Tanning and burning

Lots of questions …

Are rays from the sun and tanning salons the same?

It seems like tanning booths are generally stronger than the sun… is that correct?

How does the sun’s intensity vary based on time of the year, time of the day and latitude?

Light in a tanning booth is not the same as the sun. Tanning beds have a different proportion of UVA, which cause sunburns. However, even the UVB, which promote tanning, can cause skin damage.

Wikipedia says that tanning beds can be 3-8 times stronger than sunlight.

The sun’s intensity at the surface of the earth depends on all the factors you list plus weather. The closer you are to the equator, the stronger the sun’s rays. The sun’s rays are strongest in the northern hemisphere on June 21, and weakest on December 21 (the opposite is true in the southern hemispere). The rays are most direct at noon, and less so earlier and later. Clouds can diffuse the sun’s rays, but UV rays still reach you on a cloudy days. Other atmospheric conditions affect the strength of the sun’s rays but I’m not familiar with that.

On a second look, I think you might be looking for something quantitative. What details are you looking for?

My doctor always told me to avoid the sun during peak hours- 11- 2 pm since you are most likely to burn during the middle of the day. I think a lot of ppl think that if they have a tan and not a red burn then their skin is fine but again my derm said sun bathing of any kind- blistering burn or just a tan is bad for you skin.

Yes…a little more quantitative.

For example, in the Northern Hem the relative intensity on 4/21 vs 6/21 all other things being equal.

Or Miami vs Chicago at the exact same time.

At the equator, I’d assume that the intensity doesn’t vary based on the time of the year, right?

Well, it seems to me that the intensity of the rays as they vary by latitude would be proportional to the angle of incidence, which is I think related to the sine of the latitude. So I would say

intensity = c*sin(90-latitude)

where c is some constant that I don’t know, for the northern hemisphere. This was quick & dirty so I may not have it quite right but this is the general idea.

Earth tilts on its axis 24.44[sup]o[/sup], so the intensity at the equator does vary by time of year, but not a lot. On the summer solstice in the northern hemisphere, the most intense rays would be at 24[sup]o[/sup] 26’ N, which happens to be called the Tropic of Cancer (Tropic of Capricorn is the same latitude in the south; the area between the two tropics is the band around Earth where the sun’s rays fall perpendicular at least once during the year).