A rooftop observer at a Kichijōji school has recorded every morning at 9 a.m. since 1963 whether Mt. Fuji, 83 km away, was visible to the naked eye; combined with the Kōfu observatory’s record of first summit snow since 1894, the dataset shows a large rise in visibility. Visibility mornings climbed from 22 in 1965 to 170 in 2023, decade means rose from 43 (1963-72) to 135 (2013-23), and visibility first exceeded 100 days in 1999. The observatory also checks nearer landmarks in order - Takaido incinerator (5 km), Shinjuku high‑rises (11 km), Tokyo Tower (17 km; 329 mornings in 2023) and the Chichibu range (50 km) - so the last landmark seen on a given morning indicates air‑clear distance.
Visibility is strongly seasonal (winter peaks: Jan 62%, Feb 48%, Dec 58% of 9 a.m. mornings), and summit first‑snow dates have shifted later in 30‑year means (roughly Sep 23 → Oct 5) with extremes from Aug 9 (2008) to Nov 7 (2024). Data for 1963-1993 come from published tables and 1994-2023 were digitised from observatory charts; counts are strict "was visible" observations taken at 9 a.m. Researchers link the 1973 jump to changes after the oil shock and attribute later recoveries to reduced pollution, drying and heat‑island circulation, while noting that counting from different rooftops affects comparability.
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