A Ka-band meteorological Doppler radar was co-developed by Mitsubishi Electric Corporation and Kyoto University for experimental observations of fog (Ohmori et al., 1999; Hamazu et al., 2001; Hamazu et al., 2003). The radar mounted on a truck is shown in Fig. 1. Field tests were conducted in Beppu-Bay and on the coast of Kushiro for the purpose of constructing fog detection and prediction techniques. Fig. 2 shows a sample image of observed radar echoes. We analyzed field observation data of fogs, and derived a fog detection algorithm based on criteria which indicate the status of low visibility. Then we choose three radar parameters such as echo intensity, echo top altitude and echo area rate, and introduce the logical product of these parameters as fog detection criteria.Fig. 3 shows a block diagram of the fog detection algorithm. From some estimation results by using the algorithm to field data observed in Beppu-Bay and Kushiro coast, it is confirmed that the proposed algorithm is effective for fog detection. Figs. 4 and 5 show fog detection results of data observed in Beppu-Bay and Kushiro coast, respectively.Solid curves show visibility range reference measured by visibility meter, and X marks indicate the time periods when the proposed algorithm detected fogs.These results show that the reference visibility and the fog detection results based on radar observations are well in agreement.