In Japan, comparatively many cities face the valley of the mountain. In such cities, in order that cold air may blow from the mountain, it is predicted at summer night that the thermal environment in the built-up area is improved. Field measurements of the temperature distribution in the built-up area facing the mouth of the valley are carried out at summer night. The influence the cold air that flowed out from the valley of the mountain into the built-up area affects the temperature distribution is considered. The process of heat exchange in the built-up area is examined.