Santa Rosa Frost Dates and Growing Season

NOAA 1991-2020 normals for Santa Rosa 3 Wnw, 4.7 km from Santa Rosa, elevation 50 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Santa Rosa 3 Wnw. 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 10 and the first fall freeze around December 27, a growing season of about 355 days.

Median at 36°F: last February 3, first December 12
Median at 32°F: last January 10, first December 27
Median at 28°F: last January 1, first December 29

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 8	Feb 28	Feb 22	Feb 11	Feb 3	Jan 27	Jan 19	Jan 8	Dec 28
32°F	Feb 24	Feb 6	Jan 29	Jan 20	Jan 10	Jan 6	Dec 31	Dec 23	Dec 13
28°F	Feb 1	Jan 19	Jan 9	Jan 6	Jan 1	Dec 28	Dec 23	Dec 20	Dec 11
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 22	Nov 30	Dec 4	Dec 7	Dec 12	Dec 18	Dec 25	Jan 1	Jan 12
32°F	Dec 3	Dec 8	Dec 13	Dec 20	Dec 27	Jan 1	Jan 5	Jan 15	Feb 1
28°F	Dec 8	Dec 15	Dec 19	Dec 23	Dec 29	Jan 1	Jan 5	Jan 9	Jan 24
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	350	339	332	324	316	307	299	290	276
32°F	365	365	365	362	355	346	338	330	314
28°F	365	365	365	365	365	365	365	363	351
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: 95% of years
32°F or colder: 63% of years
28°F or colder: 23% of years
24°F or colder: 5% 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: 643
Cooling degree days, base 65°F: 4,194
Growing degree days, base 40°F: 12,680
Growing degree days, base 45°F: 10,868
Growing degree days, base 50°F: 9,080
Growing degree days, base 55°F: 7,347
Growing degree days, base 60°F: 5,710

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	207	54	334
February	114	99	414
March	52	196	612
April	10	319	759
May	0	487	952
June	0	594	1,044
July	0	651	1,116
August	0	663	1,128
September	0	519	969
October	7	367	825
November	68	169	554
December	184	77	374

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 4.7 km from Santa Rosa. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00418059 (Santa Rosa 3 Wnw). Public domain.