The UV index is a measure of how strong the sun's ultraviolet (UV) radiation is at a particular time and location. It is expressed as a number from 0 to 11+, with higher numbers indicating greater intensity of UV radiation.

The UV index is calculated using a mathematical model that takes into account several factors, including:

* The strength of the sun's UV radiation at the Earth's surface

* The time of day

* The location's latitude and altitude

* Atmospheric conditions, such as cloud cover and ozone levels

The UV index is a valuable tool for understanding the risk of skin damage from UV radiation and determining what precautions to take. The higher the UV index, the greater the risk of skin damage.

When the UV index is 3 or higher, the National Weather Service recommends taking the following precautions:

* Seek shade between 10am and 4pm

* Wear protective clothing, including long sleeves, pants, and a hat

* Wear sunglasses that block UV radiation

* Apply sunscreen with at least SPF 30 and reapply every 2 hours

More information about the UV index and your specific area can be found on the National Weather Service website.