Imperial Frost Dates and Growing Season

NOAA 1991-2020 normals for Imperial, 0.2 km from Imperial, elevation -64 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Imperial. 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 4 and the first fall freeze around December 26, a growing season of about 360 days.

Median at 36°F: last January 24, first December 14
Median at 32°F: last January 4, first December 26
Median at 28°F: last January 4, 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	Feb 21	Feb 11	Feb 4	Jan 31	Jan 24	Jan 16	Jan 10	Jan 4	Dec 28
32°F	Feb 7	Jan 26	Jan 16	Jan 8	Jan 4	Dec 31	Dec 28	Dec 22	Dec 13
28°F	Feb 6	Jan 24	Jan 11	Jan 8	Jan 4	Dec 30	Dec 28	Dec 25	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 27	Dec 1	Dec 6	Dec 10	Dec 14	Dec 18	Dec 21	Dec 25	Jan 1
32°F	Dec 6	Dec 12	Dec 18	Dec 22	Dec 26	Dec 30	Jan 2	Jan 9	Jan 25
28°F	Dec 13	Dec 23	Dec 27	Dec 30	Jan 1	Jan 5	Jan 9	Jan 23	Feb 6
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	354	346	339	333	326	319	312	304	292
32°F	365	365	365	365	360	355	349	343	330
28°F	365	365	365	365	365	365	365	365	362
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: 96% of years
32°F or colder: 61% of years
28°F or colder: 12% 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: 974
Cooling degree days, base 65°F: 3,921
Growing degree days, base 40°F: 12,072
Growing degree days, base 45°F: 10,251
Growing degree days, base 50°F: 8,448
Growing degree days, base 55°F: 6,734
Growing degree days, base 60°F: 5,216

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	281	2	196
February	168	11	265
March	76	74	463
April	20	180	611
May	2	397	860
June	0	612	1,062
July	0	801	1,266
August	0	820	1,285
September	0	646	1,096
October	7	321	780
November	109	56	399
December	314	1	165

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 0.2 km from Imperial. 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 USC00044223 (Imperial). Public domain.