Nighttime cloud cover from multi-site observations and its relationships with ground- and satellite-based measurements

Reliable characterization of nighttime cloud cover (CC) remains challenging due to the limited availability of long-term ground-based observations and the lack of coordinated multi-instrument measurements. This study presents a coordinated nighttime cloud observation framework that integrates standardized ground-based camera systems with collocated meteorological measurements, lidar-derived cloud base height (CBH), pyrgeometer-based net longwave radiation, and Himawari satellite’s CC across four sites in East Asia — Chiba, Toyama, Shiga (Japan), and Taoyuan (Taiwan). A continuous six-year nighttime CC record is available at the Chiba site that can be used to assess of seasonal variability and cloud-environment relationships. Unlike previous studies that rely on single instruments or isolated locations, this work systematically analyzes nighttime CC in relation to near-surface temperature, relative humidity (RH), cloud base height, and longwave radiation using a unified methodology. In persistently humid conditions such as Taoyuan, enhanced atmospheric water vapor suppresses the increase of CC with near-ground temperature. At the same site, net longwave radiation increases by 1.778 W m −2 per 1% increase in CC. In Chiba, a pronounced decrease in CC with increasing CBH (approximately −4.931% km −1 ). These results exhibit the value of coordinated multi-site nighttime observations using standardized ground-based instrumentation. The study provides a methodological foundation for expanding nighttime cloud monitoring networks and offers observational constraints useful for validating satellite-based nighttime CC and improving the representation of nighttime clouds in atmospheric models.