The current standard cloud height indicator (CHI) for the Automated Surface Observing System (ASOS) is the National Weather Service (NWS) CT12K laser ceilometer (CT12K). This ceilometer detects clouds to a height of 12,000 feet. In late 1998, the manufacturer discontinued production of the CT12K. The vendor agreed to maintain the existing ASOS ceilometers through 2007, with the NWS having enough stock of spare parts on hand to maintain the system through 2008. New ceilometers will need to be deployed to the ASOS network accordingly. On January 05, 2007, upon conclusion of a Commercial-Off-The-Shelf sensor evaluation, the NWS awarded a contract for development of a CT12K replacement ceilometer to Vaisala Inc. of Woburn, Massachusetts.

The evaluation of the Vaisala CL31ASOS (CL31) ceilometer has included specification compliance, sensor precision, the handling of bird related window contamination, and the elimination of false ice crystal cloud detection.

NWS Weather Forecast Offices (WFO) in the northern tier states and Alaska have observed the CT12K reporting cloud cover in conditions reported as clear skies by certified weather observers. This led to specific testing for the elimination of false reporting of sub-visible ice clouds during the evaluation of the CL31. This testing was performed at the Fairbanks, Alaska (FAI) WFO and at Mount Washington Observatory (MWO) in New Hampshire. A CL31 has also been deployed to the NWS Bismarck, North Dakota (BIS) WFO for follow-on testing during the winter 2008-2009.

Compliance testing was performed at the NWS Sterling Field Support Center (SFSC) in Sterling, Virginia; and sensor precision testing was performed at the SFSC winter test facility in Johnstown, Pennsylvania.