The UV index at any given moment is shaped by a combination of factors, from the sun’s position in the sky to the ozone overhead to the ground beneath your feet. Some of these factors change by the hour, others by the season, and a few are fixed by where you live. Understanding them helps you predict when UV exposure will be highest and when it catches people off guard.
How the Sun’s Angle Drives UV Intensity
The single biggest factor is the angle at which sunlight strikes the atmosphere. When the sun is directly overhead, UV rays take the shortest possible path through the atmosphere, so less radiation gets absorbed or scattered before reaching you. When the sun is low on the horizon, those same rays travel through a much thicker slice of atmosphere, and more UV gets filtered out along the way.
Three things determine this angle: your latitude, the time of year, and the time of day. Near the equator, the sun sits high in the sky year-round, which is why tropical regions consistently see UV index values in the “very high” or “extreme” range. At higher latitudes, the sun never climbs as high, especially in winter, and the UV index drops accordingly. Even on a single day, UV intensity roughly doubles between mid-morning and solar noon because of the changing angle. The EPA’s UV index forecast model uses exactly these inputs (latitude, date, and time of day) to calculate how directly sunlight hits each location.
Season and Time of Year
Earth’s axial tilt means the sun’s peak height shifts with the seasons. In summer, the sun climbs higher and its rays pass through less atmosphere, producing stronger UV at the surface. In winter, the opposite happens: a lower sun angle means a longer atmospheric path and more UV absorption before the light reaches you. This seasonal swing is dramatic at mid-latitudes. A city like Denver might see a peak UV index of 11 or 12 in late June but only 2 or 3 in December. Closer to the equator, the seasonal difference shrinks considerably.
The Ozone Layer’s Filtering Effect
Ozone molecules in the stratosphere absorb UV-B radiation, the wavelengths most responsible for sunburn and skin damage. The relationship is straightforward: less overhead ozone means proportionally more UV-B reaching the ground. This is why the Antarctic ozone hole, which thins the ozone layer dramatically each spring, produces dangerously high UV levels in southern regions like Patagonia and New Zealand during that period.
Ozone thickness varies naturally by latitude and season, but it also fluctuates with weather patterns. The EPA’s forecast model combines predicted ozone levels with the sun’s angle to generate each day’s UV index. Even modest changes in ozone, say 10 to 15%, produce measurable shifts in UV-B at the surface.
Altitude and Elevation
For every 1,000 meters (about 3,280 feet) of elevation gain, UV radiation increases by roughly 12%. At higher altitudes, there’s simply less atmosphere above you to absorb and scatter UV. This is why mountain environments are deceptively dangerous for sun exposure. A ski resort at 3,000 meters receives about 36% more UV than a sea-level beach on the same day, all else being equal. Combined with snow reflection (more on that below), high-altitude locations can deliver intense UV exposure even when temperatures feel cool.
Cloud Cover
Clouds reduce UV, but far less than most people assume. Up to 80% of solar UV radiation penetrates through light cloud cover. Thin, wispy clouds or scattered cumulus barely make a dent. Only thick, dark overcast skies provide substantial UV reduction, and even then, some UV gets through. This mismatch between how cool and shaded it feels versus how much UV is actually hitting your skin is one of the most common reasons people get sunburned on cloudy days.
Certain cloud formations can actually increase UV for brief moments. When the sun peeks through a gap in the clouds, the surrounding cloud edges scatter additional UV toward the ground, temporarily pushing levels above what a clear sky would produce.
Air Pollution and Aerosols
Particles suspended in the atmosphere, from wildfire smoke to industrial pollution, interact with UV radiation in two ways: they scatter it and they absorb it. During heavy pollution events, this effect is substantial. Research measuring UV levels across northern China during a dense haze episode found that aerosols reduced surface UV radiation by an average of 22.2%.
The type of particle matters. Soot (black carbon) absorbs UV across a broad range of wavelengths, while organic compounds from biomass burning absorb shorter wavelengths especially strongly. In heavily polluted cities, the UV index can read noticeably lower than a clear-sky model would predict. However, this is not a reliable form of “protection.” Pollution levels fluctuate, and the health costs of breathing polluted air far outweigh any UV reduction benefit.
Surface Reflection
Most people think about UV as coming from above, but reflected UV from the ground adds meaningfully to total exposure. Different surfaces bounce back very different amounts of UV radiation:
- Snow and ice reflect 50 to 90% of incoming UV, effectively hitting you from below as well as above. This is why snow blindness and severe sunburns are common at ski resorts.
- Sand reflects up to 40% of UV, which is why beach sunburns can be fierce even under an umbrella.
- Water reflects a moderate amount and also transmits UV below the surface, so you can burn while swimming.
- Grass and soil reflect very little, typically under 10%.
Reflected UV is particularly sneaky because it reaches skin that’s angled away from the sun: the underside of your chin, under your nose, and beneath the brim of a hat. On fresh snow at altitude, the combination of high elevation, strong reflection, and direct sunlight can push effective UV exposure to extreme levels.
How These Factors Combine in the UV Index
The UV index isn’t a raw measurement of UV energy. It’s a weighted scale designed around human skin sensitivity. Shorter UV wavelengths cause sunburn far more efficiently than longer ones, so the index weights each wavelength according to how effectively it damages skin. This weighting spans four orders of magnitude between different UV wavelengths, meaning the index is tuned specifically to predict sunburn risk rather than total UV energy.
The World Health Organization groups UV index values into five risk categories:
- Low: below 2
- Moderate: 3 to 5
- High: 6 to 7
- Very high: 8 to 10
- Extreme: 11 and above
On a clear summer day near the equator at moderate altitude, multiple factors stack: a high sun angle, thin atmospheric path, strong surface reflection, and sometimes reduced ozone. That’s how UV index readings of 14 or higher occur in places like the Bolivian altiplano. Conversely, a winter afternoon at a northern latitude under heavy cloud cover with thick ozone and low sun might produce a UV index below 1. Every factor listed above is feeding into the number you see on your weather app, and understanding them helps explain why two sunny days at different times, places, or elevations can feel similar but carry very different UV risk.