Del Mar Frost Dates and Growing Season
Frost and the growing season
NOAA's 1991-2020 freeze probabilities for San Diego Miramar Nas. 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 4 percent of years at San Diego Miramar Nas, so there is no typical freeze season.
- Median at 36°F
- last January 7, first December 30
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 18 | Feb 3 | Jan 25 | Jan 14 | Jan 7 | Jan 4 | Dec 31 | Dec 24 | Dec 15 |
| 32°F | – | – | – | – | – | – | – | – | – |
| 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 7 | Dec 14 | Dec 19 | Dec 24 | Dec 30 | Jan 3 | Jan 8 | Jan 21 | Feb 1 |
| 32°F | – | – | – | – | – | – | – | – | – |
| 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 | 365 | 363 | 356 | 347 | 336 | 322 |
| 32°F | 365 | 365 | 365 | 365 | 365 | 365 | 365 | 365 | 365 |
| 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
- 52% of years
- 32°F or colder
- 4% of years
- 28°F or colder
- 0% 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
- 1,311
- Cooling degree days, base 65°F
- 1,033
- Growing degree days, base 40°F
- 8,847
- Growing degree days, base 45°F
- 7,022
- Growing degree days, base 50°F
- 5,208
- Growing degree days, base 55°F
- 3,499
- Growing degree days, base 60°F
- 2,071
Growing degree days by month, base 50°F
Heating and cooling by month
| Month | Heating DD | Cooling DD | Growing DD 50 |
|---|---|---|---|
| January | 246 | 4 | 227 |
| February | 213 | 7 | 217 |
| March | 203 | 8 | 271 |
| April | 135 | 22 | 338 |
| May | 65 | 42 | 442 |
| June | 14 | 93 | 530 |
| July | 0 | 219 | 684 |
| August | 0 | 273 | 738 |
| September | 3 | 224 | 671 |
| October | 30 | 109 | 544 |
| November | 131 | 29 | 349 |
| December | 271 | 3 | 200 |
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 16.1 km from Del Mar. 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 USW00093107 (San Diego Miramar Nas). Public domain.