Starbase Frost Dates and Growing Season
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 17 km from Starbase. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.
More
Sources
NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00417179 (Port Isabel). Public domain.