Napa Frost Dates and Growing Season
Frost and the growing season
NOAA's 1991-2020 freeze probabilities for Napa State Hospital. 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 February 12 and the first fall freeze around December 4, a growing season of about 299 days.
- Median at 36°F
- last March 27, first November 17
- Median at 32°F
- last February 12, first December 4
- Median at 28°F
- last January 11, first December 19
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 19 | Apr 11 | Apr 5 | Mar 31 | Mar 27 | Mar 21 | Mar 14 | Mar 6 | Feb 26 |
| 32°F | Mar 13 | Mar 3 | Feb 27 | Feb 20 | Feb 12 | Feb 5 | Jan 29 | Jan 18 | Jan 3 |
| 28°F | Feb 13 | Feb 1 | Jan 26 | Jan 17 | Jan 11 | Jan 3 | Dec 29 | Dec 24 | Dec 16 |
| 24°F | Jan 22 | Jan 11 | Jan 5 | Jan 1 | Dec 30 | Dec 26 | Dec 20 | Dec 14 | Dec 7 |
| 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 | Oct 31 | Nov 4 | Nov 10 | Nov 13 | Nov 17 | Nov 22 | Nov 26 | Nov 30 | Dec 4 |
| 32°F | Nov 15 | Nov 23 | Nov 28 | Dec 1 | Dec 4 | Dec 7 | Dec 13 | Dec 20 | Jan 4 |
| 28°F | Dec 1 | Dec 5 | Dec 9 | Dec 14 | Dec 19 | Dec 26 | Jan 1 | Jan 8 | Jan 23 |
| 24°F | Dec 5 | Dec 9 | Dec 15 | Dec 21 | Dec 25 | Dec 31 | Jan 3 | Jan 8 | Jan 19 |
| 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 | 272 | 260 | 251 | 243 | 236 | 231 | 224 | 216 | 206 |
| 32°F | 338 | 325 | 316 | 306 | 299 | 289 | 279 | 270 | 257 |
| 28°F | 365 | 365 | 364 | 356 | 348 | 341 | 334 | 327 | 316 |
| 24°F | 365 | 365 | 365 | 365 | 365 | 365 | 365 | 365 | 351 |
| 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
- 98% of years
- 28°F or colder
- 71% of years
- 24°F or colder
- 22% 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
- 2,358
- Cooling degree days, base 65°F
- 643
- Growing degree days, base 40°F
- 7,412
- Growing degree days, base 45°F
- 5,620
- Growing degree days, base 50°F
- 3,959
- Growing degree days, base 55°F
- 2,554
- Growing degree days, base 60°F
- 1,444
Growing degree days by month, base 50°F
Heating and cooling by month
| Month | Heating DD | Cooling DD | Growing DD 50 |
|---|---|---|---|
| January | 485 | 0 | 50 |
| February | 349 | 0 | 98 |
| March | 295 | 2 | 183 |
| April | 197 | 12 | 267 |
| May | 99 | 45 | 411 |
| June | 23 | 113 | 540 |
| July | 5 | 141 | 601 |
| August | 4 | 147 | 608 |
| September | 14 | 131 | 567 |
| October | 89 | 49 | 425 |
| November | 305 | 2 | 163 |
| December | 493 | 0 | 47 |
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 2.8 km from Napa. 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 USC00046074 (Napa State Hospital). Public domain.