UV Index defined: what it means and how it’s measured

Weather Glossary

The Ultraviolet (UV) index is a tool used to measure the amount of UV radiation being emitted from the sun and reaching the Earth’s surface on any given day. It was developed by both the National Weather Service (NWS) and the US Environmental Protection Agency (EPA) back in 1974 in order to help communicate the adverse effects of the sun’s harmful UV rays.

There are many factors that affect UV index including:

  • Cloud cover
  • Time of Day
  • Land Cover
  • Latitude
  • Seasons
  • Altitude
  • Ozone
  • Earth Surface Characteristics 

The amount of UV radiation that reaches the Earth will vary throughout the day, but the UV index itself describes the highest predicted number at solar noon. 

Image courtesy of the EPA

Too much exposure to UV radiation from the sun can pose significant health risks such as sunburn, skin cancer, and other related illnesses such as melanoma. Those with sensitive skin should take such action in order to protect themselves.

The current scale used to report UV index has been implemented since May of 2004, and goes from 1 to 10+ from lowest to extreme exposure values. Below is the UV index scale implemented by the EPA.

Here are precautions you can take to protect yourself from UV rays:

  • Use sunscreen with at least Sun Protection Factor (SPF) 15 and reapply regularly when needed
  • Seek shade and limit exposure outdoors during midday hours
  • Wear hats with wide brims that protect the face, eyes, and neck
  • Wear sunglasses with proper UV protection

The UV index is not only useful to the public, but useful for communicators to pass the information along to help keep people safe while out in the sun.

Copyright 2020 Nexstar Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

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