Australia Weather News
As large parts of Australia swelter through yet another heatwave, it's easy to focus on the soaring temperatures.
But there's more than blistering heat to be concerned about when we head outside at this time of year.
Most of the country will also experience extreme ultraviolet or UV radiation levels at some point in the day.
And this is the case pretty much all summer long.
This is in contrast to summer in the Northern Hemisphere, where UV levels can venture into the extreme, but generally not for as long nor as often as Australia experiences.
This is perhaps most noticeable in the timeit takes for skin to burn: under the stinging Australian summer sun, unprotected skin can turn red in minutes.
There are a few reasons why skin might burn faster here than elsewhere — and why we have such high rates of skin cancer.
And it's notbecause of the hole in the ozone layer.
Why is UV so high in Australia?
The main reason for our high UV is because, during the Southern Hemisphere summer, our planet is closer to the source of the radiation, the Sun.
Earth's path around the Sun isn't a perfect circle. It's slightly egg-shaped.
Near the start of July, Earth's at thepointy end of the egg, around 152 million kilometres from the Sun.
But soon after New Year in January, Earth's at therounder endof the egg. When it's in that part of the orbit, it's around 147 million kilometres from the Sun.
So during our summer, when Earth's tilt means Australia is pointing at the Sun, we're 5 million kilometres closer to it. That means we cop more intense UV (and other sunlight).
There's also less stuff above us to filter the UV before it reaches us on the ground.
Air pollution and very thick clouds can block UV, but the Australian continent has — compared to continents on the other side of the equator — relatively clean air and clear skies, according to David Whiteman, a medical epidemiologist at QIMR Berghofer.
Plants and trees can also reflect incoming UV radiation when they emit chemicals that react with molecules in the atmosphere. While Australia has some large forests, they are mostly found in the Northern Hemisphere.
Together, Professor Whiteman says, these factors mean Australia's UV is "probably about 10 per cent higher on average" than the equivalent latitude in the Northern Hemisphere.
"That would mean for people living in Brisbane it is higher than for people living in Miami in the US, and for people in Melbourne, it's higher than for people living in Athens, Greece."
While a 10 per cent increase in UV is significant, and might account for that sting in our summer sun, reasons for Australia's high melanoma rates are more lifestyle-related, he says.
"You look at cricket and our affinity with surfing and life-saving and being at the beach and … lawn bowls, there are so many parts of the Australian recreational lifestyle that are focused on the outdoors in the summer in the middle of the day.
"That leads to a population being bathed in sunlight for longer periods of time."
What else can increase my UV exposure?
Where you live in Australia can affect the amount of UV you might encounter too.
Northern Australia, for instance, gets more UV than the south.
Earth's tilt — which gives us the seasons — means regions closer to theTropic of Capricorn, which runs through Western Australia, Northern Territory and Queensland, directly face the Sun and receive more radiation at the height of summer.
Altitude, too, means more UV. Up on a tall mountaintop, the air is thinner, with fewer molecules in the atmosphere to block the radiation on its way to the ground.
A combination of these — being at altitude and towards the equator — produced the highest UV levels recorded on Earth, Professor Whiteman says.
"Up in the Andes, right on the equator … at 6,000 metres, the UV index recorded up there was in the 20s.
"Extraordinarily high levels of UV are on planet Earth at the equator at altitude."
What about the hole in the ozone layer?
Most ozone in Earth's atmosphere is in a layer between 10 and 30 kilometres above the ground, where it helps block UV radiation from the Sun.
Each year, the layer of ozone in the atmosphere above Antarctica becomes thinner. This "hole" is a natural phenomenon that forms around August and "closes" by December.
In the 1970s, scientists grew concerned about the hole's expansion, which was largely driven by ozone-depleting chemicals such as chlorofluorocarbons or CFCs.
The Montreal Protocol, an international agreement that came into effect in the late 1980s, phased down the production and use of ozone-depleting substances.
In the decades since, the "hole" in the ozone layer that human activities contributed to has been repairing.
But even at its largest, the hole in the ozone layer never really extended over parts of the world where people lived, Professor Whiteman says.
"So it was really restricted to the deep Southern Ocean and maybe parts of Tierra del Fuego [at the southernmost tip of South America] and maybe the most southern islands off New Zealand."
But that's not to say the threat to human health wasn't real, he adds.
"It certainly could have gone that way had we not done anything [to help the hole repair]."
ABC