Observations of fogs at Misawa Fishing Port in the Aomori district were made with a tethered balloon system and a doppler sodar system from 15 June to 11 July in 1993. Minute analyses were made of time changes of fogs for a long period from the evening on 7 July to the next noon and for a short period from 6 to 13JST on July. The results show that the atmosphere at the cloud top becomes stable from the view point of CTEI (cloud-top entrainment instability) about 2 hours before the intrusion of the fog and becomes neutral (or unstable) just before fog dissipation.