What is the UV index?

Developed by the National Weather Service (NWS) and Enivronmental Proctection Agency (EPA), the UV Index rates the potential of solar ultraviolet radiation to cause sunburn on a scale of 1-11+. The daily UV index is the sunburn potential around solar noon (because of daylight savings time, solar noon occurs around
1 PM in the summer). This helps people determine appropriate sun-protective behaviors.

NWS and EPA typically make forecasts of the next day's UV index, SERC uses UV monitoring data to assess the sunburn potential at our location in “real-time”.

How and Why Do We Monitor the UV Index?

Our research is focused on understanding the impacts of UV radiation on phytoplankton and other marine organisms, but the measurements can also be useful for people planning a day out in the sun.

To measure the amount of certain UV radiation reaching the ground each minute during the day, SERC scientists developed an Instrument called the SR18 radiometer. The instrument sits atop our research tower at our main site in Edgewater, Maryland (image at right). The radiation we measure with the SR-18 represents the part of the UV spectrum most responsible for sunburn.

[More on the SR-18, the ecological impacts of UV and what we're doing in this area >>]

How is the UV index Calculated?

Not all UV is created Equal

Although most people tend to think of UV in two varieties, UV-A and UV-B, UV really represents an entire spectrum of radiation with wavelengths variyng from 280 nanometers (nm) to 400nm. As wavelengths change, so too does radiation's ability to do damage. UV-B represents radiation between 280 and 320nm and is responsible for sunburn to human skin. Within that range, radiation of shorter wavelengths is more damaging than that in the longer range.

For example, a person will get a sun burn more quickly when exposured to radiation with a wavelength of 295 nm than when exposed to 305 nm wavelength radiation.

How does that relate to the UV Index?

The proportion of radiation of each wavelength can vary dramatically with atmospheric conditions. So on two given days, the total UV radiation reaching us may be the same, but the balance between the most damaging wavelengths and the least damaging wavelengths may be very different, and therefore, the potential for sunburn may be different too.

The UV index takes these differences into account and a series of calculations is made to determine the damage potential of the UV reaching the earth at a given moment.

In simple terms, each wavelength is given a value based on its ability to burn skin. The amount of radiation reaching the earth in each wavelength at a given point in time is multiplied by its value and all of the results are then added together to form a number that represents the UV Index. Because that number can range from zero to more than 275, it is divided by 25 to give us a more manageable number resulting in a scale from 0 to 11.

[For more details on calculating the UV index]

How does the measured UV index compare with the forecast?

For several reasons, the measured noon UV index can be lower than the forecast, sometimes by as much as 20%. The forecast is based on a model computation that applies to radiation expected under clear skies. A factor is then made to adjust for forecasted cloud cover, but this is approximate. Just as important, the forecasted UV Index does not include the effects of atmospheric pollutants or haze which can substantially decrease UV intensity, especially in urban areas. On the other hand, neither the forecast nor SERC's measurement takes into account variable surface reflection (e.g., sand, water, or snow), which can substantially increase your exposure at the beach or on ski-slopes.

Learn More:

To learn more about UV index forecasts and UV health effects see the EPA Sunwise site:

Find more about how the atmosphere shields us from UV, what causes atmospheric pollution and much more in the “Change is in the Air” exhibit at the Smithsonian National Museum of Natural History:

For more information on the SERC UV monitoring program, visit the UV lab's home page, or or contact Dr. Patrick Neale.