Rio Dell Frost Dates and Growing Season

NOAA 1991-2020 normals for Scotia, 1.8 km from Rio Dell, elevation 136 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Scotia. 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 1 and the first fall freeze around December 19, a growing season of about 328 days.

Median at 36°F: last March 14, first November 23
Median at 32°F: last February 1, first December 19
Median at 28°F: last January 3, first December 31

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 9	Apr 1	Mar 28	Mar 22	Mar 14	Mar 8	Feb 28	Feb 22	Feb 1
32°F	Mar 8	Feb 28	Feb 20	Feb 12	Feb 1	Jan 21	Jan 8	Dec 31	Dec 17
28°F	Jan 31	Jan 23	Jan 14	Jan 9	Jan 3	Dec 31	Dec 29	Dec 24	Dec 15
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	Nov 1	Nov 7	Nov 13	Nov 18	Nov 23	Nov 28	Dec 3	Dec 10	Dec 23
32°F	Nov 19	Nov 30	Dec 5	Dec 12	Dec 19	Dec 27	Jan 3	Jan 13	Feb 10
28°F	Dec 7	Dec 14	Dec 23	Dec 29	Dec 31	Jan 4	Jan 10	Jan 17	Jan 28
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	306	288	275	264	255	247	240	231	218
32°F	365	361	348	336	328	320	311	300	284
28°F	365	365	365	365	365	365	365	362	349
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: 100% of years
32°F or colder: 84% of years
28°F or colder: 20% of years
24°F or colder: 0% 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: 3,488
Cooling degree days, base 65°F: 55
Growing degree days, base 40°F: 5,697
Growing degree days, base 45°F: 3,926
Growing degree days, base 50°F: 2,346
Growing degree days, base 55°F: 1,119
Growing degree days, base 60°F: 348

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	477	0	55
February	413	0	61
March	423	0	84
April	349	1	118
May	267	3	203
June	175	4	279
July	109	8	364
August	79	13	400
September	102	18	366
October	211	8	264
November	368	0	112
December	516	0	40

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 1.8 km from Rio Dell. 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 USC00048045 (Scotia). Public domain.