Solar Radiation: Types and Atmospheric Absorption

Posted on Mar 3, 2025 in Physics

Solar Radiation

Solar radiation is the main source of energy received by the atmosphere. The radiation from other celestial bodies is very small. Light and heat are forms of energy transmitted by electromagnetic waves that propagate through space. The length of these waves is so small they are measured in microns. Of all the radiation from the sun, only the part called the visible range is perceived by the human eye. Electromagnetic energy is transmitted in wave form, and radiation is precisely the emission of these waves.

The main source of energy reaching the Earth is the Sun, which is an immense mass of gas and fire. The Earth rotates around its axis, completing a rotation in 24 hours, and around the Sun in 365 days at a speed of 100,000 Km/H.

  • When the duration of the night is equal to the day all over the Earth, it is called an Equinox and occurs on March 21st and September 22nd.
  • When the Sun reaches its greatest declination, inversely illuminating both hemispheres, it is called a Solstice and happens on June 21st and December 21st.

Solar Radiation and Visible Radiation

The spectrum of sunlight is divided into three types of light:

  • Ultraviolet light: It is not visible to humans and represents 8% of the spectrum, with short wavelengths (λ < 400nm).
  • Visible light: Visible to the human eye, it consists of red, orange, yellow, green, blue, and violet light, representing 56% of the spectrum, with wavelengths between 400 nm and 800 nm.
  • Infrared light: It is not visible to human eyes and consists of long, invisible, caloric waves, accounting for 36% of the spectrum, with wavelengths between 800nm and 400μm.

Absorption of Solar Radiation by the Atmosphere

The most important atmospheric absorbers of solar radiation are:

  • Oxygen (O2): Absorbs ultraviolet radiation less than or equal to 200 μm (0.2 μm), unfolding into atomic oxygen (O), giving rise to ozone (O3).
  • Ozone (O3): Absorbs ultraviolet radiation between the wavelengths 220 nm (0.22 μm) – 290 nm (0.29 μm). Both O2 and O3 are solely responsible for the absorption of ultraviolet radiation and absorb almost 100% of ultraviolet radiation less than 0.29 μm. For this reason, only a small fraction of UV radiation reaches the lower portion of the atmosphere.
  • Water vapor: Absorbs infrared radiation at wavelengths below 800 nm (0.8 μm) – 2000 nm (2.0 μm) and 2300 nm (2.3 μm) – 3600 nm (3.6 μm). Water vapor is the main absorber of radiation.
  • Carbon dioxide (CO2): Absorbs infrared solar radiation between wavelengths of 2400 nm (2.4 μm) – 4600 nm (4.6 μm).

Variation of Solar Radiation

The amount of solar radiation varies with:

  • The Solar Constant (S): The amount of energy reaching the upper limit of the atmosphere (exosphere). Its average value is 2 Cal/cm2/min. Observations indicate that solar radiation does not vary significantly. It is defined as the solar constant, which is the amount of radiation reaching the upper limit of the atmosphere.
  • The transparency of the atmosphere: Areas with greater cloudiness and air pollution receive less direct radiation.
  • The daily duration of sunlight: Varies with latitude and seasons.
  • The effect of varying the angle of incidence: At noon, the Sun’s intensity is greater than in the mornings and afternoons. Slanting rays cover a larger area than vertical rays, thus producing less heat.

Forms of Energy Transfer

  • Radiation: A body at a certain temperature transmits energy in the form of electromagnetic waves, which varies according to the absolute temperature of the body.
  • Black Body: A body that absorbs all the energy that reaches it and emits all of its energy. It is also known as an ideal body. The amount of energy radiating from a black body is given by the Stefan-Boltzmann law.
  • Conduction: It is a function of the temperature gradient (dT/dS). The relationship between the amount of heat transferred per unit area of cross-section per unit time…