Silverton Frost Dates and Growing Season

NOAA 1991-2020 normals for Silverton, 0.4 km from Silverton, elevation 9286 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Silverton. A median date has an even chance of a later freeze in spring, or an earlier one in fall.

NOAA does not state freeze dates for this station.

Median at 28°F: last June 22, first September 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.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	–	–	–	–	–	–	–	–	–
32°F	–	–	–	–	–	–	–	–	–
28°F	Jul 5	Jul 1	Jun 28	Jun 25	Jun 22	Jun 20	Jun 17	Jun 14	Jun 9
24°F	Jun 17	Jun 14	Jun 10	Jun 6	Jun 3	May 31	May 29	May 24	May 19
20°F	May 31	May 26	May 21	May 19	May 15	May 11	May 8	May 5	May 1
16°F	May 17	May 12	May 7	May 4	May 2	Apr 30	Apr 27	Apr 23	Apr 17

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	–	–	–	–	–	–	–	–	–
32°F	–	–	–	–	–	–	–	–	–
28°F	Aug 26	Sep 1	Sep 3	Sep 6	Sep 9	Sep 11	Sep 14	Sep 16	Sep 20
24°F	Sep 9	Sep 13	Sep 16	Sep 19	Sep 21	Sep 24	Sep 26	Sep 28	Oct 1
20°F	Sep 20	Sep 24	Sep 27	Sep 29	Oct 1	Oct 4	Oct 6	Oct 9	Oct 15
16°F	Sep 29	Oct 2	Oct 5	Oct 9	Oct 12	Oct 14	Oct 17	Oct 21	Oct 26

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	32	25	21	18	16	14	12	11	8
32°F	62	55	50	45	40	36	32	28	24
28°F	95	89	85	80	77	73	70	64	55
24°F	127	120	115	111	108	104	101	96	90
20°F	158	151	146	142	137	132	128	124	115
16°F	181	175	170	165	161	158	154	149	142

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: 11,035
Cooling degree days, base 65°F: 0
Growing degree days, base 40°F: 1,684
Growing degree days, base 45°F: 978
Growing degree days, base 50°F: 447
Growing degree days, base 55°F: 116
Growing degree days, base 60°F: 8

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Growing DD 50
January	1,573	0
February	1,351	0
March	1,259	0
April	972	0
May	716	5
June	435	66
July	264	203
August	327	146
September	521	26
October	851	0
November	1,200	0
December	1,567	0

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 0.4 km from Silverton. 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 USC00057656 (Silverton). Public domain.