Weather classifications for high temperatures in Japanese cities

We present weather classifications for high temperatures in major Japanese cities. We created a self-organizing map (SOM) of summer pressure patterns in Japan and generated frequency distributions of hot days on the SOM node space for each city. Through hierarchical clustering, we identified several weather zones, which tended to coincidently experience hot summer days with daily maximum temperatures of 30°C or more, or extremely hot days with daily maximum temperatures of 35°C or more. The obtained weather zones for hot summer days partially followed a local climate classification previously proposed in geography. In contrast, weather zones for extremely hot days lacked geographical continuity and were rather related to whether the sites are inland or coastal. This suggested that extremely hot days were frequently caused by the foehn phenomenon.