Data Sources

All data on this site comes from public NOAA datasets, with no post-processing beyond unit conversions and formatting.

Climate normals (monthly averages)

Source: NOAA 1991-2020 U.S. Climate Normals.
Normals are the 30-year average of observed temperature and precipitation for each month. NOAA publishes new normals every 10 years; the current set runs from January 1991 through December 2020. Our averages come from each city's primary weather station — typically the main airport or longest-record site.

All-time records

Source: NOAA Global Historical Climatology Network - Daily (GHCN-Daily).
The hottest, coldest, and wettest dates come from daily observations going back to the start of each station's period of record (often the 1870s-1940s).

Frost dates

Source: NOAA Freeze/Frost Occurrence tables, derived from GHCN-Daily.
The last spring frost and first fall frost are calculated as the date each year when the overnight low last (or first) crossed 32°F. We report the 10th / 50th / 90th percentile — the early, median, and late dates — to give gardeners a proper risk window rather than a single "safe" date.

Weather stations

Source: NOAA GHCN station inventory (ghcnd-stations.txt).
Each city is linked to one primary station — usually its main airport (e.g. Phoenix Sky Harbor Intl, Chicago O'Hare). Airport stations have the longest uninterrupted records, which makes their normals the most reliable.

All NOAA data is in the public domain. We convert metric units (°C, mm) to US customary (°F, inches) where appropriate.