Sunnyvale Frost Dates and Growing Season
Frost and the growing season
NOAA's 1991-2020 freeze probabilities for Moffett Federal Airfield. 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 6 and the first fall freeze around December 23, a growing season of about 353 days.
- Median at 36°F
- last February 6, first December 6
- Median at 32°F
- last January 6, first December 23
- Median at 28°F
- last January 1, first December 31
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 | Mar 6 | Feb 27 | Feb 19 | Feb 12 | Feb 6 | Jan 31 | Jan 26 | Jan 15 | Jan 3 |
| 32°F | Feb 4 | Jan 26 | Jan 18 | Jan 11 | Jan 6 | Jan 1 | Dec 29 | Dec 24 | Dec 16 |
| 28°F | Jan 22 | Jan 14 | Jan 8 | Jan 5 | Jan 1 | Dec 31 | Dec 27 | Dec 22 | Dec 16 |
| 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 20 | Nov 25 | Nov 30 | Dec 2 | Dec 6 | Dec 10 | Dec 14 | Dec 19 | Dec 30 |
| 32°F | Dec 3 | Dec 9 | Dec 15 | Dec 19 | Dec 23 | Dec 28 | Jan 1 | Jan 6 | Jan 16 |
| 28°F | Dec 13 | Dec 18 | Dec 24 | Dec 28 | Dec 31 | Jan 2 | Jan 8 | Jan 14 | Jan 22 |
| 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 | 340 | 329 | 319 | 312 | 306 | 299 | 290 | 280 | 270 |
| 32°F | 365 | 365 | 365 | 358 | 353 | 347 | 342 | 335 | 323 |
| 28°F | 365 | 365 | 365 | 365 | 365 | 365 | 365 | 365 | 362 |
| 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
- 99% of years
- 32°F or colder
- 70% of years
- 28°F or colder
- 14% 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
- 2,264
- Cooling degree days, base 65°F
- 427
- Growing degree days, base 40°F
- 7,289
- Growing degree days, base 45°F
- 5,478
- Growing degree days, base 50°F
- 3,761
- Growing degree days, base 55°F
- 2,298
- Growing degree days, base 60°F
- 1,179
Growing degree days by month, base 50°F
Heating and cooling by month
| Month | Heating DD | Cooling DD | Growing DD 50 |
|---|---|---|---|
| January | 443 | 0 | 65 |
| February | 332 | 0 | 107 |
| March | 285 | 1 | 188 |
| April | 205 | 9 | 254 |
| May | 116 | 26 | 375 |
| June | 39 | 63 | 474 |
| July | 11 | 95 | 549 |
| August | 8 | 99 | 557 |
| September | 18 | 93 | 525 |
| October | 83 | 40 | 422 |
| November | 275 | 2 | 185 |
| December | 449 | 0 | 62 |
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 4.2 km from Sunnyvale. 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 USW00023244 (Moffett Federal Airfield). Public domain.