Why do two apps show a different
UV index?
Because the UV index is one standard scale, but the number on your screen is a forecast, not a measurement. Different apps read different forecast models, refreshed at different times, on different grids, with different cloud handling, so they can land a point or two apart for the same place and hour.
You glance at your iPhone weather, open a second app, then check a website, and three numbers stare back: 7, 8, and 6. None of them is lying. The disagreement is not a bug, and the fix is not to pick the scariest figure. It is to understand that you are looking at three predictions of the same physical thing, and to know which prediction is freshest and most local.
This guide explains UV-index data and where it comes from. It is about methodology, not health. It does not tell you what is safe for your skin, how long to stay out, or set any personal exposure limit. If a skin condition, medication or other factor affects your sun sensitivity, talk to a dermatologist.
One scale, many forecasts
The UV index is a single, internationally agreed scale. It was defined by the World Health Organization together with the World Meteorological Organization and others, and it runs from 0 upward, with 11+ marked as extreme. A UV index of 8 means the same intensity of sunburn-causing radiation whether you read it in Sydney or San Diego. The scale is not where the disagreement lives.
The disagreement lives in the value plugged into that scale. That value is a forecast: a computer model estimates how much ultraviolet radiation will reach the ground at your location at a given hour, accounting for the sun's angle, the thickness of the ozone layer, altitude, and expected cloud. The US EPA, which publishes a daily UV index forecast for the United States, describes it plainly as a next-day prediction. A prediction is exactly the kind of thing two careful sources can compute slightly differently.
Five reasons the numbers drift apart
1. Different forecast models
National weather services, Apple, Open-Meteo and others do not all run the same underlying atmospheric model. Each model makes its own assumptions about ozone and aerosols and arrives at its own UV estimate. Two competent models predicting the same sky will often agree to within a point but rarely match to the decimal.
2. Different run times
Models are re-run on a schedule, often every few hours. If your weather app last fetched its forecast at 7 a.m. and another app refreshed at noon, they are quoting two different vintages of the same prediction. A morning forecast that did not anticipate the afternoon's clouds will read higher than a midday one that did.
3. Different grid resolution
A forecast covers the world in a grid of cells. A coarse grid might average a whole metro area into one value; a fine grid resolves your neighborhood. If one app reports the cell over the city center and another the cell over your coastal suburb, the altitude and reflectivity differ, and so does the number.
4. Different cloud handling
Clouds are the hardest part. Thick cloud can cut surface UV substantially, thin or broken cloud far less, and bright clouds near the sun can even scatter extra UV downward. Each model handles this differently, and small differences in predicted cloud become visible differences in the UV index. This is usually the single biggest source of a one-to-two-point gap.
5. Different rounding and timing windows
One source may show the peak for the whole day; another the value for the current hour. One rounds 7.5 down, another up. None of these is an error, but stacked together they widen the apparent gap between two screens.
Where common UV data comes from
Knowing the plumbing helps you read the numbers. A few sources do most of the work behind consumer apps:
- National weather services. Government meteorological agencies (the US National Weather Service, and equivalents abroad) produce official UV index forecasts. The EPA's daily figures are an example of this public, standardized layer.
- Apple WeatherKit. Apple's WeatherKit service supplies hourly weather data, including UV index, to apps on iPhone. It is the source behind a great many iOS weather features.
- Open-Meteo and similar APIs. Open-Meteo aggregates national weather-model output into a free, well-documented forecast API that includes the UV index, widely used as a primary or fallback source.
Two apps can both be reputable and still differ simply because one reads WeatherKit and the other reads a national model through an aggregator. Neither is wrong; they are different forecasts of the same sky.
What makes one source more trustworthy
Since you cannot make a forecast a measurement, judge sources by how current and how honest they are:
- Hourly updates. UV is naturally lowest near sunrise and sunset and highest around solar noon, so a single daily number hides most of the day. A source that refreshes hourly tracks the real curve, and the changing clouds, far better than one stale figure.
- Location-specific grids. A reading tied to your exact coordinates beats one averaged across a whole city, especially near coasts, mountains and elevation changes.
- Transparent provenance. A source that tells you which model it used, and when it last updated, lets you weigh the number. A source that presents a bare figure as certainty gives you nothing to check it against.
How Honest Tan handles this
Most apps hand you one number and hope you do not ask where it came from. Honest Tan does the opposite: it reads the hourly UV index from Apple WeatherKit for your location, falls back to Open-Meteo when WeatherKit is unavailable, and tells you which source is behind the reading. That is the point of naming the source. When you can see that today's figure is a fresh, location-specific WeatherKit forecast, a slightly different number in another app stops being a mystery and becomes what it always was: a second model's reasonable guess.
Honest Tan also plots the day as an hourly curve rather than a single headline number, so the shape of the forecast is visible and the peak is obvious. A forecast you can place in context is worth more than a forecast presented as a fact. The sky always has the final say, and Honest Tan says so out loud.
Related reading
- Does cloud, altitude, snow or water change the UV index?
- When is the UV index highest during the day?
- What UV index counts as high? The 0 to 11+ scale explained.
Straight answers.
Which UV index app is most accurate?
No single app is universally most accurate, because every app shows a forecast, not a measurement. The more trustworthy source is the one that updates hourly, uses a location-specific grid rather than a city-wide average, and names where its number comes from. A source that hides its provenance gives you no way to judge it. Honest Tan reads from WeatherKit with an Open-Meteo fallback and tells you which is in use.
Why does my iPhone weather show a different UV index than another app?
Because they likely pull from different forecast models, run at different times, on different grid resolutions, with different cloud handling. The scale itself is identical and standardized by the WHO and WMO, but the underlying value is a model prediction, and two reasonable models can disagree by a point or two for the same place and hour.
Is a higher UV reading the correct one?
Not necessarily. A higher number is not automatically more accurate or safer to trust. The differences usually come from how each model handles clouds and when its forecast was last refreshed, not from one source being right and another wrong. Prefer the most recently updated, location-specific source rather than the highest or lowest figure.
Can the UV index change during the day?
Yes. It is naturally lowest near sunrise and sunset and highest in the hours around solar noon, so a single daily figure hides most of the story. Cloud cover also shifts the real value hour to hour. This is why an hourly forecast is more useful than one number for the whole day.
How often should a UV index forecast update?
The more often the better. National weather models typically refresh every few hours, and good apps re-fetch the hourly forecast for your exact location frequently. A reading that has not refreshed since this morning can lag the sky by the afternoon, which is one common reason two apps disagree.
Sources: World Health Organization UV Index, US EPA UV Index Scale, Apple WeatherKit, Open-Meteo. The UV index is a forecast, not a measurement, and the sky has the final say.
Honest Tan explains UV-index data and shows a forecast. It is not a medical device and cannot tell you what is safe for your skin or how long to stay out. Burns are not goals.