Westminster Frost Dates and Growing Season
Frost and the growing season
NOAA's 1991-2020 freeze probabilities for Santa Ana Fire Stn. 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 3 percent of years at Santa Ana Fire Stn, so there is no typical freeze season.
- Median at 36°F
- last December 31, first December 27
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 1 | Jan 17 | Jan 9 | Jan 3 | Dec 31 | Dec 27 | Dec 21 | Dec 13 | Dec 8 |
| 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 6 | Dec 12 | Dec 17 | Dec 22 | Dec 27 | Dec 30 | Jan 3 | Jan 9 | Jan 27 |
| 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 | 365 | 365 | 365 | 358 | 343 |
| 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
- 29% of years
- 32°F or colder
- 3% 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
- 927
- Cooling degree days, base 65°F
- 1,335
- Growing degree days, base 40°F
- 9,533
- Growing degree days, base 45°F
- 7,708
- Growing degree days, base 50°F
- 5,886
- Growing degree days, base 55°F
- 4,111
- Growing degree days, base 60°F
- 2,544
Growing degree days by month, base 50°F
Heating and cooling by month
| Month | Heating DD | Cooling DD | Growing DD 50 |
|---|---|---|---|
| January | 199 | 13 | 280 |
| February | 170 | 13 | 264 |
| March | 129 | 21 | 357 |
| April | 79 | 40 | 411 |
| May | 31 | 75 | 508 |
| June | 4 | 139 | 585 |
| July | 0 | 267 | 732 |
| August | 0 | 308 | 773 |
| September | 0 | 266 | 716 |
| October | 12 | 148 | 601 |
| November | 89 | 41 | 402 |
| December | 214 | 5 | 257 |
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 13.1 km from Westminster. 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 USC00047888 (Santa Ana Fire Stn). Public domain.