Ultraviolet Radiation may be high when the temperature is cool. Don’t get burned. Did you know that the level of ultraviolet (UV) radiation begins to rise in the morning, levels off around noon, then falls gradually throughout the day, yet the temperature may continue to rise until the evening?

Jim Miller, a senior meteorologist at the National Meteorological Center, National Weather Service (NWS), says “UV radiation is not temperature-dependent.”

Therefore, you can’t always gauge your solar radiation exposure by how hot it is.

A study in Australia found that more people were sunburned on cooler days because they were tricked into thinking they could safely spend more time outdoors, says Mary O’Connel, Director of Skin Cancer Control Division of the American Cancer Society.

Solar radiation–visible, infrared and ultraviolet–is the electromagnetic radiation emitted by the sun. Ultraviolet light, composed of UVA, UVB and UVC, is divided into wavelengths that are measured in nanometers (nm) and are shorter than visible light. UVA is found along the 320-400 nm range of the electromagnetic spectrum, UVB, along the 280-320 nm range, and UVC, along the 190-280 nm range.

Ultraviolet radiation (UVR) becomes more intense the closer to the equator you travel. Likewise, the higher into the mountains you go, the thinner and cleaner the air, the more UVR will strike you. UVR also fluctuates with the seasons, time of day, and atmospheric variables, such as clouds, smog and haze.

UVA and UVB vary, depending upon the angle of the sun to the surface of the earth and the relationship of the earth to the sun. What’s more, UVB is also dependent upon the effect of the ozone and atmospheric variables, such as haze, says Craig Long, meteorologist at the NWS. UVC is filtered by the ozone layer.

“Clouds are not good absorbers; most of the UVR is reflected back up to the atmosphere. But cloud formations, such as puffy, light cumulous clouds don’t do a good job of blocking UVR, so you’re likely to receive full sun exposure,” says Long.

How Radiation From the Sun is Forecast and Detected:

Two distinct methods are performed to gather data for the UV index, a numerical scale that represents the dose rate of UVR. First, the NWS makes forecasts, using the National Oceanic Atmospheric Administration (NOAA) satellite to measure the total stratospheric ozone amount. Next, based on the readings, calculations and revisions are made. Also, weather conditions are taken into account, such as temperature, air circulation, and cloud amounts. Then, the NWS predicts the amount of UV radiation expected to occur the next day at solar noon, when the sun is directly overhead. Finally, this information is then translated into an open-ended UV index.

“We have to be careful about not presenting the index with an upper limit,” cautions Jim Miller, “because at high altitudes, for example, the UVR level could be as high as 12 or 13. And, in the future, the ozone level could change.”

In addition, “when there are clouds, we tend to over forecast, predicting a dose rate of 3 on the UV index when it is actually 1, and when it’s a clear day, we under forecast, predicting 3 when it is 4,” reports Long.

The NWS doesn’t participate in direct measurement and detection of surface UVR. Other government agencies, such as NOAA, the Environmental Protection Agency , the United States Department of Agriculture , the National Science Foundation, and hospitals, such as the M. D. Anderson Cancer Center in Houston, affiliated with the University of Texas at Austin, help us to validate our forecasts, says Long.

These collaborative efforts have assisted the NWS in determining the accuracy of its forecasts. How accurate are the forecasts? The forecasts are correct 25% of the time, within 1 index number 65% of the time, and within 2 index numbers, 85% of the time, says Long.

M.D. Cancer Center’s Surface UVR Detection MD. Anderson Cancer Center is one of the groups with which the NWS exchanges data. Unlike the NWS’s predictions of UVR levels for the next day, M.D. Anderson Cancer Center detects and measures UVR directly … in real time, four times a day.

Instrumentation

There are two pieces of equipment that help measure UVR–one on the rooftop and one in the building. The broadband detector, which is placed on the rooftop, is a UV light meter with a quartz dome. First, sunlight passes through the dome, strikes a black filter called a solar blind filter, and visible light is filtered out. Next, UV light illuminates phosphor beneath the filter. (The phosphor represents skin’s reaction to UV radiation.)

After that, a photo-cell converts UV light to an electrical impulse, which travels along a cable to a recorder inside the building. Then, the recorder measures UV intensity as minimal erythema dose (MED). ( MED represents how long it takes the skin to begin burning, and erythema is the reddening of the skin caused by the dilation of blood vessels.) Finally, MED is recorded as a numeric value on a scale from one to 10+.

This summer the NWS, National Meteorological Center, Camp Springs, MD., will again send the daily UVR intensity forecasts to the NWS offices throughout the U.S. The NWS offices in turn will issue forecasts and advisories to 58 cities across the nation.

The following table of index numbers represent the range of UVR intensity possible on a given day.

UV Index and Exposure Levels:

  • 0-2 Minimal
  • 3-4 Low
  • 5-6 Moderate
  • 7-9 High
  • 10+ Very High

About the Author

Diana Clarke is a California credentialed teacher and freelance writer. Her sun protection articles have appeared in publications, such as the San Jose Mercury News, Saratoga News and a high school health magazine, Listen Magazine.

Written by: Diana Clarke