The amount of ultraviolet (UV) radiation from the sun reaching the Earth’s surface, and accordingly the UV index, depends on several factors, the most important of which are time of day and season.

UV radiation is usually strongest for a few hours around noon and less strong during the early morning and the late afternoon/evening (see Figure 1).

Figure 1: Daily sun profile.

Source: Courtesy of Ameco Solar at solarexpert.com

In summer, about 20–30% of the total daily amount of UV radiation is received between 11:00 and 13:00, and 75% between 9:00 and 15:00 (sun time, not local time). Seasonal variations in UV radiation reaching the Earth’s surface are substantial in temperate regions but much less nearer to the equator.

Other important factors that influence the strength of UV radiation at the Earth’s surface are:

• Geographical latitude (annual UV doses decrease with increasing distance from the equator).
• Altitude (in general, each 300 metres increase in altitude increases the sun-burning effectiveness of sunlight by about 4%).
• Surface reflection (e.g. snow, water) (snow reflects up to 85% of UV, and water about 5–10%; reflected UV can damage skin as much as direct UV).
• Clouds (the amount by which the UV radiation reaching the Earth’s surface is reduced by clouds depends on the thickness, density, and shape of the clouds).
• Air pollution (like clouds, urban smog from air pollution modifies the amount of UV radiation reaching the Earth’s surface).