Understanding the UV Index

Why can a skier burn on a freezing, clear day in March? The answer sits in one number on your weather app. A UV index of 10 means 250 milliwatts of skin-weighted ultraviolet power reach every square meter of ground. This guide explains what that number measures, how scientists build it, and how to read a forecast with confidence.

What Is Ultraviolet Radiation?

Ultraviolet radiation is invisible sunlight with wavelengths from 100 to 400 nanometers, just shorter than the violet light you can see. The WHO splits it into three bands, and each band behaves differently in air and in skin.

UVC covers 100 to 280 nanometers, and the atmosphere filters it out completely. UVB spans 280 to 315 nanometers, and ozone absorbs most of it on the way down. UVA runs from 315 to 400 nanometers and passes through the air with little loss.

That filtering creates a surprise. UVA makes up about 95 percent of the UV reaching the ground, yet the small UVB share drives most sunburn. UVA reaches deeper skin layers and adds to aging, and both bands raise skin cancer risk.

UVB is not purely harmful, since it also starts vitamin D production in skin. Ozone controls how much UVB gets through, which is why every UV forecast begins with an ozone estimate.

How Is the UV Index Calculated?

The UV index is the sun’s skin-weighted UV power at ground level, in milliwatts per square meter, divided by 25 and rounded. Each index point therefore stands for 25 milliwatts of weighted UV.

The weighting comes from the McKinlay-Diffey erythema action spectrum, a curve showing how strongly each wavelength reddens skin. UVB at 298 nanometers gets a weight of 1.0, while UVA at 340 nanometers gets only 0.001.

That 1,000-fold gap explains the surprise above. UVA dominates the raw light, but weighted UVB dominates the final number. The EPA method sums the weighted values across 280 to 400 nanometers.

Here is the math in one line. A weighted total of 200 milliwatts per square meter, divided by 25, gives an index of 8. Some texts multiply watts by 40 instead, and 0.2 times 40 also equals 8.

The WHO, the World Meteorological Organization, and partner agencies share this standard. An index of 8 therefore means the same dose rate in Denver, Sydney, or Madrid.

What Do the UV Index Categories Mean?

The UV index sorts readings into five color-coded categories, from low at 0 to 2 up to extreme at 11 or more. Each step up means unprotected skin burns sooner.

The EPA folds the five bands into three advice steps: minimal, needed, and extra protection. The WHO also treats 3 as the point where protection starts to matter.

The scale has no fixed top. NOAA notes that readings reach 15 or 16 in the tropics at high elevation under clear skies.

The scale gives the dose rate, but your skin type sets how fast you burn. To turn a reading into minutes, estimate your sunburn time with the UV Index Calculator, which also lists skin types and protection steps.

Why Does the UV Index Change Through the Day and Year?

The UV index tracks the sun’s height, so it peaks at solar noon and in summer, when sunlight crosses the least atmosphere. A low sun sends its rays through a long, slanted path of air and ozone that absorbs more UVB.

Solar noon is rarely 12:00 on the clock. During daylight saving time, it often falls closer to 1 p.m., so the true peak arrives later than many people expect.

Season and latitude follow the same rule. Near the equator the midday sun stays high all year, while a winter sun far from it stays low. Our guide to how sunrise, sunset, and daylight hours change through the year explains why the sun rides higher in summer.

The EPA offers a quick field check called the shadow rule. When your shadow is shorter than you are, the sun sits more than 45 degrees high and UV exposure is higher.

Latitude turns that rule into a calendar. At 40 degrees north, the noon sun climbs to about 73 degrees in late June but only about 27 degrees in late December. Your midwinter shadow there stays longer than you all day, which matches the low winter readings.

How Do Altitude, Clouds, and Reflection Change UV?

Altitude raises the UV index, clouds usually lower it, and bright ground such as snow and sand bounces extra UV back toward you.

Altitude Adds UV

The WHO puts the rise at about 10 percent for every 1,000 meters of elevation. The EPA forecast model uses a gentler 6 percent per kilometer. A hiker at 3,000 meters therefore gets roughly 18 to 30 percent more UV than a swimmer at sea level.

Clouds Subtract Less Than You Think

The EPA method passes 89 percent through scattered cloud, 73 percent through broken cloud, and 31 percent through overcast. A clear-sky 10 drops only to 7 under broken cloud and still reaches 3 under overcast.

Bright Surfaces Reflect It Back

Grass, soil, and water reflect less than 10 percent of UV, dry sand about 15 percent, and sea foam about 25 percent. Fresh snow reflects up to 80 percent and almost doubles a person’s exposure, according to the WHO.

How Do You Read a UV Index Forecast?

A daily UV forecast predicts the reading at solar noon, so treat it as the day’s peak rather than an all-day value. NOAA issues it as a next-day forecast for the moment the sun stands highest.

The model starts with sun angle and forecast stratospheric ozone, measured by NOAA satellites. It then adjusts for expected cloud and for elevation, using the factors above.

Read the number with the clock in mind. The WHO notes that summer UV runs highest in the 4 hours around solar noon, and hourly forecasts show that curve rising and falling.

Then adjust for your setting. A forecast for the nearest town does not include your summit or the snow at your feet, so push your estimate upward for both. The EPA also issues a special UV Alert for an area when its forecast runs higher than normal.

FAQs About the UV Index

Sources

This article explains UV science for general education, drawing on WHO, EPA, and NOAA guidance. Sunburn times and protection steps live on the calculator page. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated September 26, 2026.