Data & Methodology

This page explains exactly where every number on UV Index Checker comes from, how the calculations work, and — just as important — what the numbers can't tell you. If you spot something that looks wrong, email [email protected].

What the UV Index Is

The UV index is an international standard developed by the World Health Organization, the World Meteorological Organization, and partner agencies. It measures the strength of skin-damaging ultraviolet radiation at the Earth's surface on an open-ended scale where higher means stronger:

The EPA publishes the same scale for the United States.

Where Our Live UV Values Come From

All current UV readings and forecasts on this site come from the OpenUV API, a UV data service whose model is driven by satellite-derived inputs — primarily real-time ozone column data (from instruments like OMI on NASA's Aura satellite) combined with the sun's angle for your exact coordinates and time. This is the same class of source data behind official UV forecasts.

When you search a city or open a city page:

• We request the current UV value, today's maximum and its time, hourly forecasts for today and tomorrow, sun times, and per-skin-type safe exposure times for that location's exact latitude and longitude.
• Responses are cached on our server for up to 30 minutes per location. The "Updated X min ago" label in the tool shows the actual age of the data you're seeing.
• City pages use each city's fixed coordinates (city center), so a reading may differ slightly from your exact neighborhood.

How Burn Times Are Estimated

The "Safe Exposure Time" section estimates how long unprotected skin can be in the sun before burning begins. It uses the Fitzpatrick skin phototype scale (types I–VI), the standard dermatological classification of how skin responds to UV.

• Where available, we use OpenUV's per-skin-type safe exposure times, which are computed from the live UV level.
• We rescale those figures to today's maximum UV so the number is useful for planning your day, not just the current minute.
• SPF figures shown are simple multiples (SPF 15 ≈ 15× longer, SPF 30 ≈ 30× longer) and assume proper, generous application with reapplication every two hours — real-world protection is usually lower.

Surface Reflection Adjustments

UV reaches your skin both directly and reflected off surfaces. The surface selector in the tool applies multipliers consistent with figures published by the WHO:

These are approximations — actual reflection varies with the angle of the sun, water conditions, snow age, and more.

City Page Climate Data

The seasonal UV ranges, average UV figures, and sunny-day counts on city pages are editorial estimates compiled from historical climate patterns for each city — latitude, elevation, and typical cloud cover. They describe what's typical, not what's happening right now; the live tool at the top of every page is always the current source of truth.

Known Limitations

• Forecasts are clear-sky-leaning. Sudden cloud cover can lower actual UV below forecast; thin haze blocks far less UV than it appears to.
• Coordinates are city-center. Elevation differences within a metro area (foothills, high suburbs) can raise UV several percent above the displayed value.
• Burn times are population estimates. Medications, skin conditions, and individual variation can dramatically shorten them.
• Cache lag. A displayed value can be up to 30 minutes old; the freshness label tells you exactly.

Not Medical Advice

UV Index Checker is an informational data tool, not a medical service. Nothing on this site is a substitute for professional medical advice. If you have a history of skin cancer, photosensitivity, or take medications that increase sun sensitivity, consult a healthcare provider about your personal sun exposure limits. For evidence-based prevention guidance, see the Skin Cancer Foundation and the EPA SunSafety program.

Curious who runs this site? See the About page.

Last reviewed: July 2026.