The Automated Surface Observing System (ASOS) includes an icing detector which has been shown to provide reliable estimates of ice accretion (mass or thickness) during surface icing events. This paper summarizes the development work accomplished over three winter testing seasons, from 1995 to 1998; the paper describes data-collection procedures, and presents an algorithm which relates raw data from the ASOS "Freezing Rain" sensor to measurements of ice accretion. The paper focuses on a potential ASOS product that would provide quantitative estimates of ice thickness, analogous to the current reporting of liquid precipitation amounts (i.e., the METAR/SPECI "P" group). Case studies illustrate the reliability and inherent uncertainties of the quantitative ice accretion estimates from the ASOS