Bonsall Frost Dates and Growing Season

NOAA 1991-2020 normals for Vista, 6 km from Bonsall, elevation 430 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Vista. 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 35 percent of years at Vista, so there is no typical freeze season.

Median at 36°F: last February 16, first December 25
Median at 32°F: last January 30, first January 7
Median at 28°F: last December 30, 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 6	Mar 15	Mar 2	Feb 25	Feb 16	Feb 6	Jan 22	Jan 6	Dec 27
32°F	Mar 16	Mar 4	Feb 24	Feb 15	Jan 30	Jan 9	Dec 31	Dec 26	Dec 20
28°F	Mar 16	Mar 10	Mar 3	Feb 12	Dec 30	Dec 23	Dec 20	Dec 16	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	Dec 4	Dec 10	Dec 14	Dec 19	Dec 25	Dec 30	Jan 4	Jan 20	Feb 9
32°F	Dec 14	Dec 19	Dec 24	Dec 30	Jan 7	Jan 28	Feb 15	Feb 27	Mar 14
28°F	Dec 13	Dec 16	Dec 20	Dec 22	Dec 30	Feb 9	Mar 3	Mar 10	Mar 16
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	365	353	340	327	315	307	297	282	258
32°F	365	365	365	365	365	365	365	349	318
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: 89% of years
32°F or colder: 35% of years
28°F or colder: 10% 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: 1,593
Cooling degree days, base 65°F: 786
Growing degree days, base 40°F: 8,319
Growing degree days, base 45°F: 6,495
Growing degree days, base 50°F: 4,701
Growing degree days, base 55°F: 3,054
Growing degree days, base 60°F: 1,725

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	293	4	185
February	267	4	165
March	238	5	235
April	166	13	298
May	86	27	406
June	20	65	495
July	2	174	637
August	1	216	680
September	4	179	626
October	43	78	499
November	160	22	313
December	315	2	162

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 6 km from Bonsall. 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 USC00049378 (Vista). Public domain.