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Do I need sunscreen on a day with extreme UV levels if it's cloudy?

It's been pretty overcast lately but I noticed that the UV index rating on the daily weather forecast is extreme. Wouldn't clouds make the UV index lower?

Sun and clouds

Patchy clouds can intensify UV levels because radiation is reflected off the clouds' edge. (Source: iStockphoto)


Cloudy skies are no reason to become complacent about using sun protection on an extreme UV day — even when temperatures are low, says Vernon Carr, national manager of public weather services at the Bureau of Meteorology.

"A lot of people think that if it's a hot day they're going to get a lot of UV so they need to use sunscreen, but on a cold day they'll be okay. It doesn't work like that. UV radiation's got nothing to do whatsoever with the temperature," says Carr.

You can't see or feel UV radiation, a type of electromagnetic radiation from the sun. There are three types of UV radiation: UVA, UVB, and UVC.

Ozone high in the Earth's atmosphere absorbs all the UVC, which has the shortest wavelength and is the most dangerous; most of the UVB, which causes sunburn and accumulative skin damage, but no UVA.

"The one we're really concerned about here for the UV index is UVB," says Carr.

"There's a bit of research around that says that [UVA] can cause damage to the skin too. But there is still a lot of work to be done about the impact of UVA."

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UV index

The UV index is an international scale from 1 to 20 that indicates the amount of UV radiation that penetrates the atmosphere and reaches the Earth's surface.

"Level three is where UV radiation can start to cause skin damage. That's when we advise people to take sun protection measures," says Carr.

He says the UV index published on weather forecasts is based on clear sky days and doesn't take into account clouds, rain, pollution or reflectors. The figure given is the predicted maximum for the day, which is around midday depending upon latitude and local time zones.

"At the moment the maximum UV for everywhere in Australia is in the extreme range around the middle of the day," he says.

In summer on a clear sky day, the UV level rises above level three by about 9am and doesn't drop below level three until about 5pm.

The Bureau of Meteorology derives this figure by computer modelling of the ozone levels.

"The computer program predicts the amount of ozone in the high atmosphere over the next few days. Once you've got that answer you can derive the UV level at the ground," he says.

Summer UV index levels

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Cloud factor

There hasn't been much research about the effect of cloud and the types of cloud on UV so at the moment the Bureau only gives a maximum clear sky value, and the times of the day when the UV index is expected to be above level three, he says.

"UV is affected by many things. One of those is cloud, but it depends upon what type of cloud it is, the thickness of the cloud and whether it is deep right through the atmosphere."

UV radiation can penetrate through thin cloud, so you can still get high levels of UV at ground level on overcast days, he says.

Patchy clouds can also intensify UV levels because radiation is reflected off the clouds' edge.

"In other words you get a mirror effect and the UV can bounce off the clouds and focus on the ground. You can get quite high UV levels at the ground during partly cloudy days when there are breaks in the cloud.

"You have to be very, very careful on those sorts of days not to assume you won't get those sorts of extreme levels."

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Reality check

The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) uses detectors to monitor UV radiation at ground level in Australia's major cities, so you can see UV levels in real time.

"If you look at these real time traces you can actually watch the effect of the cloud during the day," he says.

"If it's a clear day it's very smooth and just goes up steadily throughout the day and decreases in the afternoon, but when it's cloudy, or overcast or raining you'll see the levels very much lower and fluctuating a lot, a bit like an earthquake on a trace."

Even in a summer that has been mild at best, dreary at worst along the east coast, monthly summaries of ARPANSA's data shows that at the peak of the day Sydneysiders, for example, have been exposed to very high to extreme UV levels on most days.

"It's great that we can show people the ARPANSA equipment and the UV levels are close to what we predict," says Carr.

Check out the BOM website for more information about UV radiation and sun protection.

Vernon Carr was interviewed by Genelle Weule.

Tags: skin-cancer, physics, weather

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Published 24 January 2012