Research · Japanese professional baseball
NPB Temperature, Wind and Scoring: Heat Adds Runs, Cold Takes Them
Eight seasons of Japanese professional baseball played in the open air, matched to hourly archive weather at each ground and measured against its own season's scoring average. Temperature moves runs in both directions; only the strongest wind band moves them at all; rain does nothing measurable.
Headline numbers
Games analysed
3,460
2018–2025, 43 open-air grounds
Dry games only. Domes and roof-unknown games excluded.
Hot games (80°F+)
+0.61 runs
t = 3.54 on 1,121 games
Against the 70–79°F control.
Strong wind (20 mph+)
+1.44 runs
t = 2.70 on 108 games
Against calm conditions. Modest sample — see limits.
Average across the sample: 7.73 runs a game at 74.1°F and 8.6 mph.
Temperature, band by band
Each game is measured against its own season's scoring average, then grouped by game-time temperature and compared with the mild 70–79°F control. The ladder is orderly: warmer means more runs, colder means fewer, with the gap widening at both ends.
| Temperature at first pitch | Games | Avg temp | Runs | Runs vs control | t |
|---|---|---|---|---|---|
| Under 50°Fbelow the 40-game minimum | 35 | 46.9°F | 5.94 | -2.23 | -4.58 |
| 50–59°F | 277 | 56.3°F | 7.11 | -0.66 | -2.40 |
| 60–69°F | 807 | 65.7°F | 7.39 | -0.32 | -1.76 |
| 70–79°Fcontrol band | 1,220 | 75.0°F | 7.76 | baseline | baseline |
| 80°F and over | 1,121 | 84.3°F | 8.15 | +0.61 | 3.54 |
Roughly |t| ≥ 2 is our usual noise boundary. The 80°F+ and 50–59°F bands clear it; the 60s do not. The under-50°F band is the largest gap on the table but sits below our 40-game minimum, so it is shown for completeness and treated as directional evidence only — never in a headline and never in anything we price.
Wind speed, band by band
Raw wind speed, same season adjustment, compared against calm games. Nothing below 20 mph is distinguishable from calm. The top band is.
| Wind speed at first pitch | Games | Avg wind | Runs | Runs vs control | t |
|---|---|---|---|---|---|
| Calm (under 5 mph)control band | 894 | 3.2 mph | 7.60 | baseline | baseline |
| 5–9 mph | 1,439 | 7.4 mph | 7.48 | -0.15 | -0.88 |
| 10–14 mph | 763 | 12.0 mph | 7.95 | +0.21 | 1.04 |
| 15–19 mph | 256 | 17.0 mph | 8.23 | +0.42 | 1.39 |
| 20 mph and over | 108 | 24.3 mph | 9.41 | +1.44 | 2.70 |
This is a speed-only cut. It does not tell you which way the wind was blowing relative to the field, and we are not publishing that: the field orientations of the open-air Japanese grounds could not be verified to our standard, with the open map database, satellite imagery and an automated read of that imagery disagreeing by 30 to 150 degrees at several parks. Until we can measure orientation properly, direction stays unpublished.
Rain, kept separate
The main analysis is dry games only, so rain cannot be carrying the temperature or wind results. The 516 games with measurable rain averaged 7.85 runs and came in +0.08 runs against dry games once season-adjusted (t = 0.40). That is a null result and we claim no rain effect on NPB scoring.
Season by season
| Season | Dry open-air games | Runs | Avg temp | Avg wind |
|---|---|---|---|---|
| 2018 | 457 | 9.16 | 73.8°F | 10.0 mph |
| 2019 | 451 | 8.80 | 72.6°F | 8.6 mph |
| 2020 | 344 | 8.63 | 74.1°F | 7.8 mph |
| 2021 | 404 | 7.65 | 70.0°F | 8.8 mph |
| 2022 | 453 | 7.40 | 73.8°F | 8.9 mph |
| 2023 | 455 | 7.12 | 75.6°F | 9.3 mph |
| 2024 | 450 | 6.68 | 75.6°F | 8.3 mph |
| 2025 | 446 | 6.57 | 76.5°F | 6.5 mph |
This is the single biggest reason every figure on the page is season-adjusted rather than pooled. NPB scoring fell from 9.16 runs a game in 2018 to 6.57 in 2025 — a drift far larger than any weather effect measured here. A raw pooled cut would credit the weather with league-wide era change, so each game is compared with its own season instead.
What actually holds up
- Heat adds runs. The 80°F+ band is the largest well-sampled effect on the page: +0.61 runs on 1,121 games, t = 3.54.
- Cool weather suppresses, mildly. The 50s cost about 0.66 runs (t = -2.40). The 60s do not clear the noise boundary.
- Only strong wind moves anything. 20 mph and over is worth +1.44 runs, but on 108 games — real evidence, lightly sampled.
- Rain is a null. No measurable effect, and we do not claim one.
- No market claim. With no archive of closing NPB totals, nothing here says whether the market already prices these effects.
Method and limits
- Sample: 3,460 dry regular-season and interleague NPB games, 2018–2025, at 43 grounds where the weather reaches the field, each with a first-pitch weather reading and a final score.
- Exclusions: 2,728 games in total — 2,514 domed stadium; 214 retractable roof, no per-game state. One open-sided ground with a partial roof is included, because wind and temperature still reach the playing surface there.
- Weather: hourly archive reanalysis at each ground's own coordinates, read at the local first-pitch hour in Japan time.
- Every figure is season-adjusted: each game is measured against its own season's average before bands are compared, so era drift cannot carry the result.
- Minimum sample is 40 games per band, a threshold already in use across our earlier studies and set before this study was run. The under-50°F band does not clear it and is labelled accordingly.
- No Over/Under hit rates and no closing-line comparison: we hold no archive of closing NPB totals. Nothing here is a betting-market claim.
- No wind-direction result, by choice. Park field bearings could not be verified independently, so direction-relative analysis is not published.
References & data sources
- Open-Meteo ERA5 historical reanalysis — archive-api.open-meteo.com (hourly game-window weather at each venue’s own coordinates)
- Nippon Baseball Data Repository — github.com/armstjc/Nippon-Baseball-Data-Repository (NPB schedules, venues and final scores, 2018–2025)
- OverUnderWeather NPB game archive — 6,705 completed regular-season and interleague games, 2018–2025, matched to venue coordinates and first-pitch weather.
- Every dated change we have made to a published study is listed on our corrections log.
Last recalculated
Figures on this page are computed directly from our stored game archive each time the page is served. Last computed 2026-09-21T09:48:06Z.
Where this is applied
These findings feed the game-time read on every NPB venue page, using the same sample-size and significance rules published here.
OverUnderWeather Research Desk
Independent data analysis on public sports and weather records. Studies match historical game outcomes to archived game-window weather, apply pre-set sample and significance rules, and publish null results when that is what the data show. Editorial policy and corrections log.
Frequently asked
Does temperature change scoring in Japanese professional baseball?
Yes, in both directions. Games at 80°F and above score about 0.61 runs more than the 70–79°F control (t = 3.54 on 1,121 games), and games in the 50s score about 0.66 runs fewer (t = -2.40). Every figure is season-adjusted, because NPB scoring fell steadily across the window.
Is the coldest band reliable?
No, and we do not treat it as a finding. Games under 50°F show the largest gap of all (-2.23 runs, t = -4.58), but on only 35 games — below the 40-game minimum that was set before this study was run. It is directional evidence only and is excluded from headlines and from anything we price.
Does wind speed matter?
Only at the top of the scale. Wind of 20 mph and over is worth about +1.44 runs against calm conditions (t = 2.70 on 108 games), while every band below 20 mph is within noise. Note that 108 games clears the 40-game minimum but is still a modest sample, so we treat it as a real but lightly-evidenced effect.
Why is there no wind-direction result?
Because we could not verify the field orientation of the open-air Japanese ballparks to our own standard. The open map database, satellite imagery and an automated read of that imagery disagreed by 30 to 150 degrees at several grounds. Rather than publish a direction-relative result built on numbers we do not trust, we publish the speed-only cut and leave direction open.
Are domes included?
No. 2,728 games at fully enclosed domes are excluded outright, as are games at the retractable-roof ground where no per-game roof state exists. Open-sided grounds where the weather reaches the field are included.
Are Over/Under hit rates included?
No. We hold no archive of closing NPB totals, so this study reports scoring only and makes no claim about whether the market prices these effects. Closing totals are being captured going forward; a market check will follow once the sample is honest.
Does rain lower scoring?
Not measurably. The 516 games with measurable rain came in +0.08 runs against dry games (t = 0.40) — a null result. Rainy games are held out of the temperature and wind analysis and reported separately.