Insolation

1. Introduction

The Sun is the source of almost all the energy received by the earth. Next, the earth radiates the energy received from the Sun into space. The amount of heat absorbed in various parts of the world is different, resulting in atmospheric pressure differences. It allows the heat to be transferred from one part of the planet to another by the wind.

Solar Radiation:

1. Most of the energy received by the earth’s surface is in short wavelengths. 
2. This energy is known as incoming solar radiation, which the earth receives. In short, it is termed insolation. 
3. As the earth’s shape is a geoid that resembles a sphere, the sun’s rays fall obliquely at the top of the atmosphere, and the world intercepts a tiny portion of the sun’s energy. 
4. On average, the earth receives 1.94 calories per sq. cm per minute at the top of its Atmosphere.  
5. The solar output varies slightly in a year due to the distance between the earth and the Sun. 
6. Aphelion: It is the earth’s position during its revolution around the Sun (152 million km) on 4th July. 
7. Perihelion: The earth is closest to the Sun on January 3rd (147 million km). On the 3rd of January, the planet receives somewhat more yearly insolation than on the 4th of July.
8. Other factors, such as the distribution of land and sea in the world and air circulation, obscure the effect of solar output change. 
9. This variation in the solar output does not significantly affect daily weather.

Variation of Insolation at the Earth’s Surface: 

1. The amount and intensity of solar radiation vary by day, season, and year. 
2. Various factors causing variations in Insolation:
   1. The rotation of the earth on its axis. 
   2. The angle of inclination of the Sun’s rays. 
   3. The length of the day. 
   4. The transparency of the atmosphere.
   5. The configuration of the earth’s land in terms of its aspect. 
3. The earth’s axis makes an angle of 66½ degrees with the plane of its orbit around the Sun, which has a more significant influence on the amount of insolation received at different latitudes.
4. Another factor determining the amount of insolation received by the earth is the angle of inclination of the sun’s rays that depends on the latitude of a particular place.
5. The higher the latitude of a place, the less is the angle made with the earth’s surface that results in slant sun rays. The area covered by vertical beams is always less than the slant rays. 
6. If more area is covered, the energy gets distributed, and the net energy received per unit area decreases.
7. The slant rays pass through a greater depth of the Atmosphere, which results in more absorption, diffusion, and scattering.

Passage of the solar radiation through the atmosphere:

1. The earth’s atmosphere is highly transparent to shortwave solar radiation, also known as incoming solar radiation.
2. Ozone gas, water vapor, and other gasses absorb much of the near-infrared radiation within the troposphere.
3. The very small-suspended particles in the troposphere scatter the visible spectrum both to space and towards the earth’s surface, thus, adding color to the sky. 
4. The red color of the rising and setting sun and the blue color of the sky result from light scattering in the atmosphere.

Spatial distribution of insolation at the earth’s surface:

1. At the surface, insolation ranges from around 320 Watt/m2 in the tropics to about 70 Watt/m2 at the poles.
2. The subtropical deserts are those parts of the planet that receive maximum insolation because the cloudiness is the least. 
3. Less insolation is received by the Equator comparatively than in the tropics. 
4. At the same latitude, the insolation adds to the continent more than oceans. 
5. The middle and higher latitudes receive less radiation in the winter season than in the summer.