An observational study of a thermal belt on hillsides

Abstract

A thermal belt or a warm zone on a hillside, developing under calm weather conditions during the night, is one of the well-known local climatic phenomena. Although many models describing its fundamental concept have been constructed, very little observational data exists to support the models directly. The difficulty in studying local climatic phenomena lies mainly in collecting data. In the present study, an infrared radiation thermometer installed on the roof of a high building was used to obtain data for analyzing the air temperature structure in complex terrain. The altitude data set (Digital National Land Information, Japan) also was utilized to examine the dependence of air temperature on topography by applying multiple regression analysis. It is confirmed by observation that a thermal belt on a hillside is produced by nocturnal drainage winds and by a cold-air lake which is formed over the basin by the drainage wind and radiative cooling. The height and intensity of a thermal belt are subject to local and temporal variations due to topographical environments and weather conditions, and can vary from hill to hill, even in situations where they are located face to face. These results show that a thermal belt is a phenomenon produced dynamically by the movement of cold air.