Millbrae Frost Dates and Growing Season

NOAA 1991-2020 normals for San Francisco Intl AP, 3 km from Millbrae, elevation 8 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for San Francisco Intl AP. 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 12 percent of years at San Francisco Intl AP, so there is no typical freeze season.

Median at 36°F: last January 8, first December 24
Median at 32°F: last December 29, first December 26

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	Feb 14	Jan 30	Jan 22	Jan 15	Jan 8	Jan 3	Dec 30	Dec 25	Dec 16
32°F	Jan 13	Jan 7	Jan 3	Dec 31	Dec 29	Dec 26	Dec 23	Dec 17	Dec 11
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	Dec 5	Dec 10	Dec 15	Dec 19	Dec 24	Dec 28	Jan 1	Jan 9	Jan 24
32°F	Dec 11	Dec 17	Dec 21	Dec 24	Dec 26	Dec 30	Jan 1	Jan 6	Jan 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	364	359	353	347	340	334	319
32°F	365	365	365	365	365	365	365	365	362
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: 70% of years
32°F or colder: 12% of years
28°F or colder: 0% 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: 2,475
Cooling degree days, base 65°F: 200
Growing degree days, base 40°F: 6,850
Growing degree days, base 45°F: 5,031
Growing degree days, base 50°F: 3,281
Growing degree days, base 55°F: 1,793
Growing degree days, base 60°F: 739

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	426	0	69
February	323	0	108
March	296	1	174
April	234	5	221
May	170	14	309
June	97	23	377
July	58	26	432
August	44	41	462
September	46	56	461
October	100	33	398
November	260	2	196
December	422	0	74

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 3 km from Millbrae. 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 USW00023234 (San Francisco Intl AP). Public domain.