Altadena Frost Dates and Growing Season

NOAA 1991-2020 normals for Pasadena, 4.8 km from Altadena, elevation 864 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Pasadena. 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 13 percent of years at Pasadena, so there is no typical freeze season.

Median at 36°F: last January 17, first December 28
Median at 32°F: last January 8, first January 7

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 27	Feb 17	Feb 6	Jan 27	Jan 17	Jan 11	Jan 5	Dec 31	Dec 18
32°F	Feb 17	Jan 27	Jan 14	Jan 11	Jan 8	Jan 4	Jan 2	Dec 28	Dec 20
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 12	Dec 18	Dec 22	Dec 28	Jan 1	Jan 7	Jan 20	Feb 7
32°F	Dec 20	Dec 27	Jan 1	Jan 4	Jan 7	Jan 10	Jan 14	Jan 23	Feb 14
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	361	353	344	335	320	308	292
32°F	365	365	365	365	365	365	365	365	363
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: 13% 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: 1,236
Cooling degree days, base 65°F: 1,604
Growing degree days, base 40°F: 9,493
Growing degree days, base 45°F: 7,669
Growing degree days, base 50°F: 5,867
Growing degree days, base 55°F: 4,182
Growing degree days, base 60°F: 2,739

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	261	7	219
February	216	12	221
March	164	25	328
April	100	52	402
May	53	87	499
June	7	180	623
July	0	327	792
August	0	380	845
September	2	324	773
October	23	167	609
November	127	42	365
December	283	3	192

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 4.8 km from Altadena. 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 USC00046719 (Pasadena). Public domain.