Asymmetry in the Morning and Evening Occurrence of Noctilucent Clouds in Japan

Noctilucent clouds (NLCs) are the highest clouds in the Earth’s atmosphere, occurring at altitudes approximately 82–85 km in polar regions, and are typically observed during summer in high-latitude regions (50°–60° in latitude). In recent years, NLC occurrences have been reported to expand toward lower latitudes. Particularly, the number of NLC detections from midlatitudes (below 50°) is increasing. In Japan, the NLC imaging network has been updated since the first NLC detection from Hokkaido in 2015. Four more events were observed in 2020, one in 2021, and four in 2025, resulting in a total of 10 NLC observations at present. The event on June 14, 2020, was particularly notable, reaching a southernmost latitude of 44.6°N, which is the first observation of NLCs extending over the main island of Hokkaido. Except for one case, all events in Hokkaido occurred in the early morning hours (2:00–3:00 LST), indicating a strong local-time dependence on the occurrence. In this study, we investigated the occurrence characteristics of NLCs from Hokkaido and their relationship with the background atmospheric conditions. By analyzing the NLC formation environment and advection history using temperature and water vapor data from Aura/Microwave Limb Sounder (MLS) and temperature data from the JAGUAR-DAS Whole neutral Atmosphere Reanalysis (JAWARA), we obtained the following key findings. In the longitudinal sector including Hokkaido, a low-temperature condition favorable for NLC conservation is formed by atmospheric tidal waves prior to the morning hours. This creates a favorable environment for NLC survival during its advection from high latitudes. This tidal activity creates a distinct observational advantage for morning NLC observations, consistent with the fact that most events in this study were observed in the morning.