What is happening above is DNA absorbs the energy of a UVB ray. If there happen to be two adjacent thymine base pairs, they will combine to form a pyrimidine dimer/thymine dimer. This is unusable, and the body responds in multiple ways. It triggers apoptosis, or cell death. Skin also peels in order to remove the now-dead cells. Finally, a potentially beneficial side effect is the production of melanin to defend the body against future DNA damage.

Clouds can only block UVB rays. When it is completely overcast, only about 1/10 of the UVB rays go through. However, there is an effect called the broken-cloud effect which results in higher UVB radiation on average. This is because the UV rays reflect off the broken clouds, meaning even shady areas receive UV radiation.

Glass also blocks UVB rays well. In cars, the windshield is specially coated to block UVA rays, but the windows are not treated, so UVA rays still permeate through.

How does tanning work?

There are two methods of tanning in humans: the oxidation of melanin, and melanogenesis (creation of more melanin). Melanin is a pigment found in skin that is very effective in absorbing light (99.9% of UV radiation).

Oxidation of Melanin

UVA radiation causes melanocytes to release its melanin which reacts with oxygen to produce the tanning color. It is important to note that the actual amount of melanin does not change because of this.

Melanogenesis

UVB radiation, which causes direct DNA damage, also results in the production of more melanin, or melanogenesis. Melanin absorbs UV radiation very well, and a tan can actually provide protection of SPF 3. The tan that results lasts for a few months.

How does skin cancer occur?

Direct DNA Damage

Direct DNA damage is the result of UVB radiation in which thymine dimers form, described above. However, instead of cell death, the cell may be mutated to create a tumor.

Basal-Cell Carcinoma (BCC)

This is the most common kind of skin cancer. It grows very slowly and is unlikely to result in death.

Squamous-Cell Skin Cancer (SCC)

This is more dangerous than BCC, but not as dangerous as melanoma. If treated quickly, the prognosis is generally good, but still 51,000 out of 2.2 million affected die every year.

Indirect DNA Damage