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Comment by colechristensen | Hacker News Reader

I hear this a lot but do not remember a single sunburn on an overcast day despite having very light skin. Is this a southern latitude thing?

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Latitude makes a huge difference. The UV index is 13 in Mexico City today vs 5 in London. Depending on your skin type, you would burn in 10 minutes in Mexico City vs 30-45 minutes in London.

This is because sunlight hits the Earth at more of an angle the farther north and south you get, so it passes through the atmosphere for a longer time and more UV gets absorbed before hitting you. This is the same reason you don't burn as much in the early morning and late afternoon.

I’ve never had a sunburn from a cloudy day in Australia.

Mexico City is near the equator (1,342.38 mi). London (3,558.31 mi) and Australia (Sydney: 2,339.14 mi) aren't? Though Darwin is closer (860.93 mi) to the equator than Mexico City. So I guess you could be right but if you're like most people and live in NSW or Perth, you probably live a lot further away.

The altitude of Mexico City would also have an effect, compared to a location like Singapore at sea-level.

Time-of-day would have far more bearing than latitude, since most human population is within ±45.

I was of the mindset that elevation was more of a factor in UV index than latitude, but I just learned differently. Thank you.

I found a nice discussion of the calculation: https://www.epa.gov/sunsafety/calculating-uv-index-0 It hand waves a bit on how latitude is factored in but states clearly that every km of elevation increases exposure by about 6%.

Mexico City: 2,240 m London: 25 m

So 12% more UV for Mexico City based on elevation. The elevation is important but not the primary factor.

Anecdotally, I have been in Mexico City. I did most of my walking in the mornings and still would sunburn on an sixty minute walk. I did not use any protection.

I'm black, and thought I couldn't burn.

Then I visited Panama.

You learn something new everyday, whether you want to or not. :P

Clouds do tend to diminish UV, but not significantly so as to make you UV-invulnerable.

In the UV tracking app I use, the largest cloud-effects I’ve seen are in the range of 1-3 points decrease. So, UV index of 5-7 instead of 8.

Depending on the baseline UV level during exposure, that difference could be significant (5 index down to 2) or not that significant (14 down to 11), in terms of expected minutes until sunburn.

I've had a few living at 49degrees north and it happens in spring with temperatures around 20. You feel comfortable, light cloud cover makes the sun unnoticeable and you're playing tennis for four hours around noon. Right around dinner time you notice that you've got a pretty solid burn that just crept up on you.

As I understand it (and this seems too pat) UV-B "burns" and UV-A does the cancerous damage. I believe it's to do with depth of penetration.

So you can get skin damage without burn.

I got sunburned on an overcast day in Ireland, of all places.

Walking around a whole day outside without anything covering my head turned out to be a bad idea.

agreed. I live in Canada and can't recall ever getting a burn on an overcast day.

The max UV index in Toronto tomorrow is a 6. On a sufficiently overcast day, that may become a 3, maybe even lower. That would have a very significant effect on odds of sunburn.

Equally overcast on a high UV day in Bangkok on the other hand...

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