UV ray intensity and the calendar

It stands to reason that UV rays and therefore the danger of sunburn is greatest on or about June 21 (in the northern hemisphere), the solstice. And to extrapolate, the period from June 1 to July 15, the first half of the summer, would be the most intense 6-week period. However, it seems that July 15 to August 31 is sunburn season. Is this because:

  1. I just think it is, because the temperature is hotter during that time
  2. I just think it is, because that’s the period of more summer activity (mostly due to #1)
  3. It really is, because of some other atmospheric factors besides the angle of the sun
  4. Other

Your supposition is wrong. Summer is warmer because of the tilt of the earth, not proximity to the sun. The Earth’s orbit is circular, for the most part.

http://science.nasa.gov/headlines/y2001/ast03jul_1.htm

Sunburns happen because it is warmer though…and people have their clothes off. Or at least less clothes. :slight_smile:

:confused: Where did the OP make any supposition to the contrary? :confused:

If your skin is exposed to the sun, the determining factors in how quickly you burn are sun angle and cloud cover. Here in Great Lakes country, it tends to be clear more often in July and August than in June, so this compensates to some degree for the lower sun angle. I’m not sure what the cloud cover is like in Virginia. And obviously, the warmer it is, the more you’re going to uncover.

I think UV intensity varies minute to minute according to how high the sun is in the sky, because air is somewhat opaque to UV and the lower the sun is the longer its path through air.

It also should vary with the local barometric pressure, because the barometric pressure is the weight of all the air above you. If it is lower (either because you’re on a mountaintop or because of the weather), the UV will be stronger.

It also would vary with the tranlucency and transparency of the air. I don’t think air’s translucency to UV varies much because its composition doesn’t vary much, and the biggest variation is water vapor which is pretty transparent to UV. But the transparency would vary with clouds and precipitation, changing the importance of direct and indirect light in different ways.

People are exposed more when they take their clothes off, but in general the winter typically offers less exposure because the sun’s angle is lower in the sky.

Finally - and maybe this is the answer OP was looking for - does atmospheric ozone vary seasonally in populated climes? It certainly does in the polar regions.