The three data sources
Crowds — the National Park Service publishes monthly recreation visits for every park unit (NPS Visitor Use Statistics — monthly recreation visits, public domain). We average the 5 most recent complete reporting years, 2021–2025, so a single odd year can't skew a month.
Weather — NOAA's U.S. Climate Normals (1991–2020, public domain): monthly average highs, lows, precipitation, and snowfall from the station nearest each park's main visitor area. Every park page names its station and how far it sits from the reference point, because a valley station cannot speak for a summit.
Daylight — calculated from each park's latitude with the standard solar formula. No dataset needed; the sky is reliable that way.
The formula
- Crowds — how far a month sits below the park's busiest month in the visitation records. The peak month scores 0, a month with half the peak's visitors scores 50.
- Weather — 100 when the average daytime high lands in the 55–75°F comfort band, falling 3 points per degree outside it. Months averaging 95°F or hotter are capped at 20, nights below 15°F cost 15 points, months averaging over 4 inches of rain lose up to 20, and months that are both hot (85°F+) and soaked (6 inches+) lose another 25 — the sticky-season penalty.
- Daylight — hours of daylight at the park's latitude, scaled from 8 hours (0) to 16 hours (100).
- Roads & access — typical monthly snowfall where the weather station reports it (25 inches or more scores 0); where it doesn't, how far average highs sit below 40°F stands in. This carries real weight by default because an empty month is often empty for a reason — mud season shouldn't win on crowds alone. Months with almost no recorded visits are treated as closed and score 0 here, and a near-empty month right next to the closed season gets access capped low — that quiet usually means the services are shut too.
- A park page blends crowds 30%, weather 40%, daylight 10%, roads & access 20%. Scores are shown as bands — Excellent (77+), Good (52–76), Tough (under 52) — never as decimals, because 30-year normals don't support false precision.
What the score honestly can't see
Wildflower timing, fall color, mosquito hatches, wildfire smoke, road construction, this year's snowpack. The score is climatology and visitation shape — a strong prior, not a forecast. Park pages carry the famous seasonal road facts ("Going-to-the-Sun typically opens late June"), but always check the park's own current-conditions page before you drive.
One more honest note: a weather station is a point, and parks are not. Where the mismatch is famous (the Grand Canyon rim versus the canyon floor), the page says so plainly.
Who this is for
Anyone deciding when to take the trip. The data is public, the parks belong to everyone, and the answer to "when should I go" shouldn't depend on which blog shouted loudest. ParkDates is independent and not affiliated with the National Park Service or NOAA — just grateful for their data.