What does the UV index measure?

The UV index measures the strength of ultraviolet (UV) radiation reaching the ground at a specific location and time. It runs from 0 (no UV) to 11+ (extreme). The scale was standardised by the World Health Organization, the World Meteorological Organization, the United Nations Environment Programme, and the International Commission on Non-Ionizing Radiation Protection — so a UV index of 7 means the same thing in California as it does in Sydney.

What UV index is dangerous?

Any UV index of 6 or higher is considered dangerous for unprotected skin. Fair-skinned people (Fitzpatrick Type I–II) can burn in under 20 minutes at UV 6–7 and in under 10 minutes at UV 8+. Darker skin types tolerate more exposure but still accumulate damage. Health authorities recommend SPF 30+, shade, and protective clothing whenever the UV index is 3 or higher.

Does cloud cover affect the UV index?

Yes — but not as much as people think. Thin and broken clouds can actually scatter UV and slightly increase the UV index at ground level. Heavy overcast reduces UV by roughly 30–50%, but it doesn't eliminate it — burns on overcast days are common. The UV index forecast in a weather or UV app already accounts for forecast cloud cover at your location.

Is UV worse at higher altitudes or near water?

Yes. UV intensity increases by about 10–12% per 1,000 metres of elevation gain, which is why mountain sunburn is so common. Water, sand, and snow also reflect UV — snow reflects up to 80%, sand around 15%, and water roughly 10% — effectively raising the UV dose your skin receives. A UV forecast app like Sunwise factors location and altitude into the index it shows.

How does the UV index relate to burn time?

Burn time is approximately inversely proportional to the UV index. If a Type II skin burns in 30 minutes at UV 5, the same skin will burn in roughly 15 minutes at UV 10 and roughly an hour at UV 2.5. Sunwise calculates burn time in real time using the current UV index at your location and your Fitzpatrick skin type, so you don't have to do the math yourself.

UV Index Guide: What the Numbers Mean for Your Skin

The UV index is a scale from 0 to 11+ that measures the strength of ultraviolet radiation from the sun at your location. Higher numbers mean greater risk of sunburn and skin damage. But the UV index alone doesn't tell the full story — your skin type determines how quickly you'll actually burn. If you want to see the UV index on your phone right now, read our guide on how to check the UV index on iPhone.

UV index scale explained

UV 0–2 — Low

Minimal risk for most people. You can safely enjoy the outdoors without sunscreen for extended periods. Fair-skinned individuals (Fitzpatrick Type I–II) may still want to apply SPF 30+ if spending more than an hour outside.

UV 3–5 — Moderate

Unprotected sun exposure can cause sunburn in 30–45 minutes for fair skin. Wear sunscreen, a hat, and sunglasses during midday hours. Darker skin tones (Type IV–VI) can generally spend longer outside but should still take care.

UV 6–7 — High

Sunburn can occur in under 20 minutes for fair skin. Seek shade during peak hours (10 AM – 4 PM), wear protective clothing, and reapply SPF 30+ sunscreen every two hours.

UV 8–10 — Very High

Skin damage happens fast — fair-skinned people can burn in as little as 10 minutes. Limit midday outdoor time, cover up, and use SPF 50+ sunscreen. Even darker skin tones should take precautions at these levels.

UV 11+ — Extreme

Maximum risk. Avoid direct sun exposure during midday hours if possible. If you must be outside, wear long sleeves, a wide-brim hat, and SPF 50+ sunscreen — regardless of skin type.

How skin type affects your burn time

The Fitzpatrick scale classifies skin into six types based on how it reacts to UV exposure. Knowing your type helps you understand how long you can safely stay in the sun without protection — which is exactly what a personalized UV forecast app like Sunwise calculates for you automatically. Not sure which type you are? Take the Fitzpatrick skin type quiz — it walks through the standard skin phototype questionnaire with a scoring rubric.

Type	Description	Approx. Burn Time
I	Very fair skin, light eyes, always burns, never tans	~10 min at UV 10
II	Fair skin, burns easily, tans minimally	~15 min at UV 10
III	Medium skin, sometimes burns, tans gradually	~20 min at UV 10
IV	Olive skin, rarely burns, tans easily	~30 min at UV 10
V	Brown skin, very rarely burns, tans darkly	~45 min at UV 10
VI	Very dark skin, almost never burns	~60 min at UV 10

Why a generic UV index isn't enough

Most weather apps show a single UV index number for your area. But a UV index of 6 means very different things for someone with Type I skin versus Type V skin. A person with very fair skin could burn in 15 minutes at UV 6, while someone with darker skin might be safe for over an hour.

That's why Sunwise pairs the UV index forecast with your personal skin type to calculate an actual burn time — giving you a specific, actionable number instead of a vague risk level. Whether you're checking the UV index from your iPhone, iPad, a Home Screen or Lock Screen widget, or the UV index on Apple Watch, you always know exactly how long you can safely spend in the sun. Planning a tanning session? Read our guide on the UV index for tanning. If you're comparing options, see our roundup of the best UV index apps for iPhone — and if you want to know why two UV apps in the same place can show different numbers, our UV index accuracy guide breaks down the upstream data sources. If your goal is simply to not burn, see our practical sunburn prevention guide — a six-step skin-type-aware checklist that turns the UV index into a clear plan.

UV index FAQ

What does the UV index measure?: The UV index measures the strength of ultraviolet (UV) radiation reaching the ground at a specific location and time. It runs from 0 (no UV) to 11+ (extreme). The scale was standardised by the World Health Organization, the World Meteorological Organization, the United Nations Environment Programme, and the International Commission on Non-Ionizing Radiation Protection — so a UV index of 7 means the same thing in California as it does in Sydney.
What UV index is dangerous?: Any UV index of 6 or higher is considered dangerous for unprotected skin. Fair-skinned people (Fitzpatrick Type I–II) can burn in under 20 minutes at UV 6–7 and in under 10 minutes at UV 8+. Darker skin types tolerate more exposure but still accumulate damage. Health authorities recommend SPF 30+, shade, and protective clothing whenever the UV index is 3 or higher.
Does cloud cover affect the UV index?: Yes — but not as much as people think. Thin and broken clouds can actually scatter UV and slightly increase the UV index at ground level. Heavy overcast reduces UV by roughly 30–50%, but it doesn't eliminate it — burns on overcast days are common. The UV index forecast in a weather or UV app already accounts for forecast cloud cover at your location.
Is UV worse at higher altitudes or near water?: Yes. UV intensity increases by about 10–12% per 1,000 metres of elevation gain, which is why mountain sunburn is so common. Water, sand, and snow also reflect UV — snow reflects up to 80%, sand around 15%, and water roughly 10% — effectively raising the UV dose your skin receives. A UV forecast app like Sunwise factors location and altitude into the index it shows.
How does the UV index relate to burn time?: Burn time is approximately inversely proportional to the UV index. If a Type II skin burns in 30 minutes at UV 5, the same skin will burn in roughly 15 minutes at UV 10 and roughly an hour at UV 2.5. Sunwise calculates burn time in real time using the current UV index at your location and your Fitzpatrick skin type, so you don't have to do the math yourself.

Try Sunwise

Sunwise is a free UV forecast app for iPhone, iPad, and Apple Watch that delivers personalized burn times based on your skin type. It includes hourly and 7-day UV index forecasts, Apple Watch complications, Home Screen widgets, and Siri integration.