Downey Frost Dates and Growing Season

NOAA 1991-2020 normals for San Gabriel Fire Dept, 16.3 km from Downey, elevation 363 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for San Gabriel Fire Dept. 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 15 percent of years at San Gabriel Fire Dept, so there is no typical freeze season.

Median at 36°F: last January 17, first December 26
Median at 32°F: last January 4, first January 2

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 20	Feb 9	Feb 2	Jan 24	Jan 17	Jan 9	Jan 4	Dec 29	Dec 20
32°F	Jan 28	Jan 22	Jan 13	Jan 8	Jan 4	Jan 1	Dec 30	Dec 27	Dec 19
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 6	Dec 12	Dec 18	Dec 22	Dec 26	Dec 30	Jan 4	Jan 12	Jan 30
32°F	Dec 18	Dec 25	Dec 28	Dec 31	Jan 2	Jan 5	Jan 10	Jan 20	Jan 26
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	360	354	346	338	331	322	307
32°F	365	365	365	365	365	365	365	365	360
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: 75% of years
32°F or colder: 15% 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: 1,145
Cooling degree days, base 65°F: 1,516
Growing degree days, base 40°F: 9,496
Growing degree days, base 45°F: 7,672
Growing degree days, base 50°F: 5,861
Growing degree days, base 55°F: 4,142
Growing degree days, base 60°F: 2,663

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	243	6	233
February	211	5	217
March	150	16	333
April	95	43	398
May	40	90	515
June	4	188	635
July	0	321	786
August	0	358	823
September	1	306	755
October	21	147	591
November	115	34	370
December	265	3	208

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 16.3 km from Downey. 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 USC00047785 (San Gabriel Fire Dept). Public domain.