The Automated Surface Observing System (ASOS) uses the Vaisala Model CT 12K as its cloud height indicator (ceilometer). The Vaisala Model CT 12K can detect clouds up to 12,000 feet. Using the output from the Vaisala CT 12K, the sky condition algorithm will process the cloud height information properly; however, some improvements can be made in the algorithm to enhance the reporting of cloud bases and vertical visibility during fog, virga, and precipitation events. Most of these improvements can be made with the current Vaisala CT 12K; however, laser beam ceilometer technology has advanced in recent years. In addition, new generation cloud height sensors may provide much more reliable information in the lower atmosphere during adverse weather conditions. The ASOS program started a concept exploration effort in August 1996 with the test and evaluation of 25,000 foot ceilometers.
This paper focuses on the test and evaluation conducted since January 1998. Twenty-five thousand foot ceilometers from three manufacturers, Belfort, Handar, and Vaisala are under test and evaluation at Sterling, Virginia. Also, the Vaisala CT 12K ceilometers are under evaluation for sky condition algorithm enhancements. This paper includes an analysis of the test results, including improvements to the current sky condition algorithm, and an analysis of the 25,000 foot ceilometer's performance as compared to the Vaisala CT 12K and the observer