Keystone Frost Dates and Growing Season

NOAA 1991-2020 normals for Dillon 1 E, 5.1 km from Keystone, elevation 9065 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Dillon 1 E. 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 18, first September 6

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 6	Jun 30	Jun 25	Jun 21	Jun 18	Jun 15	Jun 12	Jun 9	Jun 5
24°F	Jun 14	Jun 8	Jun 5	Jun 2	May 31	May 29	May 25	May 22	May 18
20°F	May 29	May 23	May 20	May 17	May 15	May 12	May 9	May 6	May 3
16°F	May 17	May 12	May 8	May 6	May 3	May 1	Apr 29	Apr 25	Apr 19

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 24	Aug 30	Sep 2	Sep 4	Sep 6	Sep 9	Sep 12	Sep 15	Sep 19
24°F	Sep 5	Sep 10	Sep 13	Sep 17	Sep 19	Sep 22	Sep 25	Sep 27	Oct 1
20°F	Sep 17	Sep 22	Sep 26	Sep 28	Oct 1	Oct 3	Oct 5	Oct 8	Oct 13
16°F	Sep 27	Oct 1	Oct 4	Oct 6	Oct 10	Oct 13	Oct 16	Oct 19	Oct 23

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	30	24	21	18	16	14	12	11	8
32°F	65	58	52	47	41	37	33	29	24
28°F	97	89	86	81	78	74	69	64	58
24°F	127	121	116	112	108	105	100	94	87
20°F	155	149	144	141	136	133	129	124	117
16°F	178	171	166	162	158	155	151	147	140

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: 10,736
Cooling degree days, base 65°F: 0
Growing degree days, base 40°F: 1,736
Growing degree days, base 45°F: 1,024
Growing degree days, base 50°F: 483
Growing degree days, base 55°F: 139
Growing degree days, base 60°F: 14

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Growing DD 50
January	1,510	0
February	1,302	0
March	1,225	0
April	966	0
May	721	6
June	425	78
July	262	207
August	319	155
September	512	37
October	856	1
November	1,163	0
December	1,477	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 5.1 km from Keystone. 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 USC00052281 (Dillon 1 E). Public domain.