San Luis Obispo Frost Dates and Growing Season
Frost and the growing season
NOAA's 1991-2020 freeze probabilities for San Luis Obispo Poly. A median date has an even chance of a later freeze in spring, or an earlier one in fall.
The last spring freeze typically comes around January 16 and the first fall freeze around December 25, a growing season of about 349 days.
- Median at 36°F
- last February 28, first December 1
- Median at 32°F
- last January 16, first December 25
- Median at 28°F
- last January 9, first January 6
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 | Apr 12 | Apr 3 | Mar 21 | Mar 7 | Feb 28 | Feb 22 | Feb 15 | Feb 1 | Jan 9 |
| 32°F | Feb 26 | Feb 17 | Feb 6 | Jan 30 | Jan 16 | Jan 6 | Dec 31 | Dec 24 | Dec 11 |
| 28°F | Feb 22 | Feb 11 | Feb 1 | Jan 18 | Jan 9 | Jan 2 | Dec 29 | Dec 25 | Dec 17 |
| 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 | Nov 5 | Nov 16 | Nov 21 | Nov 27 | Dec 1 | Dec 7 | Dec 12 | Dec 19 | Jan 4 |
| 32°F | Nov 25 | Dec 4 | Dec 13 | Dec 17 | Dec 25 | Dec 31 | Jan 7 | Jan 26 | Feb 12 |
| 28°F | Dec 14 | Dec 22 | Dec 29 | Dec 31 | Jan 6 | Jan 13 | Jan 24 | Feb 4 | Feb 17 |
| 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 | 331 | 316 | 300 | 287 | 276 | 266 | 257 | 245 | 228 |
| 32°F | 365 | 365 | 365 | 361 | 349 | 340 | 329 | 318 | 304 |
| 28°F | 365 | 365 | 365 | 365 | 365 | 365 | 365 | 365 | 349 |
| 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
- 99% of years
- 32°F or colder
- 68% of years
- 28°F or colder
- 20% of years
- 24°F or colder
- 4% 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
- 2,031
- Cooling degree days, base 65°F
- 418
- Growing degree days, base 40°F
- 7,513
- Growing degree days, base 45°F
- 5,692
- Growing degree days, base 50°F
- 3,929
- Growing degree days, base 55°F
- 2,370
- Growing degree days, base 60°F
- 1,171
Growing degree days by month, base 50°F
Heating and cooling by month
| Month | Heating DD | Cooling DD | Growing DD 50 |
|---|---|---|---|
| January | 329 | 2 | 153 |
| February | 289 | 4 | 149 |
| March | 264 | 6 | 217 |
| April | 221 | 12 | 246 |
| May | 158 | 22 | 329 |
| June | 76 | 39 | 413 |
| July | 28 | 67 | 504 |
| August | 17 | 86 | 535 |
| September | 31 | 90 | 509 |
| October | 82 | 71 | 454 |
| November | 196 | 19 | 278 |
| December | 341 | 1 | 144 |
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 2.5 km from San Luis Obispo. 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 USC00047851 (San Luis Obispo Poly). Public domain.