Colma Frost Dates and Growing Season

NOAA 1991-2020 normals for San Francisco Oceanside, 6.9 km from Colma, elevation 8 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for San Francisco Oceanside. A median date has an even chance of a later freeze in spring, or an earlier one in fall.

A freeze (32°F or colder) happens in only 22 percent of years at San Francisco Oceanside, so there is no typical freeze season.

Median at 36°F: last January 15, first December 26
Median at 32°F: last December 30, first December 25

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 7	Feb 28	Feb 7	Jan 26	Jan 15	Jan 7	Jan 1	Dec 25	Dec 11
32°F	Mar 13	Feb 5	Jan 23	Jan 5	Dec 30	Dec 24	Dec 16	Dec 9	Dec 2
28°F	–	–	–	–	–	–	–	–	–
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 28	Dec 5	Dec 12	Dec 21	Dec 26	Jan 2	Jan 11	Jan 24	Feb 22
32°F	Nov 26	Dec 6	Dec 12	Dec 20	Dec 25	Jan 1	Jan 12	Jan 28	Feb 13
28°F	–	–	–	–	–	–	–	–	–
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	365	365	365	357	347	338	326	308	275
32°F	365	365	365	365	365	365	365	357	336
28°F	365	365	365	365	365	365	365	365	365
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: 67% of years
32°F or colder: 22% of years
28°F or colder: 5% of years
24°F or colder: 1% 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: 3,260
Cooling degree days, base 65°F: 29
Growing degree days, base 40°F: 5,895
Growing degree days, base 45°F: 4,074
Growing degree days, base 50°F: 2,303
Growing degree days, base 55°F: 862
Growing degree days, base 60°F: 174

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	392	0	91
February	322	0	108
March	331	0	141
April	308	1	145
May	298	0	169
June	246	0	204
July	216	0	250
August	172	3	296
September	153	8	305
October	175	14	305
November	258	3	197
December	389	0	94

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 6.9 km from Colma. 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 USC00047767 (San Francisco Oceanside). Public domain.