Ross Frost Dates and Growing Season
Frost and the growing season
NOAA's 1991-2020 freeze probabilities for Kentfield. 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 18, a growing season of about 334 days.
- Median at 36°F
- last March 5, first November 29
- Median at 32°F
- last January 16, first December 18
- Median at 28°F
- last December 30, first December 24
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 5 | Mar 27 | Mar 18 | Mar 11 | Mar 5 | Feb 28 | Feb 22 | Feb 11 | Jan 29 |
| 32°F | Feb 25 | Feb 15 | Feb 5 | Jan 28 | Jan 16 | Jan 10 | Jan 3 | Dec 29 | Dec 21 |
| 28°F | Jan 23 | Jan 10 | Jan 5 | Jan 2 | Dec 30 | Dec 26 | Dec 21 | Dec 15 | Dec 9 |
| 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 12 | Nov 17 | Nov 22 | Nov 25 | Nov 29 | Dec 2 | Dec 6 | Dec 11 | Dec 21 |
| 32°F | Nov 30 | Dec 4 | Dec 8 | Dec 13 | Dec 18 | Dec 23 | Dec 31 | Jan 5 | Jan 19 |
| 28°F | Dec 4 | Dec 9 | Dec 14 | Dec 18 | Dec 24 | Dec 29 | Jan 2 | Jan 8 | Jan 21 |
| 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 | 302 | 289 | 280 | 272 | 265 | 259 | 251 | 243 | 229 |
| 32°F | 365 | 357 | 349 | 342 | 334 | 326 | 317 | 306 | 292 |
| 28°F | 365 | 365 | 365 | 365 | 365 | 365 | 365 | 360 | 346 |
| 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
- 86% of years
- 28°F or colder
- 27% 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
- 2,546
- Cooling degree days, base 65°F
- 458
- Growing degree days, base 40°F
- 7,040
- Growing degree days, base 45°F
- 5,244
- Growing degree days, base 50°F
- 3,578
- Growing degree days, base 55°F
- 2,202
- Growing degree days, base 60°F
- 1,158
Growing degree days by month, base 50°F
Heating and cooling by month
| Month | Heating DD | Cooling DD | Growing DD 50 |
|---|---|---|---|
| January | 488 | 0 | 41 |
| February | 360 | 0 | 87 |
| March | 319 | 1 | 160 |
| April | 232 | 7 | 227 |
| May | 135 | 27 | 357 |
| June | 46 | 70 | 474 |
| July | 13 | 102 | 553 |
| August | 10 | 113 | 567 |
| September | 26 | 98 | 522 |
| October | 105 | 40 | 400 |
| November | 316 | 1 | 150 |
| December | 494 | 0 | 40 |
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 1.1 km from Ross. 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 USC00044500 (Kentfield). Public domain.