This report documents the findings of a human factor study conducted to estimate the effects of the Graphical-Weather Service (GWS) on general aviation (GA) aircraft utility, pilot situational awareness, and the weather dissemination workload on ground personnel. GWS is a data link application, being developed at MIT Lincoln Lbaoratory through the sponsorship of the Federal Aviation Administration, that will provide near-real time graphical weather information to the General Aviation pilot in the cockpit. Twenty instrument-rated pilots participated in the study. Subjects were presented with recorded actual weather information in the context of a series hypothetical pre-flight briefings and accompanying "flights." GWS images were accessible on a Macintosh TM Computer. The study design enabled the analysis of the effects of GWS and the determination of whether those efforts were influenced by the experience level of the pilot/user. Objective and subjective measures of effectiveness were collected. Results indicate that GWS had a substantial effects on weather-related decision-making. This was true for pilots with varying levels of instrument experience. Subject confidence in the ability to assess the weather situation was markedly increased when GWS was used. Subjects with GWS made fewer calls for weather information to weather dissemination ground personnel, thus indicating a potential decrease in ground personnel workload. Subjects found GWS to be very useful and were enthusiastic about receiving data link services in the GA cockpit in the future.