Insolation: Angle of Distribution

Insolation refers to the sunlight received by the earth. It is reckoned as the amount of solar energy received per unit area of the surface of the Earth. Solar radiation garnered by the Earth’s surface changes according to latitude, season, ground, aspect, hillshade, or slope, transparency of the atmosphere, and duration of the day. Throughout the whole day, maximum insolation is experienced when the sun is overhead, that is, at noon. It directly influences the temperature of the place. The temperature increases proportionately with the increase in insolation.

What are the factors affecting insolation?

The Earth is tilted on its axis, which causes the non-uniform distribution of insolation on its surface. The amount of sunlight amassed by the specific geographical area varies according to the place and time. The insolation reduces when moving from the tropical regions toward the polar locations. While it is higher in the summer season as compared to winters or colder climates. The amount of insolation received in a place is determined by the factors mentioned below:

•     Duration of the day
•     Transparency of the atmosphere
•     Distance of the Earth from the Sun
•     The angle of incidence of the sun’s rays
•     Solar constant
•     Topographical variations
•     Earth’s rotation on its axis

Let us discuss each influencer of insolation thoroughly to get a deeper understanding of the concept:

• Duration of the day

The day’s length factors in the amount of insolation received by a specific area on the earth’s surface. The longer the day, the more is the exposure of the area to solar energy, and thus, the higher would be the insolation received. On September 23 and March 21, which are autumn and spring equinoxes, the sun perches overhead at the equator at noon. Since the length of day and night is equal on these days, thereby bringing about maximum insolation, which decreases as we move towards the polar regions. This is because the sun’s rays fall vertically on the equator, while as the latitudes decrease, the rays get increasingly slanting. Hence, less solar energy is collected in the poleward areas. 

• Transparency of the atmosphere

The composition of the atmosphere is not uniform throughout. Instead, there are different layers of varying composition and thickness. The atmosphere constitutes a blend of different gases such as Oxygen, Nitrous Oxide, Hydrogen, Krypton, Methane, Helium, Carbon Dioxide, Xenon, Argon, and Neon. It also comprises water vapours in their gaseous form. These elements of composition render varying transparency to the atmosphere, which directly influences the incoming sun’s radiation and hence, insolation distribution on the Earth’s surface. 

• Distance of the Earth from the Sun

The earth revolves around the sun on an elliptical path, which consistently alters the distance between the sun and the earth on a yearly basis. As the revolution of the planet brings about seasonal variations, the amount of solar energy received on different parts of the Earth’s surface also changes and thus the distribution of insolation. The farthest spot of Earth is 152 million km from the sun, which is called aphelion and falls on January 3 every year. During the aphelion phase, the northern hemisphere, as it faces the sun, accumulates 7% less solar radiation than the southern hemisphere.

• The angle of incidence of the sun’s rays

Since the Earth is a geoid and not a perfect sphere, the incoming sun’s rays strike its surface at varying angles at diverse geographical locations. The angle of incidence at which the ray would strike the surface is determined by the latitude of the area. As the latitude changes, the angle of incidence also changes. Vertical rays from the sun wreath more surface of the planet compared to the slanting ones. As the coverage augments, the energy distribution becomes greater, thereby leading to less insolation in the place. Furthermore, the rays that strike smaller angles travel more of the space to reach the earth than those hitting at bigger angles. 

• Solar Constant

The insolation garnered at the apex of the Earth’s atmosphere is taken as the solar constant. It strikes at the top of the thermopause perpendicularly to the incoming solar beam. Typically the top of the atmospheric layer amasses 1368 W/ m2 in the form of short waves. Therefore, this average distance is expressed as a solar constant that changes over 1 W/m2 by regular disturbances and explosions on the sun’s surface. The solar surface explosions are linked to sunspots which erupt and regularly reduce, presenting a cycle of about 11 years.

• Topographical variations

Earth’s surface is not flat and featureless. Rather it is covered in several kinds of them such as mountains, deserts, valleys, plateaus, waterfalls, canyons, etc. These elements render different elevations and dip to the planet’s exterior and play a role in controlling the amount of insolation received by the surface of the Earth. The south-facing slopes of the northern hemisphere get more warm winds and sunlight, while north-facing slopes are warmer and wetter. 

• Earth’s rotation on its axis

When rotating on its axis, Earth tends to make an angle of 66.5 degrees with its orbital plane about the sun. This factor greatly influences the amount of insolation an area on the earth’s surface receives at varying latitudes. Seasonal variations on the Earth are solely not the result of the planet’s proximity to the sun but the tilting axis of Earth as well.

Conclusion

The concept of insolation in geography and the factors affecting insolation distribution. Insolation refers to the amount of solar energy received by a unit area on the Earth’s surface. It is never uniform throughout the earth’s expanse, rather varies depending on several factors such as earth’s distance from the sun, angle of inclination, length of the day, transparency of the atmosphere, topographical variations, and so forth.