly 35 units are reflected in space even before reaching Earth’s surface.
  
• Of these 35 units, Clouds reflect nearly 27 units of Insolation and 2 units of it by snow and ice-covered areas. 
  
• Snow and Ice have the highest albedo.
  
• The remaining 65 units are absorbed, 14 units within the atmosphere and 51 units by the earth’s surface. 
  
• The Earth radiates back 51 units in the form of terrestrial radiation. Of these, 17 units are radiated to space directly and the remaining 34 units are absorbed by the atmosphere. 
• 48 units (14 units directly and 34 units from terrestrial radiation) absorbed by the atmosphere are also radiated back into space. 
• Thus, the total radiation returning from the earth and the atmosphere respectively is 17+48 =65 units which balances the total of 65 units received from the sun. 
• Through this process, Earth gets neither too warm nor too cold.
  
• This is termed as heat budget or heat balance of the Earth.