A climatological study of fog in Japan based on event data

A climatological study is carried out to understand the characteristics of fog in Japan. This study uniquely focuses on fog density and fog type using atmospheric visibility data from surface observations. The main results are summarized below, within the following three contexts: 1) the characteristics of fog and dense fog, 2) fog type, and 3) long-term change in the number of foggy days.1)Most of the foggy and densely foggy regions in Japan are inland mountainous areas, basin areas, and the Pacific coast of eastern and northern Japan. Fog density varies seasonally. Although the warmer seasons have the highest frequency of fog occurrence in Japan, the density of fog tends to be higher in the colder seasons. Land cover also affects the density of fog. In the urban area, fog rarely forms, with dense mist sometimes forming.2)The most common type of fog in Japan is radiation fog, which accounts for half of all fog events. The dominant fog types of the inland areas, the Pacific coast, and the western part of Japan are radiation fog, advection fog and rain fog, respectively.3)The numbers of foggy days decrease at many of the observation sites during a 40-year period from 1966 to 2005. Inland and basin sites showed the largest decrease. Urban areas were once affected by fogs, but now experience mist instead. In contrast, most coastal sites showed no clear trend of decrease and some sites even showed an increase in the numbers of foggy days. •The climatological features of fog and dense fog in Japan are summarized from hourly visibility data.•We focus on the relationship between the fog density and the surrounding environment including topography and land use.•The Pacific coast of Hokkaido is unique area because dense fog events are rarely reported although fog events are frequent.