Frost dates explained: NOAA's last and first frost probabilities and how to use them
NOAA gives each frost date at three probabilities. In Columbus, Ohio, the last spring frost (32°F) comes after April 18 in half of years, after May 5 in one year in ten and after April 4 in nine years in ten; the frost-free season lasts 191 days in a typical year.
First published · revised
Three probabilities, one date each
"After April 16 in half of years" means that in half of the years 1991-2020 the station's last 32°F night of spring came after April 16. The one-in-ten date is later and safer; the nine-in-ten date is earlier, after which frost is still likely.
Frost and hard freeze
In Columbus a hard freeze (28°F) comes after April 5 in half of springs and before November 6 in half of autumns, so the season without a hard freeze is longer than the frost-free season.
Why your garden may differ
The station's thermometer stands about five feet above open ground. Cold air pools in low spots and frost forms on grass on nights the thermometer stays above 32°F, so many gardeners add a week to the spring date.
Questions
What is the difference between a frost and a hard freeze?
NOAA reports both: 32°F (the usual frost date) and 28°F (a hard freeze, which damages most vegetation; tender plants are already hurt at 32°F), as well as 36°F, 24°F, 20°F and 16°F.
Which date should I plant by?
PlainClimate gives the odds, not advice. Extension services commonly suggest waiting for tender plants until after the date by which frost has stopped in nine years out of ten (the 10% date), and treating the median as a coin toss. A station measures air about 5 feet up, so ground frost can form on nights the station reads 33-36°F.
Source: NOAA NCEI 1991-2020 annual normals (frost probabilities).