Harbor Bluffs Frost Dates and Growing Season

NOAA 1991-2020 normals for St Petersburg Intl AP, 13.8 km from Harbor Bluffs, elevation 11 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for St Petersburg Intl AP. A median date has an even chance of a later freeze in spring, or an earlier one in fall.

A freeze (32°F or colder) happens in only 23 percent of years at St Petersburg Intl AP, so there is no typical freeze season.

Median at 36°F: last January 23, first January 12
Median at 32°F: last January 17, first January 14

Last freeze in spring

The chance of a later freeze than the date shown. At 10 percent, one spring in ten still freezes after the date; planting after the 10 percent date for 32°F is the cautious choice.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	Feb 17	Feb 8	Feb 2	Jan 29	Jan 23	Jan 18	Jan 13	Jan 7	Jan 1
32°F	Feb 6	Feb 1	Jan 26	Jan 21	Jan 17	Jan 13	Jan 10	Jan 6	Jan 1
28°F	–	–	–	–	–	–	–	–	–
24°F	–	–	–	–	–	–	–	–	–
20°F	–	–	–	–	–	–	–	–	–
16°F	–	–	–	–	–	–	–	–	–

First freeze in fall

The chance of an earlier freeze than the date shown. At 10 percent, one fall in ten freezes before the date.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	Dec 22	Dec 31	Jan 4	Jan 7	Jan 12	Jan 17	Jan 22	Jan 28	Feb 6
32°F	Dec 27	Jan 3	Jan 6	Jan 10	Jan 14	Jan 19	Jan 22	Jan 28	Feb 4
28°F	–	–	–	–	–	–	–	–	–
24°F	–	–	–	–	–	–	–	–	–
20°F	–	–	–	–	–	–	–	–	–
16°F	–	–	–	–	–	–	–	–	–

Growing season length

Days between the last spring and first fall freeze at each threshold, with the chance of a season at least that long.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	365	365	365	362	353	346	338	331	318
32°F	365	365	365	365	365	365	365	360	345
28°F	365	365	365	365	365	365	365	365	365
24°F	365	365	365	365	365	365	365	365	365
20°F	365	365	365	365	365	365	365	365	365
16°F	365	365	365	365	365	365	365	365	365

How often it gets that cold

The share of years with at least one night at or below each temperature.

36°F or colder: 63% of years
32°F or colder: 23% of years
28°F or colder: 3% of years
24°F or colder: 0% of years
20°F or colder: 0% of years
16°F or colder: 0% of years

Degree days

A degree day is one degree of the daily mean temperature beyond a base, for one day. Heating degree days count cold (the fuel a house burns), cooling degree days heat, growing degree days the warmth a crop needs; each crop has its own base.

Heating degree days, base 65°F: 447
Cooling degree days, base 65°F: 3,701
Growing degree days, base 40°F: 12,380
Growing degree days, base 45°F: 10,558
Growing degree days, base 50°F: 8,745
Growing degree days, base 55°F: 6,968
Growing degree days, base 60°F: 5,270

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	159	53	368
February	93	71	399
March	42	138	561
April	7	247	690
May	0	417	882
June	0	513	963
July	0	561	1,026
August	0	572	1,037
September	0	511	962
October	3	373	835
November	33	155	572
December	109	91	451

What this means

NOAA computes these from the daily lows at the station over 1991-2020, fitting the dates of each year's last and first freeze to a distribution, so a date can be stated at any level of risk. 36°F is a frost risk for tender plants on a clear, calm night, since the ground cools below the air above it; 32°F is a freeze; 28°F is a hard freeze that kills most annuals.

The station is 13.8 km from Harbor Bluffs. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USW00012873 (St Petersburg Intl AP). Public domain.