The sun produces energy through nuclear fusion, constantly radiating this energy outward into space. This flow of energy, known as solar radiation, is the primary source that sustains life and drives the climate systems on Earth. Scientists classify this radiation based on its wavelength and energy levels, forming the electromagnetic spectrum. This spectrum includes a vast range of energy, only a small portion of which is visible to the human eye.
The Full Range of Solar Output
The sun emits radiation across the entire electromagnetic spectrum, ranging from extremely short-wavelength, high-energy rays to very long-wavelength, low-energy waves. At one end of the spectrum are gamma rays and X-rays, which possess the shortest wavelengths and the highest photon energy. These forms of radiation are produced by high-temperature processes and energetic events within the sun’s outer atmosphere.
Moving along the spectrum, the energy transitions through ultraviolet (UV) radiation, visible light, and then to infrared (IR) radiation. The longest wavelengths emitted by the sun are microwaves and radio waves, which have the lowest energy. Although the sun generates this complete energy range, high-energy radiation (X-rays and gamma rays) and radio waves constitute only a tiny fraction of the total power output. The vast majority of solar energy is concentrated within the UV, visible, and infrared portions of the spectrum.
How the Atmosphere Filters Radiation
The Earth’s atmosphere acts as a selective filter, absorbing or reflecting much of the sun’s radiation before it can reach the surface. This protective function is accomplished by various gases at different atmospheric layers. High-energy radiation, specifically X-rays and gamma rays, is absorbed almost entirely in the upper atmosphere, known as the thermosphere and mesosphere, primarily by molecules of nitrogen and oxygen.
The atmosphere’s filtering capacity is a function of the radiation’s wavelength, with shorter, higher-energy wavelengths being blocked more efficiently. The ozone layer, located in the stratosphere, is particularly effective at absorbing ultraviolet radiation. This layer prevents all of the sun’s UVC and most of its UVB radiation from penetrating to the ground.
Ultraviolet Radiation and Its Effects
Ultraviolet (UV) radiation is the shortest wavelength portion of the solar spectrum that has significant biological effects on humans and makes up about 8% of the total solar radiation reaching the Earth’s surface. UV radiation is divided into three categories based on wavelength, which determines its energy and its ability to penetrate the skin. The shortest and most energetic is UVC radiation, spanning a wavelength range of 100 to 280 nanometers (nm). UVC is the most damaging type of UV, but it is completely absorbed by the ozone layer and atmospheric oxygen before reaching the ground.
UVB radiation, with wavelengths between 280 and 315 nm, is medium-energy and is the primary cause of sunburn and delayed tanning. Although most solar UVB is filtered by the atmosphere, the amount that reaches the surface is biologically very active and affects the superficial skin layer, the epidermis. This radiation causes direct damage to cellular DNA, which is strongly linked to the development of skin cancer.
The longest wavelength UV is UVA, which ranges from 315 to 400 nm and accounts for approximately 95% of the UV radiation that successfully reaches the Earth’s surface. UVA has a lower energy level than UVB but penetrates deeper into the skin, reaching the dermis layer. This deep penetration is responsible for the immediate tanning effect and is a major contributor to premature skin aging, including wrinkling. While UVA was once thought to be relatively harmless, it is now known to cause indirect DNA damage by generating reactive molecules like free radicals within the skin cells.
Visible Light and Infrared Energy
The radiation that passes through the atmosphere to the Earth’s surface is dominated by visible light and infrared energy, which are both longer-wavelength, lower-energy forms of the spectrum. Visible light occupies the narrow band between approximately 400 and 700 nm. This is the only portion of the electromagnetic spectrum that the human eye can perceive, and it provides the light necessary for vision.
Visible light is also fundamental to plant life, as it is the energy source that powers photosynthesis, the process by which plants convert light energy into chemical energy. Just below the visible range, with wavelengths greater than 700 nm, is infrared (IR) radiation. Infrared energy is primarily perceived as heat and makes up nearly half of the solar radiation that reaches the planet’s surface.
This energy effectively warms the Earth’s surface, which then radiates heat back into the atmosphere. The longer wavelengths of IR radiation are readily absorbed by water vapor and carbon dioxide molecules in the air. This absorption process plays a significant role in regulating the planet’s temperature.