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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 Ci

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.

megablast 5y ago
I’ve never had a sunburn from a cloudy day in Australia.
- godelski 5y ago
  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.
D1CA1890 5y ago
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.
SapporoChris 5y ago
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 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.

megablast 5y ago

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

godelski 5y ago
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.

D1CA1890 5y ago

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.

SapporoChris 5y ago

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 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.