Relationships among the Urban Heat Island Phenomenon, Cold Air Pool, and Mountain Breeze on Clear, Calm Nights during Autumn and Winter in Nagano City

Abstract

The purpose of this paper is to clarify the relationships among the urban heat island （UHI） phenomenon, cold air pool, and mountain breeze, focusing on the urbanization index and altitude, for the entire city of Nagano. We analyzed these phenomena by composing the temperature curve based on the sunset time for 100 days with clear, calm nights in autumn and winter. Approximately 2.5 hours after sunset, a clear temperature distribution associated with the UHI was seen. Several hours post-sunset, a cold-air pool also developed, and the temperature distribution associated with the UHI was maintained at the bottom of the cold-air pool until sunrise. In Nagano’s city center, an urban boundary layer was formed when the mountain breeze was blowing. It is possible that the urban boundary layer was maintained due to the influence of the mountain breeze. In addition, it was suggested that although the cold-air flow from the rural areas to the urban area slightly weakens the intensity of the UHI, that flow contributed to maintaining the temperature distribution with the UHI at the bottom of the cold-air pool 6 hours after sunset.