Foothill Ranch Frost Dates and Growing Season

NOAA 1991-2020 normals for Tustin Irvine Rch, 8.8 km from Foothill Ranch, elevation 235 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Tustin Irvine Rch. 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 5 and the first fall freeze around December 25, a growing season of about 365 days.

Median at 36°F: last February 1, first December 13
Median at 32°F: last January 5, first December 25

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 5	Feb 24	Feb 13	Feb 6	Feb 1	Jan 25	Jan 17	Jan 10	Jan 3
32°F	Feb 2	Jan 23	Jan 14	Jan 9	Jan 5	Dec 31	Dec 28	Dec 22	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 24	Dec 1	Dec 5	Dec 9	Dec 13	Dec 17	Dec 22	Dec 31	Jan 8
32°F	Dec 3	Dec 9	Dec 13	Dec 19	Dec 25	Dec 30	Jan 3	Jan 9	Jan 20
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	345	335	327	319	311	304	295	286	275
32°F	365	365	365	365	365	358	350	342	331
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: 50% of years
28°F or colder: 7% of years
24°F or colder: 1% 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: 1,287
Cooling degree days, base 65°F: 1,246
Growing degree days, base 40°F: 9,084
Growing degree days, base 45°F: 7,260
Growing degree days, base 50°F: 5,449
Growing degree days, base 55°F: 3,748
Growing degree days, base 60°F: 2,320

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	295	2	179
February	231	3	195
March	173	10	303
April	117	24	357
May	48	67	484
June	10	127	567
July	0	256	721
August	0	316	781
September	0	269	718
October	14	144	595
November	128	26	349
December	271	3	201

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 8.8 km from Foothill Ranch. 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 USC00049087 (Tustin Irvine Rch). Public domain.