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Overview

As the NWS strives to improve the accuracy of the UV Index, it remains an excellent independent source of comparison for surface UV observations. The UV Index is a dose rate of erythemally weighted UV irradiances. The U.S. National Weather Service (NWS) produces a daily forecast of the UV Index for the following day valid during the local solar noon hour for nearly 800 U.S. cities(Long, et al., 1996). Total ozone forecasts based upon observations from NOAA satellites, cloud forecasts from NCEP numerical models provide the necessary input for the daily UV Index forecasts. No surface UV observation information is used in the generation of the daily forecasts.

There are no less than seven networks taking surface UV radiation observations operated by various governmental agencies. Each network has its own specific purpose. Which directs the locations of the surface sites. For this exercise, three of the larger networks were chosen: USDA, NOAA's SURFRAD, and EPA's UVnet.

The USDA and the SURFRAD networks use the Yankee UV-B broadband instrument to make their surface observations. The EPA uses the spectral Brewer Mk IV. More information about these instruments can be found at the networks home pages.

Figure 1 shows the location of the three networks' sites within the contiguous 48 states. Note that there are several locals where more than one network is located. This offers the chance for inter-comparison of the observations.

Figure 2 shows the locations of nearly 800 cities where the UV Index forecasts are issued. Stations closest to the surface observation sites are used for this comparison. Some parings of UV observation site and UV forecast city have small distances between them. While others, especially for the sites in National Parks, are much further away.

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