All About the UV Index

What is UV (ultraviolet) radiation?

• Ultraviolet radiation is a specific part of the sun`s entire spectrum of wavelengths. The entire spectrum includes gamma rays, visible light, infrared light, and radio waves. See this chart for more details.

What are the three types of UV radiation

• UV-C has wavelengths less than 280 nm (nm stands for nanometers - 1 billion nanometers equal one meter). Although extremely dangerous to plants and animals, this part of the UV spectrum is completely absorbed by stratospheric ozone (found between 40,000 and 150,000 feet in the atmosphere) and does not reach the earth`s surface.
• UV-B ranges in wavelengths between 280 and 320 nm. Ozone absorbs much of the shorter wavelength radiation, but this absorption weakens as 320 nm is approached. Plants and animals are particularly affected by this part of the UV spectrum. UV-B effects to humans are:
  • Short-term: reddening of the skin (erythema) and reduction of vitamin-D synthesis
  • Long-term: development of skin cancer, cataracts and suppression of the immune system
• UV-A radiation ranges between 320 and 400 nm. Ozone absorbs very little of this part of the UV spectrum. UV-A radiation is needed by humans for the synthesis of vitamin-D. However, too much UV-A causes toughening of the skin, suppression of the immune system and, to a lesser degree, reddening of the skin and cataract formation.

What is the UV index?

• The UV Index is a next day forecast of the amount of skin damaging ultraviolet radiation expected to reach the earth`s surface at the time when the sun is highest in the sky (solar noon)
• The UV Index can range from 0 (when it is night time) to 15 or 16 (in the tropics at high elevations under clear skies).

What factors determine the UV index?

• Elevation of the sun in the sky - UV decreases rapidly as the sun moves toward the horizon
• Amount of ozone in the stratosphere - ozone blocks harmful UV rays
• Amount of clouds present - clouds block harmful UV rays

How is the UV index calculated?

• The National Weather Service obtains ozone amounts using a special satellite and puts these numbers into a computer model to generate a forecast of tomorrow`s ozone.
• Another model is used to determine the UV irradiances (emissions of light rays) from 290 to 400 nm, using the time of day (solar noon), day of year, and latitude.
• These irradiances are weighted to reflect the human skin`s response to each wavelength.
• National Weather Service forecast models also predict the cloud cover at the time the UV forecast is being prepared for.
• The forecast cloud conditions can be converted into a forecast transmission, which is adjusted for elevation.
• The final result is divided by 25 milliwatts/square meter. For example, if the UV index was published to be a value of 10, the energy reading at the surface of the Earth would be 250 milliwatts/square meter.

What are the dangerous consequences of overexposure to UV radiation?

• Severe sun burn following an intense short term overexposure
• Long term overexposure to UV radiation has been linked to the formation of cataracts in the eyes as well
• Two types of skin cancer:
  • Melanoma, the more deadly kind of skin cancer, occurs when the patient has been subjected to several intense short term overexposures
  • Non-melanoma skin cancers, which are almost 100% curable and occur in people who are overexposed for very long periods of time, like construction workers, farmers, or fishermen.

How can you protect yourself from UV rays?

• Follow the guidelines listed in the "UV Index Chart" in the upper right corner.

Image courtesy U.S. Environmental Protection Agency.