Solar Energy
Solar radiation does not reach the Earth's surface exactly as it is emitted by the Sun. During its journey towards the Earth and its passage through the atmosphere, solar energy experiences various physical phenomena that modify its intensity, direction and spectral composition.
These processes determine the amount of energy available in each place on the planet and are responsible for everyday phenomena such as the blue color of the sky, the formation of shadows or the differences in insolation between different regions.
The journey of solar radiation to Earth
The Sun emits energy in the form of electromagnetic radiation over a wide range of wavelengths, from ultraviolet rays to infrared radiation.
At the upper limit of the Earth's atmosphere, the average solar irradiance is approximately 1,361 W/m², a value known as the solar constant. However, not all of this energy reaches the Earth's surface due to interaction with the atmosphere.
As the sun's rays pass through the different atmospheric layers, some of the energy is reflected, absorbed or dispersed by atmospheric gases, clouds and suspended particles.
The atmosphere as a natural filter
The Earth's atmosphere acts as a filter that regulates the amount and type of radiation that reaches the ground.
This filtering is essential for life, as it removes much of the most energetic and potentially harmful radiation from the Sun.
The main processes that affect the propagation of solar radiation are:
- Reflection.
- Absorption.
- Dispersion.
The combination of these phenomena determines the Earth's energy balance and the radiation finally available on the surface.
Solar radiation reflection
Reflection occurs when a portion of the incident radiation bounces off a surface without being absorbed.
This phenomenon takes place both in the atmosphere and on the earth's surface. Clouds are one of the elements that reflect the most radiation back into space, although atmospheric aerosols, snow, ice and certain light surfaces also contribute to this.
The ability of a surface to reflect solar radiation is expressed by albedo.
The Earth has an average albedo close to 30%, which means that approximately a third of the solar energy received is returned to space.
Absorption of solar radiation
Part of the sun's energy is absorbed by atmospheric gases and by the earth's surface.
Absorption depends on the wavelength of the radiation and the properties of the materials that receive it.
Some important examples are:
- The ozone layer absorbs much of the ultraviolet radiation.
- Water vapor absorbs certain infrared bands.
- Carbon dioxide and other greenhouse gases absorb thermal radiation.
The absorbed energy is mainly transformed into heat, contributing to the warming of the atmosphere, oceans and continents.
Scattering of solar radiation
Not all sunlight that enters the atmosphere reaches the Earth's surface directly. During its journey, a part of the radiation interacts with air molecules, water droplets and suspended particles present in the atmosphere. When this happens, the sun's rays change direction and the light is distributed in multiple directions, a phenomenon known as scattering.
Although it goes unnoticed in our everyday lives, dispersion is responsible for some of the most well-known natural spectacles. Thanks to it, the sky acquires its characteristic blue color during the day and sunrises and sunsets are tinged with reddish and orange tones. It also explains why natural lighting exists even in places that do not receive direct sunlight.
One of the most important mechanisms is Rayleigh scattering, which occurs when light interacts with molecules in the air. This phenomenon affects the shorter wavelengths, corresponding to the blue and violet colours of the visible spectrum, with greater intensity. As our eyes are more sensitive to blue and some of the violet light is absorbed by the atmosphere, the result is the blue color we observe when looking at the sky on a clear day.
Direct radiation and diffuse radiation
As a result of atmospheric processes, the radiation that reaches the Earth's surface can be classified into two main components.
Direct radiation
It is the radiation that arrives from the solar disk without having undergone significant changes in direction.
It represents the most concentrated fraction of energy and generates well-defined shadows on clear days.
Diffuse radiation
It is the radiation that has been scattered by the atmosphere before reaching the ground.
Although it comes from multiple directions, it still provides a significant amount of energy, especially on cloudy days or with a high concentration of atmospheric particles.
Global radiation
The sum of direct radiation and diffuse radiation is called global radiation.
This is the total radiation available on a surface exposed to the Sun.
Influence of the angle of incidence
The amount of solar energy received also depends on the angle at which the sun's rays reach a surface.
When radiation strikes perpendicularly, the energy is concentrated on a smaller surface and the intensity is maximum.
On the contrary, when lightning strikes at a high inclination, the same energy is distributed over a larger surface and must also pass through a greater amount of atmosphere.
This phenomenon explains many aspects of the behaviour of solar radiation on Earth. For example
- The alternation between day and night occurs because the Earth's rotation continuously modifies the angle at which the sun's rays reach each point on the planet.
- The seasons of the year are a consequence of the inclination of the earth's axis, which causes the same region to receive the sun's rays more or less perpendicular depending on the time of year.
- The climatic differences between latitudes are due to the fact that areas near the equator receive more direct and concentrated radiation than the polar regions.
- The daily variation of solar radiation.
Factors that affect the propagation of solar radiation
The amount of radiation that eventually reaches the Earth's surface depends on numerous factors:
- Latitude.
- The time of year.
- The time of day.
- The altitude.
- Cloud cover.
- Atmospheric humidity.
- The concentration of aerosols and particles.
- Air pollution.
For this reason, two places located at the same latitude can receive very different amounts of solar radiation.