The sun emits electromagnetic radiation, including invisible ultraviolet (UV) radiation, which is just beyond visible violet light. While the sun is the primary UV source, not all of this radiation reaches Earth’s surface. The amount that penetrates our atmosphere varies due to natural processes.
Types of Ultraviolet Radiation
Ultraviolet radiation is categorized into three main types based on its wavelength: UVA, UVB, and UVC. UVA radiation has the longest wavelengths, ranging from 315 to 400 nanometers (nm). The majority of UVA radiation, approximately 95%, successfully penetrates Earth’s atmosphere and reaches the surface.
UVB radiation has medium wavelengths (280-315 nm). Only about 5% of UVB reaches the Earth’s surface; its atmospheric penetration depends heavily on atmospheric conditions. UVC radiation has the shortest and most energetic wavelengths (100-280 nm). Almost all UVC is absorbed by Earth’s atmosphere and does not reach the surface.
Earth’s Atmospheric Shield
Earth’s atmosphere acts as a natural shield, absorbing and scattering incoming solar radiation, including UV. The ozone layer, primarily in the stratosphere (10-50 km above Earth’s surface), plays a significant role. Ozone molecules (O3) efficiently absorb UVC radiation, preventing it from reaching the ground.
The ozone layer’s absorption of UVC is nearly complete, making it virtually harmless at the surface. It also absorbs a substantial portion of UVB, significantly reducing its intensity. Without the ozone layer, harmful UVB reaching the surface would be considerably higher.
Other atmospheric components also contribute to UV attenuation. Oxygen (O2) and nitrogen (N2) molecules absorb some short-wavelength UVC, though less effectively than ozone. Aerosols, clouds, and other particulate matter in the troposphere can also scatter or absorb UV. This scattering can redirect UV, causing it to reach the surface from different angles.
Factors Influencing Surface UV Levels
The amount of UV radiation reaching Earth’s surface varies due to several factors. The sun’s angle is a primary determinant; UV intensity is highest when the sun is directly overhead, around midday. This is because sun’s rays travel through the least atmosphere at this angle. UV levels are lower in the early morning and late afternoon as rays pass through greater atmospheric thickness.
Seasonal changes also affect the sun’s angle and surface UV levels. UV radiation is strongest during summer months when the sun is higher in the sky. Altitude also plays a role, with UV intensity increasing by approximately 10% for every 1,000 meters (3,280 feet) of elevation gain. This is due to the thinner atmosphere at higher altitudes, which provides less absorption and scattering of UV rays.
Cloud cover can significantly reduce surface UV levels, but its effect is variable. Thick, dark clouds block most UV, while thin or scattered clouds may have little impact or even enhance exposure through reflection and scattering. Ground reflection is another factor; surfaces like fresh snow can reflect up to 80% of UV, while sand reflects about 15% and water about 10%. Localized ozone depletion, often due to human-made chemicals, can also lead to temporary increases in surface UV in specific areas.
Measuring and Understanding UV Exposure
To help the public understand daily UV intensity, scientists developed the UV Index. This international standard scale indicates the strength of solar UV radiation at a particular place and time. The index is a simple numerical value, ranging from 0 (low) to 11+ (extreme).
A higher UV Index signifies greater risk from sun exposure and indicates a need for increased protective measures. For instance, an index of 0-2 suggests low risk, while 8-10 indicates very high risk. The UV Index provides actionable information, allowing individuals to make informed decisions about spending time outdoors and applying sun protection.