Sunburn is traditionally considered an indicator of the attractiveness and a symbol of good health. Small amounts of UV are beneficial for people and essential in the production of vitamin D and serotonin, which is responsible for mood and overall vitality.Prolonged human exposure to solar UV radiation may result in acute and chronic health effects on the skin, eye and immune system. Over the longer term, UV radiation induces degenerative changes in cells of the skin, fibrous tissue and blood vessels leading to premature skin aging, photodermatoses and actinic keratoses. In the most serious cases, skin cancer and cataracts can occur.
Independently assess the risk of sunlight is very difficult, often believed that simply go into the shadows and do not stay in the sun during the hours of maximum solar activity. But the level of UV radiation is influenced by various environmental factors, which also must be considered.
Do you know that:

- Sun height—the higher the sun in the sky, the higher the UV radiation level. Thus UV radiation varies with time of day and time of year, with maximum levels occurring when the sun is at its maximum elevation, at around midday (solar noon) during the summer months.
- Latitude — the closer the equator, the higher the UV radiation levels.
- Cloud cover — UV radiation levels are highest under cloudless skies. Even with cloud cover, UV radiation levels can be high due to the scattering of UV radiation by water molecules and fine particles in the atmosphere.
- Altitude — at higher altitudes, a thinner atmosphere filters less UV radiation. With every 1000 metres increase in altitude, UV levels increase by 10% to 12%.
- Ozone — ozone absorbs some of the UV radiation that would otherwise reach the Earth’s surface. Ozone levels vary over the year and even across the day.
- Ground reflection — UV radiation is reflected or scattered to varying extents by different surfaces, e.g. snow can reflect as much as 80% of UV radiation, dry beach sand about 15%, and sea foam about 25%.

All of these factors takes into account the UV-index. As part of an international effort, the UV-index was developed by WHO, the United Nations Environment Programme, and the World Meteorological Organization.

http://www.who.int/…/intersunprogra…/activities/uv_index/en/
http://www.who.int/uv/uv_and_health/en/

Current UV index (Clear Sky) and historical data are available for any geo location (lat/lon) in JSON http://openweathermap.org/api_uv
Data feed is included into Professional and Enterprise accounts http://openweathermap.org/price