The Sun emits electromagnetic radiation across a broad range of wavelengths (λ). High-energy X-rays and gamma (γ) rays—known as ionizing radiation—would make life on Earth impossible. Fortunately, they are prevented from reaching the Earth's surface by the atmosphere and the Earth's magnetic field. As a result, only non-ionizing radiation, particularly wavelengths longer than 280 nanometres (nm), reaches the ground.
The figure below illustrates the composition of the solar spectrum at sea level.
Figure. Ultraviolet (UV) radiation accounts for approximately 10% of the solar spectrum and is divided into UVB (280–320 nm) and UVA (320–400 nm). Visible light (400–760 nm), ranging from violet to red, represents about 40%, while infrared (IR; >760 nm) radiation, perceived as solar heat, makes up the remaining 50%. Beyond infrared lie the far-infrared region and radio waves. UV photons carry more energy than visible-light photons.
The intensity and relative proportions of the different components of sunlight vary according to:
- Latitude
- Season
- Time of day
- Weather conditions
Equatorial, tropical and subtropical regions receive the highest levels of solar radiation, particularly UV radiation. In temperate regions, especially beyond 40° north or south latitude, solar irradiance varies markedly with the seasons, with the greatest fluctuations occurring in the UV portion of the spectrum and, at very high latitudes, also affecting visible light.
Regardless of latitude, UV irradiance reaches its maximum between 11:00 a.m. and 3:00 p.m.
On days with thin cloud cover, most UVA and visible light continue to reach the Earth's surface, whereas UVB is substantially reduced and may become almost undetectable. During rainy days, UV exposure is generally negligible.