The Weather Football Index
The Weather Football Index (WFI) is a single 0–100 score expressing how strongly the weather is expected to affect a football game — where 0 is a climate-controlled dome and 100 is a blizzard or gale that fundamentally changes how the game can be played. It is built to be transparent, reproducible, and calibrated against decades of outcomes, not a black box.
The components
WFI is the sum of four weather components, then scaled down for roof type. Each component is bounded so no single factor can silently dominate.
Wind
up to 45 ptsThe single biggest on-field factor — it degrades the deep passing and kicking game. Sustained speed plus a gust premium and a crosswind adjustment for stadium orientation.
Precipitation
up to 30 ptsRain and especially snow affect ball security, footing, and visibility. Scaled by intensity and accumulation; frozen precipitation weighted higher than liquid.
Cold
up to 25 ptsSub-freezing air stiffens the ball and hands and lowers kick distance. Driven by wind-chill, not just air temperature.
Heat
up to 20 ptsExtreme heat and humidity drive fatigue and cramping, especially early season. Driven by heat index.
The formula
raw = wind_pts + precip_pts + cold_pts + heat_pts
wind_pts = clamp((sustained_mph − 5) × 2.2 + gust_premium, 0, 45)
precip_pts = clamp(rate_factor × 22 + snow_accum × 6, 0, 30)
cold_pts = clamp((32 − wind_chill_F) × 0.9, 0, 25)
heat_pts = clamp((heat_index_F − 88) × 1.3, 0, 20)
// roof scaling: controlled environments crush the score
roof_factor = { open/outdoors: 1.0, retractable-open: 1.0,
retractable-closed: 0.15, dome: 0.05 }
WFI = round( clamp(raw × roof_factor, 0, 100) )
Data sources
- Games, stadiums, roof type & historical conditions — nflverse (Pro-Football-Reference) for the NFL back to 1999, and cfbfastR-data (keyless nightly parquet) for FBS college.
- Live forecasts — hourly conditions at exact stadium coordinates from our own weather.city hub, built on the National Blend of Models (NBM) with wind, gust, precipitation, temperature and humidity.
- Historical reanalysis — Copernicus ERA5-Land (ARCO analysis-ready store) for temperature, precipitation and humidity, blended with ERA5 single-level wind gust, and with observed Pro-Football-Reference wind where recorded. All keyless and openly licensed.
Update frequency. Live game forecasts refresh hourly from the latest NBM cycle. The historical archive and validation tables are rebuilt after each completed week during the season. This page's spec is versioned; weight or source changes are logged below.
Does it actually work?
Component weights are checked against the historical record. Because the WFI is computed only from weather, game scoring is a genuinely independent outcome — so if the index means anything, points should fall as it rises. Across every outdoor NFL game since 1999, they do, cleanly and monotonically:
Sample sizes shrink sharply at the top of the scale — genuinely severe weather is rare, so the high band rests on relatively few games and its exact average will move as more accumulate. The direction, however, has been stable across every recut. The full table and the worst-weather games on record are on the Index board; the warm-vs-cold acclimation cut is on Stadium Weather Impact.
Limitations & honest caveats
The WFI is a decision aid, not a crystal ball. Where it is weak, we say so:
- Historical precipitation is coarse. Pre-forecast games rely on reanalysis and box-score notes; hourly rain/snow rate at a single point is one of the hardest fields to pin down. We treat frozen precipitation conservatively and flag games where accumulation is inferred rather than measured.
- Reanalysis wind is smoothed. Gridded models under-represent gusts and stadium-specific channeling. Our hybrid layer restores observed wind where Pro-Football-Reference recorded it; where it didn't, windy games can be under-scored. This is a floor on the index, not a ceiling.
- Forecast uncertainty grows with lead time. A Sunday score posted on Wednesday can shift by kickoff. Every live game forecast carries a reliability flag (below); trust the number more inside 24 hours.
- Small samples at the extremes. Blizzard- and gale-tier games number in the dozens across 25 seasons. Individual-game inferences at WFI 70+ are illustrative, not statistically settled.
- Weather is one input among many. Injuries, scheme, and matchup dwarf weather in most games. The index isolates the weather signal; it does not claim to predict outcomes on its own.
Forecast reliability
Live game scores carry a reliability tier based on lead time and cross-model agreement, so you know how much weight the number can bear:
- ● High — inside 24 h, models agree. The WFI is unlikely to move more than a band.
- ● Moderate — 1–3 days out, or some model spread. Direction is reliable; the exact number may drift.
- ● Low — 3+ days out, or forecasts diverge. Treat as a heads-up, not a hard read; re-check near kickoff.
Edge cases
- Domes & retractable roofs. Fixed domes are scaled to 5% of raw impact and retractable-closed to 15% — never exactly zero, because HVAC, open end-zones and door drafts leave a residue. A retractable-open game is scored as fully outdoor; roof state at kickoff, when known, overrides the default.
- Cold-weather CFP & neutral sites. College playoff and bowl games at domes or warm neutral sites correctly score near zero regardless of the home cities' climates — the game is played where it's played, not where the team is from.
- Blizzards & hurricanes. The scale saturates at 100 by design; a true whiteout and a merely brutal cold-wind game can both pin the meter. When a named storm or a warning-level event coincides with a game, we annotate it explicitly rather than letting a capped number imply the two are equivalent.
- Missing or conflicting observations. Games with no recorded wind and no reliable reanalysis are marked low-confidence rather than silently scored zero.
This spec is versioned. Material changes to weights or sources will be logged here. Current: WFI v0.1 — 2026-07-21.