The amount of ozone in the atmosphere varies, and so the intensity of UV-irradiance on the earth's surface does.
Strong short term variations of total ozone in the northern hemisphere are observed in the months of January, February and March. The lowest concentrations of total ozone there can be found in late fall.
The UV-Index denotes solar irradiance that may affect your health.
The actual effect on the human skin depends on skin type and tanning.
You can identify your skin type according to the following characteristics: