The National Weather Service is replacing the cup and vane anemometers that are currently used in the Automated Surface Observing System (ASOS) with sonic anemometers. The primary problem with the current cup and vane technology is its susceptibility to lock-ups in freezing precipitation conditions. When the cups and/ or vanes are immobilized by freezing precipitation they will generally remain immobilized until the temperature rises above freezing, which can lead to extended periods when wind data is inaccurate and/ or unavailable. Another problem with the cup speed measurement is that when wet snow attaches to the cups their rotation speed is slowed. The result is a wind speed measurement that is lower than the actual speed. Sonic anemometers overcome problems associated with icing and wet snow by applying heat to their transducers, thus melting ice or wet snow that would otherwise interfere with the speed and direction measurement. While sonic anemometers were originally developed to measure wind speeds that are too low for mechanical sensors to measure, the technology has evolved to the point where speeds up to 125 Knots can be measured accurately. This paper will discuss the extensive testing in the field and on operational ASOS systems in a wide variety of weather conditions to verify that the sonic anemometers will provide accurate speed and direction measurements for ASOS.