How accurate is the iPhone Weather app's UV index?

The iPhone Weather app uses Apple WeatherKit data, which is sourced from Apple's own meteorological models combined with multiple weather data providers. It rounds to whole integers (0, 1, 2…) so a true UV 6.7 reads as 7, which can look like a discrepancy versus apps that show one decimal. For everyday sun-safety decisions the iPhone Weather UV number is reliable to within ±1, but it doesn't tell you anything about your personal burn time at that UV level.

Where does Sunwise get its UV data?

Sunwise uses Apple WeatherKit for UV index and weather data. WeatherKit requests are made from your device using your GPS location, so the reading reflects your precise spot — not a city-level approximation. Apple WeatherKit is the same data source that powers Apple's own Weather app and a number of high-quality third-party weather apps, so Sunwise's raw UV number matches what you'd see in Apple Weather; the difference is that Sunwise converts that UV number into a personal burn time for your Fitzpatrick skin type.

How is the UV index actually measured?

The UV index is a modelled value derived from solar elevation angle, total atmospheric ozone, cloud cover, surface albedo (how reflective the ground is), and altitude. A small number of research-grade ground sensors measure UV directly, but consumer forecasts are computed from satellite data and atmospheric models — not from sensors on every street. That's why two apps drawing on different models can produce slightly different numbers at the same location, and why a UV forecast 7 days out is meaningfully less precise than the current hour's reading.

Does cloud cover affect the UV index?

Yes, but less than most people expect. Thick overcast cuts UV by roughly 50–70%, but thin or scattered clouds reduce UV by only 10–30% — and broken cloud can occasionally focus UV via 'cloud enhancement', pushing readings briefly above the clear-sky value. UV penetrates haze, fog, and overcast better than visible light, so a cool, grey day can still produce a sunburn. A UV forecast app that updates hourly will reflect cloud changes; one that only shows a daily peak won't.

How Accurate Are UV Index Apps?

Q: Why do different UV index apps show different numbers?

UV index apps pull from different upstream data sources (Apple WeatherKit, NOAA, OpenWeatherMap, OpenUV, ECMWF) and each source models UV slightly differently — varying cloud-cover handling, ozone inputs, altitude correction, and update cadence. Two apps in the same spot can show UV 6 and UV 8 not because one is wrong, but because they use different models. The differences are usually within 1–2 points and the qualitative band (Moderate / High / Very High) is consistent in practice.

Q: Which UV index app is most accurate?

There's no single 'most accurate' UV app — every consumer app relies on a modelled UV index rather than a direct ground sensor, so accuracy depends on the underlying data source and how well it accounts for local conditions. Apps built on Apple WeatherKit (which blends multiple meteorological models) tend to be reliable on iPhone because WeatherKit also accounts for the device's GPS altitude and local cloud cover. The bigger question for most people isn't 'is the number exact' — it's 'what does this number mean for my skin', which is where personalization matters more than two decimal places of precision.

If you've ever opened two UV index apps in the same place and seen two different numbers, you're not alone — it's one of the most common complaints about UV apps on iPhone. The short answer is that no consumer UV app reads a sensor at your exact location; every reading is modelled. This guide explains where the numbers actually come from, why they sometimes disagree, and how to judge whether the UV index on your phone is something you can plan around.

How the UV index is actually calculated

The UV index isn't measured by a sensor on the side of your building. It's a modelled value computed from five main inputs:

•Solar elevation — the angle of the sun above the horizon, which depends on your latitude, the day of the year, and the time of day.
•Atmospheric ozone — the total column ozone overhead, which absorbs UVB and varies daily and seasonally.
•Cloud cover — type and thickness of clouds, derived from satellite or ground observations.
•Altitude — UV rises roughly 10–12% per 1,000 m of elevation, so a forecast that ignores altitude under-reads UV at mountain locations.
•Surface albedo — reflective surfaces like fresh snow, water, and sand bounce UV back into the sky and can increase exposure.

Two apps can use the same five inputs and arrive at slightly different numbers because each runs its own atmospheric model, treats cloud data differently, and updates on a different schedule. That's why a UV 6 from one app and a UV 7 from another at the same time and place isn't a bug — it's modelling variance, usually within ±1.

Comparing the major UV data sources

Almost every iPhone UV app — including Sunwise — gets its numbers from one of a small handful of upstream data sources. Here's what each one is good and not-so-good at, and which apps you may recognise that use them.

Source	Used by	Strengths	Caveats
Apple WeatherKit	Apple Weather, Sunwise, and most modern third-party iOS weather apps	Multi-source ensemble; uses on-device GPS for accurate altitude and location; refreshes regularly; available on iPhone, iPad, and Apple Watch.	Rounds UV to whole integers in the public API. Not available outside Apple platforms.
NOAA / EPA UV Index	EPA's SunWise app and many US public-health resources	Authoritative US source with strong ozone modelling. Backed by the National Weather Service.	Forecast resolution is coarser (ZIP-code level) and updates less frequently than commercial APIs.
OpenUV	Some independent UV apps and developer projects	Worldwide coverage with hourly forecasts and a free API tier.	Free tier is rate-limited; quality depends on the upstream model OpenUV is pulling from.
OpenWeatherMap	Many cross-platform weather and UV apps	Broad global coverage and inexpensive API.	UV is a modelled add-on, not the headline product. Accuracy can lag behind dedicated UV sources, especially at high latitudes.
WHO / SunSmart Global UV	SunSmart Global UV app	Curated for public-health messaging with conservative protection-time guidance and global reach.	Optimised for cautious recommendations rather than precise hour-by-hour planning.

Sunwise uses Apple WeatherKit because it's the same data source as Apple's own Weather app, it accounts for your device's GPS altitude, and it's available across iPhone, iPad, and Apple Watch with consistent results. The raw UV number in Sunwise should match Apple Weather closely — the difference is that Sunwise turns it into a personal burn time.

Why two UV apps in the same spot disagree

The Reddit threads asking "why does one UV app show 5 and another show 8?" almost always come down to one of four causes:

1.Different upstream data — the apps pull from different forecast providers (e.g. WeatherKit vs OpenWeatherMap vs NOAA), each with their own model.
2.Rounding — some apps display integers only, others show one decimal. A true UV 6.6 reads as 7 in one app and 6.6 in another.
3.Update cadence — one app may refresh every 15 minutes while another is showing a 3-hour-old forecast. On a partly cloudy day this alone can move the reading.
4.Clear-sky vs cloud-adjusted — a few apps report the clear-sky UV (what UV would be if no clouds were present) and only adjust at request time; others adjust automatically. Clear-sky values are always higher.

The qualitative answer ("Moderate" vs "Very High") tends to agree across reputable apps even when the integers don't. For practical sun safety, the band is what you should plan around — and Sunwise converts the band into a personal burn time so the action is unambiguous.

How to judge a UV reading you don't trust

If you suspect an app is showing the wrong UV, three quick checks usually resolve it:

•Sanity check against the sun — UV peaks within ~2 hours of solar noon and drops sharply after. A UV reading of 8 at 7 PM in summer is almost certainly stale.
•Compare with a second source — pull up Apple Weather and a public-health source like the EPA UV index page for your location. If both agree within ±1, the reading is fine.
•Check location permissions — an app with coarse Location Services permission may be using a city-level fallback, not your precise GPS. In iOS Settings → Privacy → Location Services, make sure the app has "Precise Location" enabled.

Why personalization matters more than two decimals

Even if every UV app on your phone agreed exactly, the bare number alone doesn't tell you what to do. A UV index of 7 is ten minutes to a burn for very fair skin (Fitzpatrick Type I) and 45+ minutes for darker skin (Type V). Two people standing next to each other reading the same UV 7 should react differently.

That's the gap Sunwise focuses on. Once you set your Fitzpatrick skin type, every UV number becomes a burn time — a specific, actionable count of minutes — instead of an abstract integer. For more on what the UV scale actually means and how skin type changes the risk, see the UV index guide.

Frequently asked questions

Why do different UV index apps show different numbers?

Each app pulls from a different upstream model — Apple WeatherKit, NOAA, OpenUV, OpenWeatherMap, or the WHO's data — and each model treats cloud cover, ozone, and altitude slightly differently. Differences of ±1 between reputable sources are normal and don't mean one is wrong; they just reflect modelling variance.

Which UV index app is most accurate on iPhone?

There's no single "most accurate" app — every consumer UV reading is modelled rather than measured. Apps built on Apple WeatherKit tend to be reliable because WeatherKit uses on-device GPS for altitude and integrates multiple meteorological providers. For everyday sun-safety decisions the more useful question is whether the app turns the UV index into something actionable — which is where a personalized burn time matters more than fractional UV precision.

Is the UV index more accurate than the Weather app's reading?

Apps that pull from Apple WeatherKit (including Sunwise) share the underlying UV number with Apple Weather. The difference between apps is usually presentation and rounding, not the underlying data — so a UV-focused app with hourly graphs and Apple Watch complications isn't "more accurate" than Apple Weather, it's just more useful for sun planning.

How accurate is a 7-day UV forecast?

Hourly UV for today and tomorrow is generally reliable. Day 3–5 forecasts are useful for planning trends — "is next weekend going to be high UV?" — but specific hourly numbers can shift as cloud-cover forecasts update. Day 6–7 should be treated as directional. Sunwise refreshes the forecast as new data arrives so the closest hours are always the freshest.

Get a UV forecast you can act on

Sunwise pairs Apple WeatherKit's UV index with your Fitzpatrick skin type to produce a personal burn time on iPhone, iPad, and Apple Watch — so "UV 7" becomes "22 minutes" instead of an abstract number you have to interpret.