San Mateo County Frost Dates and Growing Season

NOAA 1991-2020 normals for Half Moon Bay, 8.7 km from San Mateo County, elevation 27 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Half Moon Bay. 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 6 and the first fall freeze around December 17, a growing season of about 328 days.

Median at 36°F: last April 3, first November 20
Median at 32°F: last February 6, first December 17
Median at 28°F: last January 5, 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	May 1	Apr 22	Apr 16	Apr 10	Apr 3	Mar 29	Mar 20	Mar 8	Feb 23
32°F	Mar 25	Mar 3	Feb 26	Feb 15	Feb 6	Jan 23	Jan 7	Dec 29	Dec 17
28°F	Feb 26	Feb 16	Feb 7	Feb 1	Jan 5	Dec 30	Dec 22	Dec 15	Dec 5
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	Oct 20	Nov 3	Nov 10	Nov 15	Nov 20	Nov 25	Dec 1	Dec 8	Dec 16
32°F	Nov 25	Dec 2	Dec 6	Dec 12	Dec 17	Dec 24	Jan 1	Jan 17	Feb 13
28°F	Dec 4	Dec 8	Dec 15	Dec 22	Dec 31	Jan 6	Feb 5	Feb 15	Feb 23
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	285	264	249	241	230	222	212	201	186
32°F	365	362	348	337	328	314	302	284	266
28°F	365	365	365	365	365	365	365	365	350
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: 82% of years
28°F or colder: 19% 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: 3,701
Cooling degree days, base 65°F: 12
Growing degree days, base 40°F: 5,437
Growing degree days, base 45°F: 3,627
Growing degree days, base 50°F: 1,952
Growing degree days, base 55°F: 703
Growing degree days, base 60°F: 120

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	422	0	80
February	385	0	68
March	402	0	86
April	365	0	98
May	315	0	154
June	244	1	207
July	194	1	271
August	165	2	302
September	187	3	266
October	249	5	222
November	340	1	124
December	436	0	73

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 8.7 km from San Mateo County. 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 USC00043714 (Half Moon Bay). Public domain.