Atherton Frost Dates and Growing Season

NOAA 1991-2020 normals for Redwood City, 4 km from Atherton, elevation 31 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Redwood City. 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 17 and the first fall freeze around December 12, a growing season of about 333 days.

Median at 36°F: last February 25, first November 25
Median at 32°F: last January 17, first December 12
Median at 28°F: last December 31, 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	Mar 31	Mar 14	Mar 7	Mar 1	Feb 25	Feb 20	Feb 13	Feb 5	Jan 25
32°F	Feb 16	Feb 5	Jan 31	Jan 25	Jan 17	Jan 10	Jan 3	Dec 29	Dec 20
28°F	Jan 22	Jan 14	Jan 8	Jan 4	Dec 31	Dec 29	Dec 26	Dec 21	Dec 14
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 8	Nov 13	Nov 17	Nov 21	Nov 25	Nov 28	Dec 1	Dec 5	Dec 14
32°F	Nov 24	Dec 1	Dec 4	Dec 8	Dec 12	Dec 17	Dec 23	Jan 1	Jan 14
28°F	Dec 5	Dec 11	Dec 16	Dec 21	Dec 26	Dec 29	Jan 1	Jan 7	Jan 15
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	309	296	286	278	272	267	259	252	241
32°F	365	353	345	338	333	326	318	310	298
28°F	365	365	365	365	365	365	359	351	341
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: 88% of years
28°F or colder: 38% of years
24°F or colder: 4% 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: 2,519
Cooling degree days, base 65°F: 475
Growing degree days, base 40°F: 7,084
Growing degree days, base 45°F: 5,282
Growing degree days, base 50°F: 3,606
Growing degree days, base 55°F: 2,219
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	474	0	50
February	358	0	88
March	312	1	163
April	231	6	227
May	132	28	361
June	45	81	486
July	8	126	583
August	8	121	578
September	29	83	504
October	116	29	378
November	322	1	144
December	484	0	44

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 km from Atherton. 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 USC00047339 (Redwood City). Public domain.