Observed-month temperature gave no clear common change in cherry bloom interval slopes at 21 Japan stations in 1953-2024

We compared two monthly temperature rules for the time from first cherry flowers to full bloom. One rule used a fixed month at each station. The other used the observed first-flower month in each year. The analysis used 1,465 paired records at 21 Japan stations in 1953-2024. Models included winter temperature, station means and station time trends. Four region and period comparisons gave slope changes from -0.502 to +0.163 days per degree C. All four adjusted intervals included zero. Both southern point estimates changed sign, but the evidence did not show a common difference between rules. The observed month differed from the fixed month in 12-44 percent of records. Small station groups, selected long records, unknown visits, coarse weather and a region boundary changed during exploration limit the result. This is a comparison of exposure definitions, not proof of reporting bias or a causal climate or pollution effect. Code, exact source hashes, the event-weather panel, numerical checks and a figure are included.

phenology · cherry · temperature · exposure definition · observation bias · Japan