How to Understand the UV Index

The UV Index was developed in 1994 by the National Weather Service and the U.S. Environmental Protection Agency (EPA). It helps you plan your outdoor activities so you can avoid exposure to intense UV radiation.

What Is the UV Index?

Every day the UV Index is calculated for the next day for every zip code in the U.S. It predicts the intensity of UV radiation at noon and is reported on a scale of 1 to 11+.

On this scale, 1 signifies the lowest risk of overexposure and 11+ indicates the highest risk of overexposure. UV Index numbers are also grouped into exposure levels ranging from low to extreme and each exposure level has a corresponding color code.

Factors that Affect the UV Index

The intensity of UV radiation, and thus the UV Index, depends on several factors:

Season: The UV Index is highest in spring and summer. It goes down in the fall and is the lowest in the winter.
Latitude: UV radiation is strongest at the equator and goes down as you move towards the north or south poles.
Altitude: Because air at higher altitudes is thinner, UV radiation goes up as you "go up" in altitude.
Time of Day: When the sun is highest in the sky, the sun's rays beat straight down on you with very few of the rays getting scattered. This means that the intensity of UV radiation is highest at the time of "solar noon," usually somewhere between noon and 1 p.m. When the sun is at an angle other than 90 degrees to the earth, some UV radiation is scattered lowering the intensity that affects your skin.

Ozone: Ozone absorbs UV radiation making it less intense. Ozone levels can fluctuate from day to day.
Cloud Cover: Thick, heavy cloud cover can block most UV radiation, however, thin clouds can let most of the UV rays through. Fluffy, fair-weather clouds are deceiving because they reflect the rays and can increase the amount of radiation reaching Earth.

Land Cover: It makes sense that structures like trees and buildings lessen the amount of UV radiation that hits your skin.
Earth Surface Characteristics: Whatever is coating the surface of the Earth around you can reflect or scatter UV radiation. Snow reflects up to 80%, while sand reflects 15% and water reflects 10%.

How to Find Your UV Index

You can find your UV Index by visiting the EPA's UV Index site. There you can look up the UV Index for your zip code. There's also a 4-day UV Index Forecast map of the United States to help you plan your outdoor activities for the next couple of days.

_Sources:

Kinney, John P, Craig Long, and Alan Geller. "The Ultraviolet Index: A Useful Tool." Dermatology Online Journal. 6(2000): 2.

Ramirez, Raymond and Jeffrey Schneider. "Practical Guide to Sun Protection." Surgical Clinics of North America. 83(2003): 97-107.

United States Environmental Protection Agency. "A Guide to the UV Index." May 2004: 1-8.

The UV Index