Data sources
All weather data on this site comes from NOAA: the National Weather Service (NWS) forecasts, plus a NOAA smoke forecast. The map and the point forecasts use different sources.
The map
The map layers come from the National Digital Forecast Database (NDFD), the gridded forecast NWS forecasters produce for the whole country on a 2.5 km (about 1.5 mile) grid. We download the national files from NOAA's open data on Amazon Web Services and sample them about every 0.1° of latitude and longitude, roughly every 7 miles.
| Layer | NDFD forecast used |
|---|---|
| Hiking suitability | Temperature, sky cover, wind speed and precipitation chance, 8 a.m. to 6 p.m. local time |
| Precip chance | 12-hour probability of precipitation, daytime periods |
| Precip amount | 6-hour precipitation amounts |
| High and low temp | Maximum and minimum temperature |
| Wind | Sustained wind speed, averaged over daytime hours |
| Cloud cover | Sky cover, averaged over daytime hours |
| New snow | 6-hour snowfall amounts, totaled for each day |
| Thunderstorm risk | The forecast weather grid's thunderstorm wording (from slight chance to widespread), highest level during daytime hours |
| Smoke | Not from NDFD: near-surface wildfire smoke from NOAA's HRRR-Smoke model, averaged over daytime hours and converted to an air quality index with the EPA's PM2.5 scale |
The map refreshes about every six hours. The note under the day timeline shows when the data you're looking at was published.
Known gaps
- NDFD doesn't publish precipitation or snowfall amounts beyond about three days, so the Precip amount and New snow layers are blank for days 4 to 7.
- NDFD's precipitation chance comes in 12-hour blocks, so the map can't show an hour-by-hour chance. The point forecast has hourly values.
- The smoke forecast runs 48 hours ahead, so the Smoke layer covers today and tomorrow only. It shows wildfire smoke, not ozone or other pollution, and it's a model forecast rather than an official air quality forecast. For official readings and forecasts, check AirNow.
- Only the contiguous US is covered for now.
Point forecasts
When you click a spot, the site asks the NWS API for that location's forecast at that moment: the written forecast, hourly values, the forecast grid cell's elevation and the nearest town. Sunrise and sunset are calculated from the coordinates. The NWS doesn't forecast air quality, so the smoke value (and its effect on the hiking score) is the map's daytime estimate for the nearest map point, for today and tomorrow. The links at the bottom of each point forecast open the same forecast on weather.gov.
Places and search
Search and the map's trailhead and peak pins use two sources, refreshed monthly:
- Peaks, passes, lakes, falls, glaciers, arches and towns come from the US Geological Survey's Geographic Names Information System (GNIS), the federal government's official list of place names.
- Trailheads come from OpenStreetMap, © OpenStreetMap contributors. OpenStreetMap data is available under the Open Database License, and so is the trailhead list derived from it, which this site publishes at data.hikeday.app/places/index.json (together with the GNIS places, which are public domain).
Trailheads near the Canadian and Mexican borders are matched to the nearest US town, so a few just across the border may still appear. Point forecasts are only available inside the US.
Other sources
- Basemap: Esri World Topographic Map (sources: Esri, HERE, Garmin, FAO, NOAA, USGS).
- Time zones: each map point's local day is worked out with the open-source tz-lookup library.
NWS forecasts are public information from the US government. HikeDay isn't affiliated with or endorsed by NOAA or the National Weather Service, and processing the data (sampling, averaging and scoring) can introduce differences from the official forecast. When in doubt, the forecast on weather.gov is the authority.