UV Index 3 or 11: What Each Number Asks You to Do Outdoors

A spring morning in southern Australia can begin at 14 degrees under a grey sky, with a light jacket and no thought of sunscreen, and end at lunchtime with a sunburnt neck. The temperature did not cause the burn, and the cloud did not prevent it. The relevant figure was the UV Index, which the Bureau of Meteorology publishes every day for locations across the country, and which measures something that neither the thermometer nor the sky reports. This article explains what that number is, what each level asks of a person outdoors, and how to use the daily sun protection times. The helper below applies that guidance to a particular day. It is general information and not medical advice.

What the forecast number is

The Bureau of Meteorology explains that its UV Index forecast is produced by the Copernicus Atmosphere Monitoring Service model. The model takes into account the time of day, the date, the latitude, the altitude and the amount of ozone in the atmosphere. Each of these affects how much ultraviolet radiation reaches the ground at a given place.

The figure published in the forecast is the daily maximum. It is calculated for the middle of the day, and it assumes that the sky is free of cloud. That assumption carries a condition which the reader should keep in mind. Because the forecast is a clear-sky figure, cloud on the day may lower the actual UV level below the number shown. The Bureau is equally explicit, however, that people still get sunburnt on cold or overcast days. A grey sky is therefore not a reason to disregard the forecast.

In practical terms, the published number is best read as the level the day is capable of reaching around midday, rather than a promise of what any particular hour will deliver.

The five categories and the line at 3

The Bureau of Meteorology groups the UV Index into five categories.

The Extreme category has no upper limit. A forecast of 11 is extreme, and so is any figure above it.

The line that matters most falls at 3. The Bureau recommends sun protection when the UV Index is 3 or above. It also recommends protection at any level when a person is working outdoors, is near reflective surfaces such as snow, or is outside for extended periods. The categories above 3 do not change the instruction to protect the skin and eyes. They describe how much ultraviolet radiation is present, and therefore how quickly exposure accumulates.

This is the sense in which a 3 and an 11 ask different things of the same person. On a phone screen the two numbers occupy the same small space, and both sit above the threshold at which protection is recommended. The difference lies in the margin for error. At 3, a missed patch of skin or a late reapplication is less consequential. At 11, the same lapse matters more, and the planning of shade and reapplication deserves correspondingly more care.

Why UV is not heat

The most common error in reading the day is to judge the sun by the temperature. The Bureau of Meteorology sets out why that judgement is unreliable.

First, the two do not peak together. UV levels peak around the middle of the day, while the temperature usually continues to rise into the afternoon. The hottest part of the day and the part with the most ultraviolet radiation are not the same hours.

Second, the difference between summer and winter UV is a matter of the sun’s angle, not the air temperature. A higher sun delivers more ultraviolet radiation, regardless of how warm or cool the air feels.

Third, and as a consequence, cool days in spring and summer catch people out. The air may feel mild while the UV Index is well above 3.

The Bureau adds a point of national relevance. Of the capital cities, only Hobart and Melbourne have average maximum UV levels that stay below 3 during the winter months. For most of Australia, sun protection is a consideration throughout the year rather than in summer alone.

Sun protection times, used properly

Alongside the daily maximum, the Bureau of Meteorology publishes sun protection times. These are the times of day during which sun protection is recommended for a given location, because the UV Index is forecast to be 3 or above during that period.

The sun protection times are the more practical of the two figures for planning a day. A walk, a training session or a job in the garden can be scheduled before or after the protection period, or the necessary protection can be prepared for the hours that fall within it. A person who intends to rake and reseed a winter-battered lawn this spring, for example, may choose an early start that finishes before the protection period begins. The same consideration applies to planting out tomatoes in the November sun, when the hours spent kneeling in a bed can pass more quickly than expected.

The times vary by location and by day. The Bureau’s website and the SunSmart app both show them for a chosen location, and the responsibility for checking them rests with the person going outside.

The location deserves attention for a further reason. Because the Bureau’s model accounts for latitude and altitude, the forecast for a person’s home suburb does not describe a day spent in another city, further north or at elevation. A person travelling for a weekend, or driving to the mountains, should look up the forecast and the protection times for the place they will be, rather than relying on the figures for the place they left.

What to do at 3 and above: the five steps

Cancer Council Australia sets out five steps for sun protection. Each step addresses a different route by which ultraviolet radiation reaches the skin or eyes, and none of them is intended to be used alone.

1. Clothing. Wear clothing that covers as much of the skin as possible.
2. Sunscreen. Apply an SPF 50 or 50+ broad-spectrum, water-resistant sunscreen. Cancer Council advises about seven teaspoons, or 35 mL, for a full-body application on an adult. Apply it at least 20 minutes before going outdoors, and reapply it every two hours, or after swimming or sweating.
3. Hat. Wear a broad-brimmed, legionnaire or bucket hat. Cancer Council states that caps and visors do not provide enough protection.
4. Shade. Seek shade where it is available. Shade reduces exposure, but it does not remove it, because reflected UV still reaches a person sitting in the shade.
5. Sunglasses. Wear close-fitting, wraparound sunglasses that meet AS/NZS 1067.

The order of the list carries a message. Sunscreen is one step of five, and it is not a suit of armour. A person who applies sunscreen and then sits for hours in the open without a hat or shade is relying on the single step most easily undone by sweat, water or a missed patch of skin. The other steps do not wear off.

Sunscreen is seldom the only bottle in the summer bag. Edura has set out how to choose a mosquito repellent for the place you are going, including which goes on first.

Below 3, and vitamin D

Cancer Council Australia states that sun protection is not recommended when the UV Index is below 3, unless a person will be outdoors for an extended period or near reflective surfaces such as snow. Below that threshold, the general recommendation does not apply, although those two exceptions remain.

Vitamin D is a reasonable question to raise alongside sun protection advice. Cancer Council’s position is that when the UV Index is 3 or above, most Australians obtain enough vitamin D from a few minutes of everyday exposure. Following the five steps during the sun protection times is therefore not, for most people, in conflict with obtaining vitamin D.

Read the number, not the thermometer

The UV Index is a forecast of a specific quantity, published daily by the Bureau of Meteorology for a specific place. It is not a measure of heat, and it does not fall simply because the sky is grey. The guidance that follows from it can be stated as a set of conditions.

• Check the day’s maximum UV Index and the sun protection times for the location concerned, not the location of the person’s home if the two differ.
• At 3 or above, use all five steps during the protection times: clothing, sunscreen, a broad-brimmed hat, shade and sunglasses.
• Below 3, protection is not generally required, except for long periods outside or near reflective surfaces such as snow.
• At 8 and above, plan shade and reapplication with particular care.
• Treat the temperature and the cloud cover as information about comfort, not about ultraviolet radiation.

The forecast is published for the day ahead. Reading it before going outdoors, and reading it for the right place, comes before every other step.

Sources: About the UV Index, Bureau of Meteorology (https://www.bom.gov.au/resources/learn-and-explore/uv-knowledge-centre/about-the-uv-index); Be SunSmart, Cancer Council Australia (https://www.cancer.org.au/cancer-information/causes-and-prevention/sun-safety/be-sunsmart)