Insolation & Heat Budget

What is insolation?

The amount of solar energy that the earth receives from the sun is known as insolation. Insolation is affected by the following four factors:

  1. Solar constant
  2. Distance from the sun
  3. Altitude of the sun
  4. Length of the day

Solar Constant

It is the intensity of solar radiation received on a unit area of horizontal surface at the average distance from the sun.

The average value of the solar constant is

2 gram calories per square centimeter per minute

Distance from the sun

Since the motion of planets is governed by Kepler’s Law the orbit of Earth around the sun is elliptical. Hence, our planet doesn’t rotate around the sun at an equal distance or at an equal time. When it is closest to the sun it moves at a faster pace, this distance is known as Perihelion. When the earth is at the farthest distance it moves at a slower pace, this distance is known as the Aphelion.


What are Perigee and Apogee?

These are the closest and farthest points of the moon from the earth in its elliptical orbit around the earth.


Altitude of the sun

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Refers to the angle that the sun’s rays form when they touch the earth’s surface. The altitude of the sun keeps changing throughout the day. On sunrise and sunset, the altitude is the lowest i.e. the rays make the lowest angle with the surface and transfer the least amount of heat. During mid-day, the sun is at the highest altitude and transfers the maximum heat. Similarly, at different latitudes, the rays of the sun fall at different angles. On higher latitudes, the suns rays are slanting, while on lower latitudes they are much more vertical.

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As shown in the figure the increase in the area that the rays have to cover changes the solar constant of the region. Also when the sun’s rays are slanting the reflectance increases resulting in a further reduced absorption rate.

Length of the day

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The length of the day varies as we travel from one latitude to another. This happens because of the movement and tilt of the earth. The tilt of the earth forms a circle of illumination due to which the sun’s rays are prolonged at some places in summer solstice while those in winter solstice receive lesser insolation.

Latitude20°40°60°90°
December 2212:0010:4809:805:3300:00
June 2112:0013:1214:5218:276 months
Length of the day in hours and minutes on Winter and Summer Solstices in the Northern Hemisphere

Why are days always longer than nights at the equator?

Answer: Because of the atmosphere.

When the rays of the sun enter the atmosphere from space, they enter a denser medium (space to atmosphere), this makes them refract. Hence, we see the sun before it actually rises and even after it sets. This is known as advanced sunrise and delayed sunset.