Sierra County climate

Sierra County Frost Dates and Growing Season

NOAA 1991-2020 normals for Portola, 25.3 km from Sierra County, elevation 4850 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Portola. 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 June 11 and the first fall freeze around September 10, a growing season of about 91 days.

Median at 32°F
last June 11, first September 10
Median at 28°F
last May 17, first October 1

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°F–––––––––
32°FJul 2Jun 23Jun 18Jun 14Jun 11Jun 7Jun 4Jun 1May 25
28°FJun 4May 31May 26May 21May 17May 12May 8May 3Apr 28
24°FMay 14May 6May 1Apr 27Apr 24Apr 20Apr 17Apr 12Apr 7
20°FApr 26Apr 19Apr 13Apr 9Apr 5Apr 2Mar 29Mar 21Mar 13
16°FApr 6Mar 31Mar 25Mar 19Mar 14Mar 10Mar 5Mar 1Feb 21

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°F–––––––––
32°FAug 22Aug 29Sep 3Sep 7Sep 10Sep 13Sep 17Sep 21Sep 28
28°FSep 13Sep 20Sep 25Sep 29Oct 1Oct 5Oct 8Oct 13Oct 18
24°FOct 3Oct 8Oct 12Oct 16Oct 20Oct 23Oct 26Oct 29Nov 3
20°FOct 15Oct 23Oct 27Nov 1Nov 3Nov 6Nov 9Nov 13Nov 19
16°FOct 30Nov 6Nov 10Nov 13Nov 16Nov 20Nov 24Nov 27Dec 3

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°F716256494540363024
32°F115107101959185797262
28°F163155148142136130125119110
24°F201193187182179174168161153
20°F238229223216211206200193183
16°F274263256251244239233227215

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
6,782
Cooling degree days, base 65°F
144
Growing degree days, base 40°F
3,531
Growing degree days, base 45°F
2,468
Growing degree days, base 50°F
1,616
Growing degree days, base 55°F
943
Growing degree days, base 60°F
438

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,04900
February87600
March82301
April648015
May4341108
June20817270
July5873481
August8147431
September2186252
October525056
November80102
December1,06200

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 25.3 km from Sierra 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 USC00047085 (Portola). Public domain.