Data & Methodology

Frost data

Frost dates come from NOAA National Centers for Environmental Information 1991-2020 U.S. Climate Normals, annual/seasonal product. For each station NOAA publishes the dates by which the first fall and last spring occurrence of 36°F, 32°F, and 28°F is reached at several probability levels, the probability of reaching each temperature at least once a year, and growing season lengths.

Spring dates on this site are shown as the chance of a frost after the date (90%, 50%, and 10%), and fall dates as the chance of a frost before the date (10%, 50%, and 90%). Where frost occurs in fewer than half of years, the site reports how often it occurs instead of an average date.

Matching ZIP codes to stations

Each ZIP code is located at the town point published by GeoNames (falling back to the Census ZIP Code Tabulation Area point) and compared with the eight nearest NOAA stations that report freeze data. The site picks the station with the best combined score of distance and, where elevation is known, elevation difference (100 meters of elevation counts like 15 km of distance), with a small penalty for stations with fewer than 15 years of record. Each ZIP page names the station used, its distance and elevation, and nearby alternatives.

Planting calendar

Planting windows start from the University of Maryland Extension vegetable planting calendar, which is designed for an average last frost of April 15 and an average first frost of October 25. Each spring window is shifted by the difference between April 15 and the local average last frost; fall windows and late-season end dates are shifted by the difference between October 25 and the local average first frost. Calendars are not shown where the frost-free season is shorter than 90 days or longer than 300 days, because shifted dates become unreliable.