8.1. SOLAR RADIATION

The earth’s surface receives most of its energy in short wavelengths. The energy received by the earth is known as incoming solar radiation which in short is termed as insolation.

Solar radiation, or insolation, is the energy that the earth receives from the sun. This energy arrives primarily in short wavelengths which are essential for life and contribute to the warmth of the atmosphere and the earth's surface.

Think of insolation as sunlight pouring through a window on a sunny day, warming the room. Just like how the sun’s rays can heat up a space, solar radiation heats up the earth.

As the earth is a geoid resembling a sphere, the sun’s rays fall obliquely at the top of the atmosphere and the earth intercepts a very small portion of the sun’s energy. On average, the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere.

Due to the spherical shape of the earth, solar rays hit the atmosphere at an angle, meaning not all of the sun's energy reaches the earth directly. Only a fraction, specifically around 1.94 calories per square centimeter per minute, makes it through to the earth’s surface.

Imagine a flashlight shining at an angle on a wall; the light is concentrated in one area and spreads out the farther it goes. Similarly, solar energy becomes less intense the more slanted it is as it hits the earth.

During its revolution around the sun, the earth is farthest from the sun (152 million km) on 4th July and nearest to the sun (147 million km) on 3rd January.

The distance between the earth and the sun changes throughout the year due to the earth's elliptical orbit. On July 4th, the earth is at its farthest point (aphelion), whereas on January 3rd, it is at its closest point (perihelion). This variation slightly affects the amount of solar energy the earth receives.

Think of a car moving around a track. Sometimes it’s closer to the center (like the sun in January) and sometimes farther away (like July). The further it goes, the less 'energy' it receives from the center.

The amount and intensity of insolation vary during a day, in a season, and in a year. The factors that cause these variations in insolation are: (i) the rotation of earth on its axis; (ii) the angle of inclination of the sun’s rays; (iii) the length of the day; (iv) the transparency of the atmosphere; (v) the configuration of land in terms of its aspect.

Several factors influence how much solar energy reaches the earth, including the rotation of the earth which affects day length, the angle at which sunlight strikes, and atmospheric conditions which can block or allow sunlight to pass. These factors interact to cause variations in insolation throughout the day and year.

Consider how the angle of sunlight changes as the sun moves across the sky. In the morning and late afternoon, the sunlight is more slanted, making the light less intense, similar to how a light dimmer adjusts brightness.

The atmosphere is largely transparent to short wave solar radiation. The incoming solar radiation passes through the atmosphere before striking the earth’s surface. Within the troposphere, water vapor, ozone, and other gases absorb much of the near-infrared radiation.

Most of solar radiation can pass through the atmosphere without obstruction. However, certain gases in the troposphere absorb some energy, particularly in the near-infrared range, which contributes to the heating of the atmosphere. This means some energy doesn’t directly reach the earth’s surface.

Imagine wearing sunglasses that let some light through but block others. The atmosphere acts like these sunglasses, allowing certain wavelengths of sunlight to pass while absorbing others.

The insolation received at the surface varies from about 320 Watts/m² in the tropics to about 70 Watts/m² in the poles. Maximum insolation is received over the subtropical deserts, where the cloudiness is the least.

Insolation isn’t evenly distributed globally; it depends on location. The tropics receive significantly more energy than polar regions. Areas like subtropical deserts generally collect the most solar energy because they have fewer clouds obstructing sunlight.

Think about how a sunny spot in the yard is warmer than a shaded area. Similarly, the tropics are like the sunny spot where solar energy is strongest compared to the 'shade' of the poles.

--