Orange County climate

Orange County Frost Dates and Growing Season

NOAA 1991-2020 normals for Chelsea 2 Nw, 8.5 km from Orange County, elevation 1440 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Chelsea 2 Nw. 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 May 9 and the first fall freeze around October 1, a growing season of about 143 days.

Median at 36°F
last May 23, first September 22
Median at 32°F
last May 9, first October 1
Median at 28°F
last April 29, first October 10

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.

Threshold10%20%30%40%50%60%70%80%90%
36°FJun 5Jun 1May 29May 26May 23May 20May 17May 13May 8
32°FMay 24May 19May 16May 13May 9May 7May 5May 2Apr 28
28°FMay 11May 6May 3May 1Apr 29Apr 26Apr 23Apr 21Apr 17
24°FApr 28Apr 24Apr 21Apr 19Apr 17Apr 14Apr 12Apr 9Apr 6
20°FApr 19Apr 15Apr 12Apr 9Apr 7Apr 5Apr 4Apr 2Mar 31
16°FApr 10Apr 7Apr 5Apr 3Apr 2Apr 1Mar 29Mar 25Mar 20

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.

Threshold10%20%30%40%50%60%70%80%90%
36°FSep 9Sep 13Sep 16Sep 19Sep 22Sep 24Sep 26Sep 29Oct 2
32°FSep 21Sep 24Sep 28Sep 30Oct 1Oct 3Oct 6Oct 8Oct 13
28°FSep 30Oct 3Oct 5Oct 7Oct 10Oct 13Oct 16Oct 20Oct 26
24°FOct 7Oct 12Oct 16Oct 20Oct 23Oct 27Oct 31Nov 3Nov 6
20°FOct 19Oct 26Oct 30Nov 1Nov 3Nov 5Nov 9Nov 13Nov 17
16°FNov 1Nov 5Nov 9Nov 12Nov 14Nov 17Nov 20Nov 23Nov 27

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.

Threshold10%20%30%40%50%60%70%80%90%
36°F139132127124120116112108103
32°F160155150147143140135132126
28°F184177172168164161158154147
24°F207202196192188185180176169
20°F225220216212209206202197190
16°F244237233229226223220216211

How often it gets that cold

The share of years with at least one night at or below each temperature.

36°F or colder
100% of years
32°F or colder
100% of years
28°F or colder
100% of years
24°F or colder
100% of years
20°F or colder
100% of years
16°F or colder
100% 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
8,572
Cooling degree days, base 65°F
187
Growing degree days, base 40°F
3,445
Growing degree days, base 45°F
2,518
Growing degree days, base 50°F
1,709
Growing degree days, base 55°F
1,033
Growing degree days, base 60°F
515

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,51900
February1,30800
March1,18604
April757122
May3955141
June14835341
July5374487
August7957443
September25815227
October623041
November93804
December1,31000

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 8.5 km from Orange County. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

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Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00431363 (Chelsea 2 Nw). Public domain.