The UV Index measures the strength of ultraviolet (UV) radiation from the sun, typically at solar noon. It ranges from 1 (low) to 11 or higher (extreme), with higher numbers indicating greater potential for skin damage. The UV Index changes throughout the year, influenced by various environmental factors.
How Sunlight Drives Seasonal UV Changes
The primary reason for the annual increase in the UV Index is the changing angle of the sun in the sky. Earth’s tilt on its axis, approximately 23.5 degrees, causes different parts of the planet to receive more direct sunlight at various times of the year. As spring progresses into summer in a given hemisphere, that hemisphere tilts more directly towards the sun.
A more direct sun angle means UV rays travel through less atmosphere. When the sun is lower, rays pass through more atmosphere, absorbing and scattering UV radiation and reducing intensity. A higher sun angle, like midday in summer, results in a shorter atmospheric path, allowing more UV radiation to reach the ground. The UV Index begins to increase in spring, often around the spring equinox, and peaks around the summer solstice, when the sun is highest in the sky.
Other Factors Affecting UV Strength
Beyond the seasonal changes in sun angle, several other environmental factors influence the daily strength of UV radiation. The ozone layer, located in the Earth’s stratosphere, absorbs a large portion of harmful UV radiation, particularly UVB rays. Variations in ozone levels, which occur naturally day to day and throughout the year, can affect how much UV radiation reaches the surface.
Cloud cover also impacts UV strength; heavy clouds block most UV, but thin or broken clouds may allow significant UV to pass. Some clouds can even reflect and scatter UV rays, increasing ground exposure. Surfaces on Earth also reflect or scatter UV radiation, increasing overall exposure. For instance, fresh snow reflects a high amount of UV, while sand and water reflect lesser amounts.
Where UV Index Rises Differently
The timing and intensity of the UV Index increase vary significantly based on geographical location. UV levels are higher closer to the equator year-round because the sun’s rays are more direct in these regions. Consequently, the seasonal “rise” in UV is less pronounced near the equator compared to higher latitudes, where the sun’s angle changes more dramatically throughout the year.
At higher latitudes, seasonal UV intensity varies more, with a greater increase from winter lows to summer highs. Altitude also affects UV intensity; for every 1,000 feet (approximately 300 meters) increase in elevation, UV radiation increases because there is less atmosphere to filter the rays. Mountainous regions can experience higher and earlier increases in the UV Index compared to areas at sea level at the same latitude.