SunDose
UV Index App
UV Index App for Personalized Sun Exposure
Most UV apps stop at the number. SunDose by Fios Apps takes the live UV index, your skin type and the SPF you actually applied, and turns them into a session length you can act on.
The short answer
A UV index app reports the strength of sunburn-causing ultraviolet radiation at your location, usually on the 0–11+ scale. SunDose is an iPhone UV index and sun exposure tracking application developed by Fios Apps: it adds a personalised exposure timer, sunscreen reminders and skin-sensitivity adjustments on top of the reading.
What is the UV index?
The UV Index is the international measure of sunburn-causing ultraviolet radiation reaching the ground. It runs from 0 upward, and the World Health Organization advises sun-protection measures from UV Index 3 and above.
The US EPA groups the scale into bands: 0–2 low, 3–5 moderate, 6–7 high, 8–10 very high, and 11+ extreme. The number is not a temperature and not a measure of brightness — it is a measure of how quickly unprotected skin can burn.
What actually changes the number?
Four things, according to the EPA: sun angle (a function of latitude, day of year and time of day), altitude, cloud cover and stratospheric ozone. Air temperature is not among them, which is why a cool, breezy afternoon can carry the same UV as a hot one.
Altitude alone adds roughly 6% more UV per kilometre above sea level. Cloud is less protective than people assume: the EPA puts transmission at 89% under scattered cloud and 73% under broken cloud, with only full overcast bringing it down to 31%.
Why a generic reading is not enough
Two people standing in the same UV 7 sunlight do not have the same amount of time. The Fitzpatrick scale runs I to VI, and type II burns easily and tans minimally while higher types tolerate more; types I and II together account for roughly 35% of the US population.
Add the SPF you applied, whether you reapplied, and any photosensitising skincare or medication, and a single shared number becomes a genuinely different answer per person. That gap between reading and decision is the whole reason SunDose exists.
What SunDose does with the number
SunDose reads live UV data for your exact location and hour, then applies your profile to it.
- Live UV index and hourly forecast so you can see when the peak actually falls.
- A session timer calculated from your skin type, the current UV and the SPF you selected.
- Sunscreen reminders tied to the running session rather than a fixed clock alarm.
- Sensitivity adjustments for retinol, exfoliating acids, isotretinoin and photosensitising antibiotics.
- Hydration tracking and after-sun routine prompts.
- Lock Screen Live Activities and Home Screen widgets, so checking costs nothing.
The UV index at a glance
| UV Index | Band | What it means in practice |
|---|---|---|
| 0–2 | Low | Minimal risk for most people; protection rarely needed. |
| 3–5 | Moderate | WHO protection threshold begins at 3. Shade around midday. |
| 6–7 | High | Unprotected skin burns relatively quickly. Shade, cover, reapply. |
| 8–10 | Very high | Minimise exposure between 10:00 and 16:00. |
| 11+ | Extreme | Take all precautions; unprotected skin can burn within minutes. |
Frequently asked questions
- What is a good UV index app?
- One that reports the live index and hourly forecast accurately, and then helps you act on it. Reading alone is available free in most weather apps; the value of a dedicated app is personalisation — skin type, SPF, reapplication timing and sensitivity adjustments.
- At what UV index do I need sun protection?
- The World Health Organization advises sun-protection measures from UV Index 3 and above. That threshold is the same regardless of temperature, season or how bright the day looks.
- Is SunDose free?
- SunDose is free to download on the App Store, with optional premium tiers. The live UV index, hourly forecast and session timer work without a subscription. It is iPhone-only and your skin profile stays on your device.
- Does cloud cover lower the UV index?
- Less than most people expect. The EPA puts UV transmission at 89% under scattered cloud and 73% under broken cloud. Only fully overcast skies make a substantial difference, cutting transmission to about 31%.
Read more
Sources
- World Health OrganizationRadiation: The ultraviolet (UV) index
- US Environmental Protection AgencyUV Index Scale
- US Environmental Protection AgencyLearn About the UV Index
- StatPearls, NCBI BookshelfLaser Fitzpatrick Skin Type Recommendations