Moraga Frost Dates and Growing Season

NOAA 1991-2020 normals for Berkeley, 12.3 km from Moraga, elevation 310 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Berkeley. 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 27 percent of years at Berkeley, so there is no typical freeze season.

Median at 36°F: last February 5, first December 19
Median at 32°F: last January 3, first January 1

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 20	Mar 5	Feb 25	Feb 15	Feb 5	Jan 27	Jan 16	Jan 7	Dec 29
32°F	Feb 14	Jan 21	Jan 12	Jan 6	Jan 3	Dec 31	Dec 28	Dec 23	Dec 14
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 27	Dec 4	Dec 9	Dec 14	Dec 19	Dec 25	Dec 31	Jan 7	Jan 26
32°F	Dec 10	Dec 17	Dec 25	Dec 28	Jan 1	Jan 4	Jan 7	Jan 14	Jan 29
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	360	346	338	329	319	309	299	287	271
32°F	365	365	365	365	365	365	365	360	347
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: 93% of years
32°F or colder: 27% of years
28°F or colder: 1% 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: 2,579
Cooling degree days, base 65°F: 202
Growing degree days, base 40°F: 6,749
Growing degree days, base 45°F: 4,936
Growing degree days, base 50°F: 3,213
Growing degree days, base 55°F: 1,765
Growing degree days, base 60°F: 731

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	431	0	71
February	335	0	106
March	304	2	172
April	246	6	213
May	177	13	301
June	91	28	387
July	65	25	425
August	53	36	448
September	56	55	449
October	111	34	388
November	272	3	189
December	437	0	66

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 12.3 km from Moraga. 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 USC00040693 (Berkeley). Public domain.