If you have read How Sunburns and Sun Tans Work, you know that your skin is sensitive to ultraviolet (UV) radiation. The UV index is a value calculated each day that helps you to know how much UV radiation will reach you if you go outside. By knowing the index value for the day, you can take appropriate precautions. These precautions help you avoid sunburn in the short term and skin cancer in the long term.

The UV index is calculated based on four factors:

• The thickness of the ozone layer over your city (detected using satellites)
• The cloud cover over your city (clouds block UV radiation to varying degrees)
• The time of year (in winter, UV radiation is lower than in the summer because of the sun's angle)
• The elevation of your city (higher elevations get more UV radiation)

The UV index value ranges between 0 and 10, with zero being minimal UV exposure risk and 10 being maximal UV exposure risk. This page lists the proper precautions to take for the different index values.

Here are some interesting links:

• How Sunburns and Sun Tans Work
• EPA: What is the UV index?
• EPA: How is the UV Index Calculated?
• Ultraviolet Index: What You Need to Know
• UV Index FAQ
• NSC: What is the UV Index?
• UV Intensity maps