are located at airports throughout the country ( These data are important in the verification of weather forecasts, in providing real-time weather information to the aviation community, and as input into data assimilation systems for numerical weather prediction. None specifically target the roadway environment. The ASOS is a fully automated system that provides an extensive suite of meteorological observations without human observers. It is sufficiently sophisticated to provide both routine hourly reports and special observations as warranted by changing conditions. The basic data given in each report include sky condition (clouds to 12,000 feet), visibility, present weather, surface pressure, temperature, dewpoint temperature, wind, and liquid precipitation amount. Although not routinely used, the ASOS has the capability to report as often as every five minutes. It was designed to support NWS warning and forecast operations and Federal Aviation Administration aviation weather needs; in addition, the system supports hydrological and climatological programs.

Despite its value to many users, the ASOS does not meet all users’ requirements, largely because there are relatively few stations and because their observations are representative only of a small area near the site. As a result various user groups have developed and installed their own specialized surface-observing systems. Representativeness of surface observations is particularly important to the roadway environment, where minor differences in the physical environment (e.g., slope and exposure) lead to dramatically different effects. Although the ASOS provides useful data, it was never intended to be used to characterize the roadway environment; therefore, additional networks that target the roadway environment are needed.

Another very similar system is the Automated Weather Observing System (AWOS), which is a suite of sensors designed to collect and disseminate weather data primarily to assist the aviation community. The systems are classified as federal, which are owned and maintained by the Federal Aviation Administration, and nonfederal, which are owned and maintained by state, local, and private organizations. There are six different AWOS sensor arrays. The most basic array of sensors report wind speed (including gusts) and direction, temperature, dewpoint temperature, pressure, and density. The other five arrays build off this basic suite by reporting such additional parameters as visibility, sky condition, present weather, or lightning detection. Over 600 AWOS sites exist throughout the United States ( As with the ASOS, the AWOS was not deployed to observe the roadway environment, although its data are useful for synoptic weather observing and forecasting purposes.

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