Occidental Frost Dates and Growing Season

NOAA 1991-2020 normals for Occidental, 2.8 km from Occidental, elevation 865 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Occidental. 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 29 and the first fall freeze around December 30, a growing season of about 347 days.

Median at 36°F: last March 24, first December 8
Median at 32°F: last January 29, first December 30

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	Apr 24	Apr 16	Apr 9	Mar 31	Mar 24	Mar 15	Mar 5	Feb 23	Feb 3
32°F	Mar 17	Mar 3	Feb 27	Feb 17	Jan 29	Jan 15	Jan 4	Dec 30	Dec 17
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 20	Nov 27	Dec 1	Dec 5	Dec 8	Dec 12	Dec 17	Dec 28	Jan 9
32°F	Dec 1	Dec 9	Dec 16	Dec 23	Dec 30	Jan 4	Jan 15	Feb 5	Feb 27
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	308	288	276	267	258	249	241	232	221
32°F	365	365	365	356	347	336	320	306	290
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: 100% of years
32°F or colder: 67% of years
28°F or colder: 10% 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: 3,087
Cooling degree days, base 65°F: 404
Growing degree days, base 40°F: 6,446
Growing degree days, base 45°F: 4,673
Growing degree days, base 50°F: 3,083
Growing degree days, base 55°F: 1,821
Growing degree days, base 60°F: 930

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	495	0	47
February	401	0	72
March	386	3	121
April	295	12	176
May	210	27	283
June	96	57	411
July	74	74	465
August	59	87	493
September	81	82	452
October	140	57	383
November	331	4	149
December	520	0	31

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 2.8 km from Occidental. 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 USC00046370 (Occidental). Public domain.