Aromas Frost Dates and Growing Season
Frost and the growing season
NOAA's 1991-2020 freeze probabilities for Watsonville Wtr Wks. 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 28 and the first fall freeze around December 13, a growing season of about 324 days.
- Median at 36°F
- last March 15, first November 26
- Median at 32°F
- last January 28, first December 13
- Median at 28°F
- last January 2, first December 26
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 14 | Apr 5 | Mar 30 | Mar 22 | Mar 15 | Mar 9 | Mar 2 | Feb 24 | Feb 4 |
| 32°F | Mar 3 | Feb 24 | Feb 12 | Feb 3 | Jan 28 | Jan 20 | Jan 11 | Dec 31 | Dec 23 |
| 28°F | Jan 30 | Jan 19 | Jan 12 | Jan 7 | Jan 2 | Dec 31 | Dec 26 | Dec 21 | Dec 13 |
| 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 8 | Nov 15 | Nov 18 | Nov 23 | Nov 26 | Nov 29 | Dec 1 | Dec 5 | Dec 12 |
| 32°F | Nov 25 | Dec 1 | Dec 4 | Dec 8 | Dec 13 | Dec 18 | Dec 24 | Jan 1 | Jan 15 |
| 28°F | Dec 4 | Dec 11 | Dec 16 | Dec 22 | Dec 26 | Dec 31 | Jan 2 | Jan 8 | Jan 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 | 297 | 279 | 268 | 260 | 252 | 245 | 237 | 230 | 218 |
| 32°F | 362 | 347 | 339 | 334 | 324 | 315 | 306 | 294 | 278 |
| 28°F | 365 | 365 | 365 | 365 | 365 | 365 | 357 | 349 | 339 |
| 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
- 100% of years
- 32°F or colder
- 91% of years
- 28°F or colder
- 38% of years
- 24°F or colder
- 2% 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
- 2,830
- Cooling degree days, base 65°F
- 138
- Growing degree days, base 40°F
- 6,434
- Growing degree days, base 45°F
- 4,627
- Growing degree days, base 50°F
- 2,937
- Growing degree days, base 55°F
- 1,527
- Growing degree days, base 60°F
- 556
Growing degree days by month, base 50°F
Heating and cooling by month
| Month | Heating DD | Cooling DD | Growing DD 50 |
|---|---|---|---|
| January | 436 | 0 | 76 |
| February | 352 | 0 | 96 |
| March | 334 | 1 | 148 |
| April | 263 | 5 | 195 |
| May | 196 | 9 | 278 |
| June | 122 | 14 | 342 |
| July | 91 | 17 | 391 |
| August | 72 | 24 | 417 |
| September | 82 | 37 | 405 |
| October | 145 | 27 | 347 |
| November | 289 | 4 | 176 |
| December | 448 | 0 | 66 |
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 12.1 km from Aromas. 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 USC00049473 (Watsonville Wtr Wks). Public domain.