I was just looking at my weather app and it said, “very high UV – limit sun exposure”. It’s mostly cloudy right now, so how do we know there’s high UV and also, does the sun’s UV rays really change in intensity?
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The Sun’s UV doesn’t change much, but the amount making it to you on the ground does. In rough order of importance, there are a bunch of factors that affect it:
* The height of the Sun in the sky at local noon (higher = more UV)
* Your altitude (higher = more UV, since there’s less atmosphere above you shielding you)
* The amount of cloud cover (lower = more UV, although typical clouds do not block all of it)
* The presence of reflective materials like snow around you (more reflection = more UV)
* The conditions of the ozone layer above you (more ozone = less UV)
* The amount of other aerosols in the air (lower = more UV)
* The current Earth-Sun distance (which varies with the seasons, though it isn’t the primary reason for them)
All these factors get fed into a computer model that estimates the amount of the type of UV that harms your skin getting through the atmosphere.
As a “fun” fact, the original UV index scale was designed to top out at 10 (direct noon sunlight in the tropics), but damage to the ozone layer since then has made much higher readings common.*
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* The scale was designed around the time CFCs – the main ozone-killing chemical – were banned. But CFCs last decades and keep doing damage to the ozone, so ozone levels are only just now bottoming out and beginning to recover. Without a ban on CFCs, the ozone layer would be nearly gone right now, and would fully vanish by the 2040s.
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