Atsusa samusa mo higan made: Statistical validation of a Japanese weather proverb across eight stations over 76 years

MIZUKI SHIRAI 

Background: Weather proverbs encode centuries of observational knowledge, yet few have been subjected to rigorous statistical testing. The Japanese proverb atsusa samusa mo higan made ("heat and cold last only until the equinox") asserts that seasonal temperature transitions coincide with the vernal and autumnal equinoxes. Despite growing international interest in weather folklore verification, no formal study has examined this widely known Japanese proverb.

Methods: We analyzed daily mean temperature data from eight Japan Meteorological Agency stations spanning latitudes 26.3-43.1°N over 76 years (1950-2025). Temperature transition points were identified using the Pruned Exact Linear Time (PELT) change point detection algorithm and segmented regression with bootstrap confidence intervals. Temporal trends were assessed via Mann-Kendall tests, and spring-autumn asymmetry was evaluated using Wilcoxon signed-rank tests. All tests used a two-sided significance threshold of α = 0.05.

Results: The proverb showed approximate consistency with autumn temperature patterns but not spring. Autumn PELT-detected transition points occurred close to the autumnal equinox (median offset: +4.0 days; interquartile range [IQR]: -2.0 to +14.0; Cohen's d = 0.25-0.51), whereas spring transitions lagged substantially behind the vernal equinox (median offset: +10.0 days; IQR: 0.0 to +16.0; d = 0.26-0.76). This spring-autumn asymmetry was statistically significant at four of eight stations after Benjamini-Hochberg correction (q ＜ 0.05). Mann-Kendall tests revealed significant increasing trends in autumn offsets at four stations (+0.12 to +0.18 days/year, q ＜ 0.05), suggesting that autumn cooling is progressively delayed. Sensitivity analyses (unrestricted PELT, block bootstrap, modified Mann-Kendall) confirmed the robustness of all primary findings. Naha (26.3°N) exhibited distinctly large offsets in both seasons, consistent with subtropical thermal inertia.

Conclusions: The proverb atsusa samusa mo higan made is approximately consistent with autumn temperature patterns but not spring, and the correspondence for autumn appears to be declining over time—a trend consistent with climate warming. These findings parallel international studies showing that traditional weather knowledge, while rooted in genuine climatic patterns, requires reinterpretation as temperatures rise.