Inverness climate

Inverness Frost Dates and Growing Season

NOAA 1991-2020 normals for Petaluma AP, 27.9 km from Inverness, elevation 20 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Petaluma AP. 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 February 28 and the first fall freeze around November 25, a growing season of about 271 days.

Median at 36°F
last April 6, first November 10
Median at 32°F
last February 28, first November 25
Median at 28°F
last January 17, first December 9

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 27Apr 21Apr 15Apr 11Apr 6Apr 1Mar 28Mar 19Mar 7
32°FApr 6Mar 26Mar 13Mar 6Feb 28Feb 23Feb 14Feb 4Jan 24
28°FFeb 24Feb 9Jan 31Jan 25Jan 17Jan 10Jan 3Dec 27Dec 17
24°FJan 23Jan 13Jan 7Jan 3Dec 31Dec 28Dec 22Dec 15Dec 6
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°FOct 27Nov 1Nov 3Nov 7Nov 10Nov 13Nov 17Nov 22Nov 28
32°FNov 9Nov 14Nov 18Nov 22Nov 25Nov 29Dec 2Dec 7Dec 16
28°FNov 20Nov 28Dec 2Dec 5Dec 9Dec 14Dec 22Jan 1Jan 12
24°FDec 1Dec 5Dec 11Dec 16Dec 21Dec 27Jan 1Jan 5Jan 12
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°F249238230224218213207200193
32°F313297287279271265256245230
28°F365355343336331323315306289
24°F365365365365365365355344337
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
100% of years
28°F or colder
88% of years
24°F or colder
41% of years
20°F or colder
6% 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
2,853
Cooling degree days, base 65°F
317
Growing degree days, base 40°F
6,595
Growing degree days, base 45°F
4,818
Growing degree days, base 50°F
3,210
Growing degree days, base 55°F
1,902
Growing degree days, base 60°F
933

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January518032
February396066
March3660120
April2782180
May17813299
June6846427
July2272515
August1880527
September3176495
October12027372
November3341140
December522037

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 27.9 km from Inverness. 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 USC00046826 (Petaluma AP). Public domain.