Thermalito climate

Thermalito Frost Dates and Growing Season

NOAA 1991-2020 normals for Oroville, 3 km from Thermalito, elevation 171 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Oroville. 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 30 and the first fall freeze around December 13, a growing season of about 321 days.

Median at 36°F
last March 8, first November 27
Median at 32°F
last January 30, first December 13
Median at 28°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.

Threshold10%20%30%40%50%60%70%80%90%
36°FApr 17Apr 5Mar 27Mar 16Mar 8Mar 1Feb 23Feb 13Feb 3
32°FMar 3Feb 22Feb 12Feb 5Jan 30Jan 22Jan 15Jan 6Dec 26
28°FFeb 4Jan 28Jan 20Jan 11Jan 5Dec 31Dec 26Dec 21Dec 14
24°FJan 26Jan 17Jan 10Jan 3Dec 30Dec 25Dec 22Dec 17Dec 11
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.

Threshold10%20%30%40%50%60%70%80%90%
36°FNov 8Nov 14Nov 18Nov 22Nov 27Dec 1Dec 4Dec 9Dec 18
32°FNov 21Nov 30Dec 4Dec 9Dec 13Dec 19Dec 25Jan 2Jan 11
28°FDec 5Dec 11Dec 16Dec 21Dec 25Dec 30Jan 2Jan 10Jan 25
24°FDec 10Dec 15Dec 20Dec 24Dec 27Dec 31Jan 3Jan 11Jan 24
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.

Threshold10%20%30%40%50%60%70%80%90%
36°F305291280271263255244233219
32°F357345336329321312304292276
28°F365365365365361354348341331
24°F365365365365365365365365357
20°F365365365365365365365365365
16°F365365365365365365365365365

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
94% of years
28°F or colder
55% of years
24°F or colder
18% of years
20°F or colder
1% 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
2,215
Cooling degree days, base 65°F
1,700
Growing degree days, base 40°F
8,616
Growing degree days, base 45°F
6,829
Growing degree days, base 50°F
5,179
Growing degree days, base 55°F
3,773
Growing degree days, base 60°F
2,620

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January505038
February356093
March2664214
April15330328
May46143563
June8312755
July0462927
August0412877
September6256701
October7277470
November2983172
December505043

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 3 km from Thermalito. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

More

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00046521 (Oroville). Public domain.