The air temperature distribution in the built-up area facing the mouth of a valley was investigated using both measurements and calculations. Surface temperature was kept higher than air temperature at every time in the night, and it is thought that the cold air drainage flowing into the built-up area has the capability that cools the built-up area. Calculations indicated that air temperature decreased in the riverside area and increased in the southern built-up area. This tendency was in good agreement with measurements taken in the present study. Since sensible heat flux had positive and negative values as a result of calculating the surface heat budget according to land uses, it was indicated to be the cases where the cold air drainage supplies both cold air to the built-up area and hot air to the forest area. From calculation in consideration of the altitude distribution, air temperature in the built-up area became low as compared with the case where altitude is not taken into consideration. It is thought that the slope of geographical feature is promoting the drainage of cold air.
