The UV index explained

Last updated 27 October 2026

What is the UV index? A single number describing how strong the sun's ultraviolet radiation is at ground level, at a particular place and moment. It starts at 0 and has no fixed upper limit, though readings above 16 are rare outside the tropics and high altitude.

It was designed to be acted on rather than understood: the higher the number, the less time unprotected skin can be exposed before it is damaged. The scale is standardised internationally, so a UV index of 8 means the same in Perth as it does in Phoenix.

The UV index scale

The World Health Organization groups the index into five bands. The colours below are the ones used throughout this site.

Sun protection is generally advised whenever the index reaches 3 or above. That threshold is the basis of the sun protection times shown on every city page.

You cannot feel it

This is the point most worth remembering. UV is not heat and not light, so there is nothing to warn you about it. A cool, breezy, overcast spring day can carry a higher UV index than a sweltering afternoon in late summer, and skin burns just as readily on it.

Temperature is a particularly poor guide in Australia, where the UV index routinely reaches Very High or Extreme in September and October while the air is still mild.

What changes the UV index

Time of day and year

The dominant factor is how high the sun sits in the sky. UV peaks around solar noon — which is not the same as midday on the clock, and is an hour off wherever daylight saving is in force. It rises and falls steeply either side, so the middle of the day carries most of the day's total exposure.

Latitude

UV is strongest near the equator and weakens towards the poles. Darwin sits at about 12° south and sees Extreme readings for much of the year; Hobart at about 43° south rarely exceeds Very High even in midsummer.

Cloud

Cloud reduces UV, but much less reliably than people expect. Thin or broken cloud may cut it only slightly, and scattered cloud can briefly increase it above the clear-sky level by reflecting additional radiation towards the ground. Only thick, continuous cloud reduces UV substantially.

This is why the sun protection times on this site are calculated from the clear-sky forecast. Cloud can break at any time, and advice that is wrong in the direction of too little protection is the dangerous kind.

Altitude

UV increases with height, by roughly 10% for every 1,000 metres of elevation, because there is less atmosphere above you to absorb it.

Reflection

Surfaces bounce UV back at you, adding to what arrives directly from the sky. Fresh snow is by far the strongest reflector and can almost double exposure. Dry sand and sea foam reflect appreciably; water, grass and soil much less. Shade under a tree or an umbrella therefore reduces UV but does not remove it.

Ozone

Stratospheric ozone absorbs most incoming UV-B. Natural variation in the ozone layer changes surface UV from day to day, and this is part of why Australia and New Zealand record higher UV than places at comparable latitudes in the northern hemisphere. The Earth also passes closest to the sun during the southern summer, which contributes a few per cent more.

Measured and forecast are not the same thing

A measured reading comes from an instrument pointed at the sky, reporting what actually reached the ground a few minutes ago. Australian city pages on this site show measured readings from ARPANSA.

A forecast is a model's expectation, and depends on getting cloud right, which is the hardest part. Tomorrow's figures, today's curve and the sun protection times are forecasts, and are labelled as such wherever they appear. The methods page states how accurate ours has measured out to be.

Protecting yourself

The advice below is the standard public-health guidance of ARPANSA, Cancer Council Australia and the US EPA. It applies whenever the index is 3 or above.

• Clothing is the most reliable protection: it does not wear off or need reapplying. Closely woven fabric covering shoulders and back does most of the work.
• Sunscreen of SPF 30 or higher, broad spectrum and water resistant. Apply generously about 20 minutes before going out and reapply every two hours, and after swimming or towelling. Most people apply far less than the amount products are tested with, which is the usual reason real-world protection falls short of the label.
• A broad-brimmed hat shades face, neck and ears, which caps and visors do not.
• Shade, particularly between mid-morning and mid-afternoon — remembering that reflected UV still reaches you.
• Sunglasses meeting a recognised standard. Eyes are damaged by UV as skin is, and the damage accumulates.

Sunscreen is the least reliable of these on its own, because it depends entirely on how much is applied and how often. It works best combined with the others rather than instead of them.

When the index is low

At 0–2 most people need no protection for ordinary outdoor activity. Some sun exposure helps the body make vitamin D, and the amount needed varies with skin type, location and season. How much is right for you is a question for a doctor, not a website — particularly if you have a condition or take medication that affects sun sensitivity.

Where these figures come from

Measured Australian readings come from ARPANSA's network of instruments. US forecasts come from the Environmental Protection Agency. Forecast curves and sun protection times are derived from the Copernicus Atmosphere Monitoring Service. The methods page explains each source, how often it is refreshed and how accurate it is.

This page is general information, not medical advice. If you are concerned about your skin, a mole that has changed, or your particular risk, speak to a doctor.