Methodology and sources
Frost and freeze data
All dates come from the NOAA National Centers for Environmental Information (NCEI) 1991–2020 U.S. Climate Normals, annual/seasonal product, downloaded as the official archive us-climate-normals_1991-2020_v1.0.1_annualseasonal_multivariate_by-station_c20230404.tar.gz from ncei.noaa.gov/data/normals-annualseasonal/1991-2020/. NOAA data are U.S. government works in the public domain.
From the 928 stations in SC, NC and GA listed in NOAA’s inventory_30yr.txt, we kept the 416 that have freeze-probability values (most others are precipitation-only). The fields used are:
ANN-TMIN-PRBLST-T32FPxx: date of the last spring daily minimum at or below 32°F, at probability levels 10–90%. Also 28°F and 36°F versions.ANN-TMIN-PRBFST-T32FPxx: date of the first fall occurrence at or below 32°F (also 28°F and 36°F).ANN-TMIN-PRBGSL-T32FPxx: growing-season length in days between them.ANN-TMIN-AVGNDS-LSTH032: average number of days per year with a minimum at or below 32°F.
How to read the probabilities
Fall: NOAA’s documentation defines the fall values as the date by which the first freeze has occurred with the given probability. The 10% date is early (only 1 year in 10 has had a freeze by then), the 90% date is late.
Spring: In the data files the 10% spring date is always the latest and the 90% date the earliest (for example, Charleston International Airport: 10% = 03/27, 50% = 03/07, 90% = 02/12). That ordering means the percentage is the chance that the last freeze occurs after the date, which is how we label it: plant tender crops after the 10% date and a late freeze is about a 1-in-10 risk. (NOAA’s 2020 documentation words these fields as “occurrence or earlier”; we follow the ordering in the actual values, which matches NOAA’s long-standing convention for spring freeze probabilities.)
Thresholds: 32°F is the usual “freeze”; 28°F is a hard freeze that damages most cool-season crops; 36°F at the shelter height often means frost can form on plants near the ground on clear, calm nights.
Completeness flags
Each value carries a NOAA completeness flag. S (standard) means 24+ years of data; R (representative) 10+ years with gaps filled from neighbors; E (estimated) means NOAA estimated the normals statistically from nearby stations. Station pages show the flag.
Matching stations to towns
NOAA station names usually describe a location relative to a town, e.g. “GAFFNEY 6 E” is about 6 miles east of Gaffney. For each station we compute straight-line distances to every incorporated place and Census Designated Place in SC, NC and GA using the internal-point coordinates in the U.S. Census Bureau 2024 Gazetteer Files (public domain). If a place matching the station’s name lies within 25 miles, the page is titled for that town; otherwise for the nearest place. The town search maps every Census place to its two nearest stations. Distances ignore elevation, which matters a lot in the mountains.
Planting calendar
See planting rules and sources.
Computed comparisons
State medians, ranks and differences on station pages are computed by this site from the NOAA values; they are not NOAA products.