The Ultraviolet Index
The National Weather Service forecasts the amount of ultraviolet radiation that will reach the earth's surface during the peak hour of sunlight. That is at solar noon, which is approximately noon local standard time (1:00 PM local daylight savings time).
The forecast is based on three items:
- The sun's height in the sky
- The amount of ozone in the atmosphere above each location
- The forecast for cloud cover at each location
The sun's height varies during the year, as does the amount of cloud cover and the amount of ozone.
Ultraviolet Index Values (Courtesy of the National Weather Service)
|Index Values||Exposure Category|
|0 to 2||Minimal|
|3 to 4||Low|
|5 to 6||Moderate|
|7 to 9||High|
|10 or greater||Very High|
Here are some of the factors that may affect your exposure to ultraviolet radiation.
- Partly cloudy or variably cloudy days will do little to reduce ultraviolet rays, while rainy or overcast days will reduce the exposure.
- The time of day greatly affects the amount of UV received. The peak exposure time is 1:00 PM Daylight Savings Time (DST). In the continental U.S., the intensity is reduced by about 50% some three hours before and after the peak exposure time.
- The greatest amount of ultraviolet radiation is in the late spring and early summer, while less is received in the late fall and early winter.
- The intensity varies with altitude and latitude. The closer one gets to the equator or the higher one goes in elevation, the higher the index value.
- Smog can reduce ultraviolet intensity. Also, the type of reflective surface can determine the amount of exposure. Water, sand, and snow all have high albedos and can reflect the ultraviolet rays, increasing the exposure.