What parameters cause sunburn? [closed]

Question

I am new to this site so if the question doesn't fit in this forum feel free to remove it.

I have read here https://en.wikipedia.org/wiki/Sunburn of all the causes of sunburn and I was mostly aware of all of them. I will list them here:

($1$) The time of day. In most locations, the sun's rays are strongest between approximately 10am and 4pm daylight saving time.

($2$) Cloud cover. UV is partially blocked by clouds; but even on an overcast day, a significant percentage of the sun's damaging UV radiation can pass through clouds.

($3$) Proximity to reflective surfaces, such as water, sand, concrete, snow, and ice. All of these reflect the sun's rays and can cause sunburns.

($4$) The season of the year. The position of the sun in late spring and early summer can cause a more-severe sunburn.

($5$) Altitude. At a higher altitude it is easier to become burnt, because there is less of the earth's atmosphere to block the sunlight. UV exposure increases about 4% for every 1000 ft (305 m) gain in elevation.

($6$) Proximity to the equator (latitude). Between the polar and tropical regions, the closer to the equator, the more direct sunlight passes through the atmosphere over the course of a year. For example, the southern United States gets fifty percent more sunlight than the northern United States.

My question is, can we compose ($1$), ($4$) and ($6$) into one statement: "Height of sun over the horizon"?

Wikipedia seems to hint that we can since it says:

Regardless of one's latitude (assuming no other variables), equal shadow lengths mean equal amounts of UV radiation.

However, the statement is kind of vague and I would like confirmation.

Another doubt I have regarding this is experience. I live in Europe where solar noon is at 1pm during summer and the day is longest on June 21 (or 22, whatever). But, people seem to get way more sunburn in, say, August 21 than in April 21, and also more at 4pm than at 10am (regardless of the month). It is also hotter in August than in April and at 4pm than at 10am. So it might be that people simply spend more time out because it's hotter and therefore get more sunburn, but it also might be that the temperature affects our skin somehow and we do get more sunburn when it's hotter.

So, taking clouds, reflective surfaces and altitude out of equation, is it true that sunburn is caused just by the height of the sun above horizon? If I spend 5 minutes at some place on a sunny day at noon in April, will I get the same amount of sunburn as I would at 3pm in June? (given that the sun is at the same height over the horizon at that times, and that 5 minute span is short enough to approximate that sun stays "fixed" in the sky)

My intuition says it should be true that height over horizon is the only parameter that should matter, but my experience shows different. Moreover, if I am correct, is there some graph/table showing how much intensity (or some other unit?) of sunburn I get with respect to height of sun? (say at zero altitude on a sunny day without reflective surfaces, or at some other, fixed, reference configuration).

Answer 1

Yes, all of those contribute to the total irradiance, which is the amount of sun power falling on a particular area, measured in Watts per square meter. You can imagine a 1m$^2$ "window" perpendicular to the sun's rays - excluding atmospheric and weather effects, the amount of sunlight passing through that window never changes. But when the sun is directly overhead, that window illuminates a 1m$^2$ patch of ground, while later in the day that same sun power is spread over a larger area. Only the angle of the sun affects how big that area is, and that is determined, as you point out, by time of day, season, and latitude.

Because of this, the length of your shadow is a good tool to estimate the angle of the sun and your risk of sunburn. I tend to be a bit sun-sensitive, but if my shadow is longer than I am tall (i.e. the sun is below 45 degrees), I'm usually fine - that's most of the day in winter, but excludes the hours around noon in the summer. That angle will of course have to be adjusted based on your personal sensitivity.

As for your experience, I would imagine that people tend to expose more skin in the hottest month of the year (August), increasing their sunburn risk over a period with higher irradiance but less exposure (June). Sunburns also will also often show up in the afternoon because they're a result of accumulated damage throughout the day. If an individual takes X hours of exposure to burn, they're more likely to hit that mark later in the day, rather than during the peak of irradiance.

Answer 2

Yes, you can roll parts 1,4 and 6 into one because, as you say, it all boils down to how high the sun is above the horizon. But then you have to take altitude into consideration. Most people live less than 2,000 feet above sea level, so they can ignore it, but Mexico City is 8,000 feet up, so sensible inhabitants would be wise to bear this in mind. Reflective surfaces? Not many people go sunbathing in the snow, and the few that do don't expose their skin for very long. The danger in snow conditions is snow blindness, which in severe cases could cause permanent damage to the eyes, so sunglasses would be a good idea if you spend much time in sunny snowfields. People shouldn't get paranoid about sun exposure; a moderate amount of sunlight is good for us and enables our skin to manufacture vitamin D. People vary considerably in their sensitivity to sunlight, and what's alright for me won't necessarily be alright for you. Darker skins, of course, are less sensitive than pale skins. People with red or pale blonde hair need to be cautious.