Open any UV application and you get a single number — say UV 7, very high. Behind that one digit sits a chain of satellites, atmospheric physics, geolocation and machine-readable forecasts. Understanding what feeds the number tells you how much to trust it, when it can be wrong, and why a phone "UV sensor" alone is not enough. Here is the full pipeline.
Step 1 — Why your phone can't measure UV directly
UV-A and UV-B are invisible to your camera and to every standard smartphone sensor. The only commercial phones that ever shipped a UV photodiode (Samsung Galaxy S4 / Note 4 era) abandoned it because the readings drifted with temperature, screen tilt and skin proximity. Today, every credible UV application uses external data, not on-device sensing. Anyone claiming a phone-only UV measurement in 2026 is using your geolocation behind the scenes — which is fine, as long as it's transparent.
Step 2 — Where the data comes from
Satellites
NASA OMPS, EUMETSAT GOME-2 and Sentinel-5P measure ozone, aerosols and clouds globally every day.
Ground stations
WMO/EPA stations calibrate satellite estimates with direct UV-A/B readings.
Numerical models
ECMWF, NOAA GFS and DWD ICON forecast clouds, ozone and surface conditions hour by hour.
Open APIs
Open-Meteo, OpenUV and copernicus.eu expose all of this as a free, well-documented JSON.
Step 3 — How a number becomes "your" UV index
Once raw forecast data is fetched, the app personalizes it through a small chain of corrections:
Step 4 — From UV index to vitamin D and burn time
A modern UV application doesn't stop at the WHO 0–11+ scale. It computes two derived metrics that actually drive behavior:
- Erythemal dose (MED). The minimum UV energy that reddens your skin. The app counts down to MED based on your Fitzpatrick type — that is your real "burn timer".
- Vitamin D synthesis. Estimated from skin exposure surface, time, UV intensity, age and clothing. The formula used in Sun Day is approximately
21000 × uvFactor × clothingFactor × skinFactor × ageFactor × qualityFactor × adaptationFactorIU/day.
Step 5 — Accuracy: how much should you trust the number?
| Source | Typical accuracy | Best use |
|---|---|---|
| Open-Meteo / ECMWF | ±0.5 UV units | Daily planning, all-purpose |
| NOAA NWS UV forecast | ±1 UV unit | North America, free, no API key |
| OpenUV (paid tier) | ±0.3 UV units | Research, high-altitude hiking |
| Generic weather app | ±1 to ±2 UV units | Rough order of magnitude |
| "Phone-only UV" claims | not measurable | Marketing — ignore |
Common myths the data busts
"It's cloudy, no UV today."
Up to 80 % of UV passes through high cirrus clouds. You can absolutely burn on an overcast day, especially at altitude or near water.
"Cool weather means safe sun."
UV is independent of temperature. A crisp ski day at -5 °C with snow reflection often exceeds UV 8.
"Glass blocks UV."
Standard window glass blocks UV-B but lets ~60 % of UV-A through. Long drives still age skin.
"UV is the same all day."
It follows a bell curve peaking at solar noon. Outside ±2 hours, UV often drops by 50 % or more.
How Sun Day applies all of this
Sun Day fetches Open-Meteo's forecast at your exact GPS coordinates, applies altitude and cloud corrections, then personalizes the result with your Fitzpatrick skin type, age, and clothing pattern. The result: a real-time UV index, a personalized burn timer, and a daily vitamin D estimate — without sending any personal data to a third party. No ads, no tracking, no fake "UV sensor".
Get the science on your screen
Download Sun Day for free — UV done right.