The UV-B levels at Earth’s surface vary with the time of day, geographic location and season. UV radiation is highest in the tropics, because the sun is higher in the sky, and decreases towards the poles where the Sun is lower. The height of the Sun above the horizon (the solar elevation angle) has an influence on the UV radiation, since the steeper angles mean longer pathways and more opportunity for the radiation to be absorbed. For the same reason, UV radiation is most intense near noon hours and least near sunrise or sunset, and is more in the summer and less in the winter. Clouds, particulate matter, aerosols and air pollutants absorb and scatter some of the UV radiation and thereby reduce the amount reaching Earth's surface.
Locations at higher altitudes (Figure 4) have thinner atmosphere overhead, therefore the radiation from the Sun is less attenuated. This increase in UV radiation varies between 5% and 20% for each kilometre of height, depending on the specific wavelength, solar angle, atmospheric pollution and other local conditions. Frequently, other factors cause even larger differences in UV radiation between different altitudes. Surface reflections, especially from snow, ice and sand, increase the UV radiation levels at a particular location, because the reflected radiation is redirected towards the surface through scattering by particles and molecules in the atmosphere. Snow reflects as much as 90% and dry beach sand and sea foam about 25% of UV radiation. Clouds also reflect an appreciable amount of radiation towards the surface, Thus while areas that are not shaded by clouds usually receive more radiation, under certain cloud conditions levels of UV radiation at Earth’s surface can actually be higher under these conditions than under clear-sky conditions.
The solar UV Index (UVI) describes the level of solar UV radiation relevant to human sunburn (erythema).
The UVI was originally used in Canada. The maximum value in the South of the country is 10 at midday in the summer, and about 1 at midday in the winter. In other locations and under different conditions the UVI can be higher or lower. In the tropics at sea level the UVI can exceed 16 and peak terrestrial values of 25 can occur at high altitudes. The higher the UVI, the greater the potential for damage, and the less exposure time it takes for harm to occur. For fair-skinned individuals a UVI of more than 10 can cause skin damage (erythema) from an exposure of about 15 minutes. Outside the protective layer of Earth’s atmosphere (altitude > 50 km), the UVI can exceed 300.
Information about UV intensities is provided to the public in terms of the internationally adopted UVI colour-scale, along with appropriate health warnings, as shown in Table 5. The colours corresponding to the various ranges are standardised throughout the world.
The UVI can be measured directly with instruments designed specifically to measure sun burning UV radiation. For clear sky conditions, the UVI can be calculated approximately from knowledge of the ozone and the solar elevation angle (Figure 5). However, the UVI at a specific location and time depends strongly on the cloud cover and on the amount of aerosols. Other influential factors include the seasonally varying Sun-Earth separation, the altitude, atmospheric pollution, and surface reflection. When the surface is snow-covered, the UVI can be up to 90% greater than for snow-free surfaces. Several countries provide forecasts of UVI that take predicted changes in ozone and cloud cover into account. Further details about the UVI can be found at