How the frost dates are calculated
Frostplot does not model or forecast anything. Every date on the site is a published value from NOAA, tied to one named weather station.
The data
The dates come from the U.S. Climate Normals 1991-2020 published by NOAA's National Centers for Environmental Information (NCEI). For each station with enough daily minimum temperature records, NCEI publishes freeze probabilities: the date of the last spring and first fall freeze at several temperature thresholds and probability levels, and the length of the freeze-free season. These normals are in the public domain.
The fields Frostplot uses from the annual/seasonal station files:
| Field | Meaning |
|---|---|
ANN-TMIN-PRBLST-T32FP10 … P90
|
Last spring date with a low of 32°F or colder, at 10% … 90% probability |
ANN-TMIN-PRBFST-T32FP10 … P90
|
First fall date with a low of 32°F or colder |
ANN-TMIN-PRBLST-T28FP*, PRBFST-T28FP*
|
The same for 28°F |
ANN-TMIN-PRBGSL-T32FP*
|
Length of the season between them, in days |
Each value carries a completeness flag and the number of years behind it. Frostplot shows the number of years and uses only stations with complete freeze data.
What the probability means
For spring, the 10% date is late: a frost still came after it in 1 year out of 10. The 50% date is the median: half of the years had their last frost later. The 90% date is early: in 9 years out of 10 frost still came after it.
For fall it runs the other way: the 10% date is early (1 year in 10 the first frost came before it), the 90% date is late.
At Detroit Metro Airport, Michigan, for example, the last 32°F frost is Apr 6 at 90%, Apr 20 at 50% and May 8 at 10%; the first fall frost is Oct 14, Oct 27 and Nov 9.
Why two thresholds
32°F (0°C) is a light freeze: it kills frost-tender plants such as tomatoes, peppers, squash, beans and basil. 28°F is a hard freeze that also damages many hardier vegetables and flowers. Hardy crops like kale, spinach and garlic often keep growing after a light freeze.
Matching a place to a station
Cities and ZIP codes are located by their center point in the U.S. Census Bureau Gazetteer files. Each place is matched to the nearest NOAA station with complete freeze data, preferring a station within 50 km (31 miles) and within 150 m (about 500 ft) of the place's elevation.
When no station meets both limits, the page says so and shows the two nearest stations, so you can see how much the dates vary. Frost depends strongly on local terrain: cold air pools in valleys, and a station at an airport may frost earlier or later than your yard.
Planting dates
Planting times are counted in weeks before or after the frost dates, using rules published by university extension services. Each crop names the publication its rule comes from, and where sources differ the range is shown. Frostplot does not invent planting rules.
Limits
Normals describe the 30 years from 1991 to 2020. A single year can be far from them. Check the local forecast before planting out tender plants.