Rubidoux Frost Dates and Growing Season

NOAA 1991-2020 normals for Riverside Citrus Exp, 5.2 km from Rubidoux, elevation 986 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Riverside Citrus Exp. 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 45 percent of years at Riverside Citrus Exp, so there is no typical freeze season.

Median at 36°F: last February 11, first December 13
Median at 32°F: last January 10, 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	Mar 12	Mar 3	Feb 24	Feb 18	Feb 11	Feb 3	Jan 24	Jan 14	Jan 1
32°F	Feb 17	Feb 3	Jan 25	Jan 16	Jan 10	Jan 5	Jan 2	Dec 29	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	Nov 25	Dec 1	Dec 5	Dec 9	Dec 13	Dec 19	Dec 24	Dec 29	Jan 8
32°F	Dec 11	Dec 20	Dec 26	Dec 30	Jan 2	Jan 7	Jan 15	Jan 24	Feb 7
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	350	338	328	321	312	303	294	284	269
32°F	365	365	365	365	365	365	357	348	334
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: 98% of years
32°F or colder: 45% of years
28°F or colder: 2% 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: 1,266
Cooling degree days, base 65°F: 1,857
Growing degree days, base 40°F: 9,717
Growing degree days, base 45°F: 7,892
Growing degree days, base 50°F: 6,087
Growing degree days, base 55°F: 4,404
Growing degree days, base 60°F: 2,973

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	259	6	219
February	222	8	209
March	185	18	301
April	117	42	375
May	38	118	546
June	4	241	687
July	0	418	884
August	0	470	935
September	1	364	813
October	30	134	569
November	133	36	354
December	278	4	198

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 5.2 km from Rubidoux. 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 USC00047473 (Riverside Citrus Exp). Public domain.