Montclair Frost Dates and Growing Season

NOAA 1991-2020 normals for Pomona/fairplex, 7 km from Montclair, elevation 999 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Pomona/fairplex. 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 8 and the first fall freeze around December 28, a growing season of about 354 days.

Median at 36°F: last February 8, first December 10
Median at 32°F: last January 8, first December 28

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 9	Mar 1	Feb 24	Feb 16	Feb 8	Jan 31	Jan 22	Jan 9	Dec 29
32°F	Feb 18	Feb 4	Jan 26	Jan 15	Jan 8	Jan 1	Dec 29	Dec 24	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 21	Nov 28	Dec 3	Dec 7	Dec 10	Dec 13	Dec 16	Dec 20	Dec 26
32°F	Dec 5	Dec 13	Dec 19	Dec 24	Dec 28	Dec 31	Jan 4	Jan 14	Feb 2
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	342	330	320	311	303	294	286	277	264
32°F	365	365	365	361	354	348	339	332	315
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: 99% of years
32°F or colder: 65% of years
28°F or colder: 7% 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: 1,457
Cooling degree days, base 65°F: 1,495
Growing degree days, base 40°F: 9,163
Growing degree days, base 45°F: 7,340
Growing degree days, base 50°F: 5,557
Growing degree days, base 55°F: 3,921
Growing degree days, base 60°F: 2,548

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	299	4	183
February	261	6	177
March	188	18	298
April	108	39	381
May	50	88	502
June	9	182	623
July	0	350	815
August	0	389	854
September	3	289	737
October	40	108	533
November	163	20	310
December	335	1	144

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 7 km from Montclair. 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 USC00047050 (Pomona/fairplex). Public domain.