How reliable are UV indexes? Genuine question. The iOS weather app is far from 100% reliable and I wonder the margin of error regarding the UV index number it provides.

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lurking_swe 4 days ago | parent | next [-]

great question! My understanding is the UV index shown online and in apps is “modeled” data, not realtime.

Basically it takes into account things like ozone measurements from satellites, latitude, forecasted cloud cover, and distance from sun (time of year).

See section “Calculating the UV index” here:

https://www.epa.gov/sunsafety/learn-about-uv-index

It’s not intended to be realtime. It’s more about understanding, generally, what the risk is.

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hollerith 4 days ago | parent | prev | next [-]

On cloudless days it it very accurate because on those days UV strength depends almost entirely on the solar altitude, which can be calculated very accurately from the location, the date and the time.

A sibling comment mentions the ozone layer, but I severely doubt it varies enough to be a source of inaccuracy.

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ComputerGuru 4 days ago | parent | prev [-]

I am in a unique position to confirm that they are a load of bunk. I have solar urticaria and develop hives in response to UV exposure, directly proportional to how much UV is getting through. I’ve developed hives in minutes while the UV index was supposedly only 4 and gone for relatively too long without erupting in hives the next day even when the UV index was supposedly 10.

	▲	lurking_swe 4 days ago \| parent [-]
		i hate to be that person that quotes chatgpt, but this seems VERY relevant to your complaint: “Solar urticaria is a rare condition where the skin reacts to specific wavelengths of light rather than the overall UV intensity. The UV index is a general measure of the total amount of erythema-causing UV radiation (mainly UVB) that can cause sunburn in the average person. But in solar urticaria, the trigger might be UVA, visible light, or even a narrow band of wavelengths — and the UV index doesn’t capture that nuance. So it’s not that the forecast is wrong — just that the UV index isn’t designed to reflect the sensitivity profile of solar urticaria.” In other words, you’re (literally) a special case. :)