Port Isabel Frost Dates and Growing Season

NOAA 1991-2020 normals for Port Isabel, 10.3 km from Port Isabel, elevation 17 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Port Isabel. 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 34 percent of years at Port Isabel, so there is no typical freeze season.

Median at 36°F: last January 17, first December 25
Median at 32°F: last January 3, first January 1
Median at 28°F: last January 2, first January 1

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 24	Feb 10	Feb 2	Jan 25	Jan 17	Jan 8	Jan 1	Dec 28	Dec 18
32°F	Feb 2	Jan 24	Jan 14	Jan 8	Jan 3	Jan 1	Dec 29	Dec 20	Dec 11
28°F	Jan 22	Jan 19	Jan 9	Jan 6	Jan 2	Jan 1	Dec 29	Dec 21	Dec 12
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 1	Dec 7	Dec 15	Dec 20	Dec 25	Dec 31	Jan 3	Jan 9	Jan 31
32°F	Dec 7	Dec 15	Dec 22	Dec 28	Jan 1	Jan 3	Jan 8	Jan 18	Feb 1
28°F	Dec 12	Dec 21	Dec 27	Dec 31	Jan 1	Jan 2	Jan 8	Jan 17	Jan 20
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	363	356	347	340	331	321	306
32°F	365	365	365	365	365	365	364	355	344
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: 76% of years
32°F or colder: 34% of years
28°F or colder: 11% of years
24°F or colder: 1% 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: 551
Cooling degree days, base 65°F: 3,837
Growing degree days, base 40°F: 12,413
Growing degree days, base 45°F: 10,596
Growing degree days, base 50°F: 8,800
Growing degree days, base 55°F: 7,048
Growing degree days, base 60°F: 5,379

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	184	38	336
February	99	78	405
March	45	152	573
April	10	277	717
May	0	447	911
June	0	546	996
July	0	589	1,054
August	0	606	1,071
September	0	486	936
October	4	378	839
November	49	172	575
December	160	69	387

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 10.3 km from Port Isabel. 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 USC00417179 (Port Isabel). Public domain.