Of the full set of observations carried out at weather stations, the ‘present weather’ observation is one of the most difficult to automate. Initially, elements such as rain and fog were automated. However, during the past ten years a new group of sensors, aimed at identifying the type and intensity of precipitation, has emerged. These sensors have come to be called Present Weather Sensors (PWSs). Most of the early tests and intercomparisons have been limited to liquid precipitation. Not much attention has been given to investigating the reasons for disagreement between the primary reference, the professional meteorological observer, and the automated observations. This paper presents an analysis of an intercomparison between observer and one type of PWS (Vaisala FD12P), in conditions of both liquid and frozen precipitation. An interaction model is introduced as a new and improved tool for the quality control of PWSs.