Frost and freeze information is available via the NCDC Climate Normals Supplement, which can be found here. For the climate supplement, a "Freeze" event is computed as any time the temperature falls to 32 degrees, and a "Hard Freeze" event is computed as any time the temperature falls to 28 degrees. The definition of what constitutes a "hard freeze" will vary from office to office, and across areas of the country. For the NWS Tallahassee area, a Hard Freeze Warning is issued when temperatures are expected to fall below 26 degrees for at least 2 hours.
The frost dates on the climate supplement are calculated using a temperature of 36 degrees. This is because frost can form at or near ground level while observed air temperatures are slightly above freezing. Air temperatures are observed at a height of 2 meters (6.56 feet) above ground level. At night, heat radiates away from the ground, leaving the air very close to the ground the coldest. Therefore, when temperatures being observed by the sensor range between 32 and 36 degrees, the temperature at ground level may be cold enough to support frost formation, given light winds and high relative humidity.
A basic graphic explaining this concept is shown below:
We have also calculated the average dates of the first autumn frost (36 degree temperature), freeze (32 degree temperature), and hard freeze (25 degree temperature, as dictated by local criteria) for our area. They are displayed in maps below.
For the Tallahassee Airport the average first frost, using the 1981-2010 period of record, is November 3rd. The average first freeze is on November 20th, and the average first hard freeze is on December 23rd. It's important to remember that in the wintertime, the Tallahassee airport is often quite a bit cooler than areas well within the city limits.