Skin Health Skin Care & Cleansing Products Print How to Understand the UV Index By Heather Brannon, MD Updated May 18, 2019 Stephen Dorey/Stockbyte/Getty Images More in Skin Health Skin Care & Cleansing Products Anti-Aging Skin Care Hair & Scalp Care Acne Psoriasis Eczema & Dermatitis Fungal, Bacterial & Viral Infections More Skin Conditions 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. The UV Index UV Index Number Exposure Level Color Code 2 or less Low Green 3 to 5 Moderate Yellow 6 to 7 High Orange 8 to 10 Very High Red 11+ Extreme Violet Was this page helpful? Thanks for your feedback! Sign up for our Health Tip of the Day newsletter, and receive daily tips that will help you live your healthiest life. Email Address Sign Up There was an error. Please try again. Thank you, , for signing up. What are your concerns? Other Inaccurate Hard to Understand Submit Article Sources United States Environmental Protection Agency. "A Guide to the UV Index." May 2004: 1-8. 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. Continue Reading Choosing the Best Sunscreen Can Reduce Risks of Sunburns and Skin Cancer The Effects of Sun on the Skin Can Unprotected Sun Exposure Be Good for You? Care for Your Skin With Dermatologist-Recommended Skincare Products What You Should Know About Phototherapy for Psoriasis Discover the Best Sunscreens to Wear If You're Going on a Cruise Discover How the Ingredients in Sunscreen Work to Protect Your Skin The Basics of Healthy Skin Care How Using Sunscreen Can Help Prevent Sun Damage and Premature Aging Why Sunless Tanning Products Are a Safer Choice Than Tanning Beds Discover the 7 Things That Cause Wrinkles in Men Are Tanning Pills Safer Than Tanning Beds or Laying out in the Sun? Understanding the Signs and Symptoms of Xeroderma Pigmentosum Acne Medications That Make You More Sensitive to the Sun Rank Your Level of Sun Damage and Prevent More Wrinkles With Sunscreen Does Tanning Really Clear Acne?