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Meteorology (Weather)/UV index and solar angle


Good afternoon Mr. Moran. I'm a weather enthusiast with a nagging question I've finally decided to ask and hopefully get answered related to UV index, latitude and time of year. What better time to do that than on my lunch break!

Latitude, time of day, time of year, and cloud cover, are the principle drivers of daily UV indexes. Here's my dilemma, encapsulated in the scenario below:

By mid February South Florida, at about 26N, will begin to see midday UV indexes around 6-8 out of the 10+ scale. Especially under mostly sunny conditions. On Feb. 15, the peak solar angle for South Florida is 51.1 degrees off the horizon at noon.

New York City, at 40.8N, will see the same solar height off the horizon on about March 24. But in late March UV indexes at that latitude under mostly sunny midday skies are only around a 5 or "moderate." UV indexes of 6 arrive by early April and 7s and 8s dominate most of April and May, with 9s rounding out the end of May, continuing through early August.

My question: If the sun is at the same angle at two places on Earth, why is there a UV intensity difference under similar sky conditions? Why is it not totally accurate to say "the late March New York sun is as intense as the mid February sun in South Florida despite identical heights above the noon horizon?"

Hi Daniel, I actually had to look this up as we weren't taught about the UV index in college. While the drivers you listed are definitely factors, other factors such as air pollution, surface albedo, and elevation, which vary by location, can also be factors.

Hope this helps.

Meteorology (Weather)

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David Moran


I can answer questions about radar and large scale meteorology. If a question looks like homework, I won't answer it.


I have been a forecaster for many years with a specialty in severe weather.

Full Member, American Meteorological Society

BS in Meteorology, University of Oklahoma BA in Mathematics, University of Oklahoma

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