Can the UV index be 0 when the sun is out?

Yes. The index tracks UVB, and when the sun is low its long path through the ozone layer removes most of it. The sun is visible and warm, UVA is present, and the erythemal number rounds to 0.

Why did my app show a UV index of 4 at night?

It was showing a stale forecast hour or the wrong time zone. Actual UV at night is zero everywhere. An app that displays a non-zero value after sunset is not checking the sun's position at all.

Is the UV index on my weather app accurate?

It is a model forecast for the nearest grid cell, usually updated a few times a day. Under clear skies at midday it is close. Under broken cloud, at low sun, or in hilly terrain it can be off by several index points.

Do I need sunscreen at UV index 0 to 2?

For most skin types, no. At index 2, reddening takes more than an hour even for fair skin, and at 0 it does not happen. If you take a photosensitizing medication or have a photosensitive condition, that rule does not apply to you.

Is the UV index a measurement of sunlight?

No. The UV index is a sunburn-weighted score: solar UV weighted by how efficiently each wavelength reddens skin, then summed. UVB dominates that weighting and UVA barely registers, so the index is a proxy for burn risk and says little about the rest of the light. Two moments with the same index can carry very different spectra.

How does Sunlight is Life calculate UV?

It runs a solar physics engine that models the actual sunlight reaching your coordinates, not the nearest city. The app shows the UVA and UVB you are receiving separately, and the readings update while it is open. It is still a model, not a measurement of your sky.

Why Does My UV Index App Say 0 When the Sun Is Out?

Peter CowanSeptember 16, 20265 min read

Your app is showing a forecast, not a measurement. Most UV apps show a model value for the nearest grid point, updated a few times a day, and many clamp it to 0 at low sun or when the forecast hour has passed. The sun on your face is real. The number is a guess about somewhere near you.

The UV index is an erythema-weighted index: it weights each wavelength by how well it reddens skin, then sums the result. That weighting is dominated by UVB, 290 to 315 nanometers. UVB is also the band the ozone layer absorbs most aggressively, so it collapses when the sun is low and the path through ozone is long. UVA, 315 to 400 nanometers, comes through almost regardless of angle. At 8 AM the sun can be bright and warm on your skin, delivering UVA and infrared, while the erythemal number rounds to 0.

"This index shows as 1 right now at a sunny day in Florida"

A sunny sky is not the same as a high sun. In Florida in December the sun peaks near 40 degrees; at 9 AM it is closer to 20. At 20 degrees the UV index is typically 1 or 2 even under a cloudless sky, because UVB has to cross roughly three times the ozone it crosses at noon in June. The app is not lying about the sky. It is reporting a band you cannot feel.

Warmth on your skin is infrared, which is a poor guide to UV.

"Weather app says it's 10 UV, this says it is 0"

The usual cause is stale time. Many UV apps pull an hourly forecast a few times a day and display whichever hour they last matched. If the forecast window has ended, or the app's clock is in a different time zone from your location, it shows 0 or holds yesterday's peak. This is also why reviewers report "UV index at 4 at night and it went higher as night went on." Actual UV after sunset is zero everywhere on Earth. An app showing anything else is not checking the sun's position.

The second cause is the clamp. To avoid showing 0.3 at dawn, some apps hard-code 0 below a set solar elevation or before a fixed clock time, so a bright low sun reads as darkness.

Clear-sky value versus cloud-adjusted value

The public UV forecast in the United States comes from the National Weather Service and is published by the EPA. It is a next-day forecast of the solar-noon value, computed from predicted ozone and elevation, then reduced by a cloud modification factor drawn from the cloud forecast. Weather data providers offer the same split at hourly resolution: a clear-sky UV index and a cloud-adjusted one.

Apps choose which to show. An app on the clear-sky series shows 9 in the rain. An app on a cloud-adjusted series for a grid cell 20 kilometers wide can show 2 while you stand in a gap in the cloud, in full sun. Neither is measuring your sky; almost no consumer app has a ground sensor.

"The closest place I can get a reading from is 30 km away"

Location is the failure mode nobody suspects. Apps that ask for approximate location get a Wi-Fi or cell-tower estimate that can sit several kilometers away. Apps that cache your last position keep showing the city you flew out of. Apps that snap to a city database show the nearest big city, which is why a reviewer in Menorca saw numbers for Perth. On a coast, in mountains, or across a time-zone line, the wrong point gives a wrong number even when the model is sound.

How to sanity-check any UV app

Check the sun's height. Shadow longer than you means the sun is below 45 degrees and UVB is weak. Shadow shorter than you under a clear sky, and the app should show at least 3.
Check the time. If the number has not changed in two hours, you are looking at a stale forecast, not live data.
Check the sky against the number. Under solid overcast, a value equal to the clear-sky peak means cloud is being ignored.
Check the location name. If it is a city you are not in, the number belongs to that city.

What Sunlight is Life does differently

Sunlight is Life computes its number for your coordinates, refreshes your physical location from GPS when the app comes to the foreground, and shows UVA and UVB separately, so a bright low sun that reads 0 on the index still registers as UVA. How the number is built is in the questions below.

Check today's UV index for your city →

The through line

A UV index of 0 with the sun on your face is not a contradiction. It is the index doing its job, tracking one narrow band that the atmosphere strips out at low angles while the rest of the spectrum arrives as usual. The morning sun that reads 0 is still setting your clock and delivering infrared to your skin. The number tells you about burning. It was never built to tell you whether the light is worth standing in.

Common questions

Can the UV index be 0 when the sun is out?: Yes. The index tracks UVB, and when the sun is low its long path through the ozone layer removes most of it. The sun is visible and warm, UVA is present, and the erythemal number rounds to 0.
Why did my app show a UV index of 4 at night?: It was showing a stale forecast hour or the wrong time zone. Actual UV at night is zero everywhere. An app that displays a non-zero value after sunset is not checking the sun's position at all.
Is the UV index on my weather app accurate?: It is a model forecast for the nearest grid cell, usually updated a few times a day. Under clear skies at midday it is close. Under broken cloud, at low sun, or in hilly terrain it can be off by several index points.
Do I need sunscreen at UV index 0 to 2?: For most skin types, no. At index 2, reddening takes more than an hour even for fair skin, and at 0 it does not happen. If you take a photosensitizing medication or have a photosensitive condition, that rule does not apply to you.
Is the UV index a measurement of sunlight?: No. The UV index is a sunburn-weighted score: solar UV weighted by how efficiently each wavelength reddens skin, then summed. UVB dominates that weighting and UVA barely registers, so the index is a proxy for burn risk and says little about the rest of the light. Two moments with the same index can carry very different spectra.
How does Sunlight is Life calculate UV?: It runs a solar physics engine that models the actual sunlight reaching your coordinates, not the nearest city. The app shows the UVA and UVB you are receiving separately, and the readings update while it is open. It is still a model, not a measurement of your sky.