The sun is the origin of ultraviolet radiation reaching the earth. This radiation is absorbed by the components of the Earth's atmosphere including ozone. At higher latitudes the rays of the sun have to take a longer path through the atmosphere before they reach the Earth. In general, the level of ultraviolet radiation is higher at the equator and diminishes towards the poles. The levels increase with elevation above sea level. Clouds, particulate matter and aerosols absorb ultraviolet radiation and diminish ultraviolet levels. The level of the ultraviolet radiation at any particular location is determined by a combination of all these factors.

The UV-B levels vary with the time of day and season. The highest level occurs when the sun is at its maximum elevation, around midday during the summer months. The radiation levels are higher in the intertropical regions.

On average, the highest ultraviolet radiation levels occur on cloudless days. Cloud cover reduces the radiation but it can still be high under partly cloudy conditions when the sun is not obscured. Evidence shows that over the time span of satellite-based ozone measurements, changes in cloud cover have been much less important than stratospheric ozone reductions in causing surface UV changes.

Higher elevations have less atmosphere overhead, as evidenced by the thinner air and lower atmospheric pressure. The increase in UV radiation varies between 10% and 20% for each kilometre of elevation, the exact number depending on the specific wavelength, solar angle, reflections, and other local conditions.

Frequently, other factors besides thickness of the atmosphere cause even larger differences in UV radiation between elevations. Snow is more common at higher elevations, and reflects as much as 90% of the ultraviolet radiation. Dry beach sand reflects about 15% and sea foam about 25%.