I am curious if anyone else can tell whether it is a high UV day just by looking outside.

There is a certain look to the sky (slightly whiter-bluer) looking against the clouds, and I am not sure if it is because I am directly seeing UV, whether there is some fluorescence, or whether it’s placebo effect.

the sun may still feel pretty intense in the morning or late afternoon, but the risk of sunburn is considerably lower

This is very misleading. By some coincidence I was just looking into this earlier today.

The lower the UV index, the smaller the window is between the amount of time needed to get enough Vitamin D and the time to get burnt (erythema). So the best thing to do is get sunlight mid-day for a shorter amount of time.

UV Radiation: Balancing Risks and Benefits https://onlinelibrary.wiley.com/doi/10.1111/j.1751-1097.2008...

  As the UVI becomes smaller, it becomes more difficult to produce sufficient vitamin D without inducing erythema. For low sun conditions typical of midday in the mid-latitude winter (SZA ∼65°), where R∼1, the skin area exposed must be more than 12% (see Eq. 5) This means that for these low sun conditions it is improbable that sufficient vitamin D can be produced from exposing the hands and face alone, since that area is less than 12% of the full body.

  Figure 6 shows that for full body exposures, there is a wide window between the time for sufficient UV and the time for too much UV. As the fraction of body that is exposed decreases, the window of optimum UV exposure times also decreases. If only hands and face are exposed, there is generally only a small window between receiving insufficient UV for vitamin D production, and too much UV for skin damage (erythema).

You would do well to calculate the numbers for your own city, as I did earlier today for mine:

https://chatgpt.com/share/6ab2f272-a32c-83ea-9fec-49031e4da3...

Isn't think only true if getting enough vitamin D is as important to you as cancer?

I really appreciate learning this fact, but I can't tell how it makes the quoted sentence misleading. The author is just talking about the surprising disparity between how warm it feels and how fast you will burn. They don't mention vitamin D.

I know they don’t mention vitamin D, but I think it’s important for people to know the context and not think that late day sunlight automatically = better

That's good to know as an additional factor to consider, but it's not enough to justify your accusation that the original statement is "misleading".

You're allowed to just say "cool, here's another important thing to think about it". You don't have to pick a fight.

it really only takes about 5 minutes of exposure to get the vitamin D your body needs. i would happily do it between 7-9AM when the UV is low than doing it at mid day.

The statement you quoted is very useful [One of the most useful heuristics], and your statement is misleading. You accumulate damage (Photoaging, cancer risk) dramatically faster at high sun angles (i.e. mid day, close to the summer solstice, closer-to-equatorial latitudes) etc than low ones.

I am not trying to be negative, but this is very important if you have pale skin. Being outdoors in morning and late afternoon is dramatically less damaging than mid day.

It kind of depends on context.

If your goal is "hang out in the sun but don't get burned", then morning/evening is better. If your goal is "get exactly as much vitamin D as I need while minimizing UV damage, perhaps using a timer", then it sounds like technically mid-day is better (assuming I'm reading the parent right).

I think I got the worst sunburns when I didn't feel hot at all.

Usually, it involves water, water cools you down, so you don't feel the heat, but it doesn't block UV, so you don't get the warning.

Overcast days are the worst offenders for sunburn because the melanin impaired don’t feel the heat of the sun to remind them they are going to roast.

I have always heard sunny snow days are the worst, because you feel cold, but the sun is still hitting you, plus reflecting off the snow to hit you twice.

Depends which part of the world you are in. I've never been sunburnt on an overcast day where I live.

Sunburn is triggered when your DNA is damaged from light absorption [...] This interferes with protein synthesis and wreaks havoc in your body over time

Not a skin-scientist, but this seems misleading: Sunburn is not caused by some bad DNA floating around, nor damaged cells making making mistakes or pumping out wrong proteins to inflame their neighbors.

Instead, your cells have techniques to measure a variety of risk factors and signals (which does include DNA-molecule weirdness [0]) and when things get too risky the well-behaved ones will self-destruct, to avoid the chance of turning into a cancer.

So the sunburn you see and feel is instead due to cells signalling "something damaged me and I can't be trusted, I'm shutting down, recycle me" to your immune-system, plus the attendant bustle of hazmat-cleanup and increased security-sweeps for cancer-leftovers and other potential causes of the damage.

[0] https://www.nature.com/articles/newbio241074a0

Does anyone know how UV indexes reported by weather apps (including forecasts) work?

I've found that some sources report lower UV index on cloudy days, whereas some seem to indicate the same index as a sunny day.

It's not clear if these sources all have a different model for UV, some being more sophisticated or conservative, and whether any of them are backed up by a UV meter.

It is a prediction of UV intensity based on specific forecast location, time of day, day of year, prevailing weather, and any anomalies affecting the ozone layer. It is calculated by NOAA.

https://www.cpc.ncep.noaa.gov/products/stratosphere/uv_index...

This depends on the country and the source. Here in Australia BOM UV values are predicted, but cancer council are observed.

Good post - the UV index is related to how short the "column of air" that the sun traverses to get to your skin - noon is the shortest distance. Scattering is a density function (more air, more scattering), so this is why it looks like cos^p when compared to normal irradiance (cos). It also explains why it is hard to get a sunburn in the temperate winter, since the sun always appears at an angle, it doesn't go directly overhead.

I've heard a quick way to determine if the UV index is dangerous is if your shadow is shorter than you. Though location matters too.

I moved from Vermont to Texas and had to purchase an umbrella with UV protection to walk around during the day. The highest the UV index hits in the Vermont's summer is 6. In Texas, it's 11+ !!! 15 minutes and you're toast