These days any weather report worth its salt will include the UV index number. I think we all understand the practicality of it. The higher the number, the higher risk of sunburn. But personally, that superficial understanding doesn’t satisfy my curiosity.
I also get to brag a little, as the UV Index in a Canadian invention, circa 1992. Many other countries picked up on the idea. It’s now been taken over and standardized by the World Health Organization.
We’re talking about ultraviolet radiation here, which got a mention last week regarding its spot on the spectrum of electromagnetic waves. There are three types of UV radiation, conveniently called UVA, UVB, and UVC depending on the wavelength. Each one affects us differently.
UVA: These are the longest waves, and thus can more easily pass through stuff. As such, nearly 99% of all UV radiation that gets through the ozone layer is UVA. UVA causes immediate tanning, and long term wrinkling and skin aging.
UVB: While making up only 2% of ultraviolet radiation reaching earth, this is the bad guy. The cause of skin cancer! Last year I wrote about how a sunburn modifies your genes!
UVC: Our ozone layer pretty much completely blocks UVC radiation from reaching us. Let’s hear it for the ozone, ladies and gentlemen!
Now, the UV index is a rating of how much UV radiation is hitting the earth on a given day.It’s calculated based on a number of factors specific to your area.
- Sun height – Midday in mid-summer would have the sun looking straight down on you, meaning the least amount of atmosphere between you.
- Latitude - Your position on the globe affects the angle of the sun.
- Cloud cover – Well that seems obvious!
- Altitude – The higher you are, the less atmosphere to protect you. Being 1000 meters above sea level equals a UV increase of about 10%.
- Ozone – The ozone fluctuates, so it can be absorbing more or less UV on any given day.
- Ground reflection – Depending on the colour and surface, you could be getting UV bounce-back. Snow will reflect 80% of UV waves.
At night, in the dark, the UV index would be zero. While there’s no upper limit to the scale, 10 would be “extreme”… blue sky, mid summer, sun directly overhead.
I’ll wrap up with a note about sunscreen. Specifically that big SPF number on the bottle. You may know it means Sun Protection Factor. That number is determined with a simple lab test.
They round up some pasty looking people and test how long it takes for their bare skin to burn under a controlled dose of UV light. Then they apply the sunscreen to another patch of skin, and measure the time to burn again. If the original burn happened in 3 minutes, and the sunscreened burn took an hour, that means the natural skin protection increased by a factor of 20. (3 minutes x 20 = 60 minutes) Thus, it gets labeled as Sun Protection Factor 20.
P.S. The illustration above was inspired by the random words found in robot-proof captcha security things. (where they ask you to decipher a blurry word… in my case “climate iseight”) Captcha Art is kind of an in thing to do these days. The drawing then reminded me I’ve been wanting to do a story on the UV index for a while.
