Big Sky Frost Dates and Growing Season

NOAA 1991-2020 normals for Big Sky 2WNW, 3.4 km from Big Sky, elevation 6590 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Big Sky 2WNW. 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 11, first September 8

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	Jun 28	Jun 21	Jun 17	Jun 14	Jun 11	Jun 8	Jun 4	Jun 1	May 26
24°F	Jun 11	Jun 3	May 30	May 25	May 21	May 18	May 14	May 10	May 6
20°F	May 20	May 14	May 11	May 8	May 5	May 3	Apr 30	Apr 27	Apr 21
16°F	May 9	May 3	Apr 30	Apr 27	Apr 24	Apr 20	Apr 18	Apr 14	Apr 9

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 23	Aug 29	Sep 1	Sep 5	Sep 8	Sep 11	Sep 14	Sep 18	Sep 23
24°F	Sep 6	Sep 13	Sep 17	Sep 21	Sep 24	Sep 27	Sep 30	Oct 3	Oct 8
20°F	Sep 22	Sep 28	Oct 1	Oct 4	Oct 6	Oct 10	Oct 14	Oct 17	Oct 23
16°F	Oct 1	Oct 5	Oct 9	Oct 14	Oct 17	Oct 21	Oct 25	Oct 28	Nov 2

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	41	35	31	28	26	23	21	18	14
32°F	73	66	61	56	50	45	42	37	31
28°F	110	102	96	91	87	82	78	72	62
24°F	145	138	133	129	124	119	115	107	99
20°F	174	167	162	157	152	148	145	140	132
16°F	195	188	183	179	174	170	165	159	151

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: 9,955
Cooling degree days, base 65°F: 11
Growing degree days, base 40°F: 2,132
Growing degree days, base 45°F: 1,372
Growing degree days, base 50°F: 778
Growing degree days, base 55°F: 345
Growing degree days, base 60°F: 91

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	1,421	0	0
February	1,218	0	0
March	1,107	0	0
April	893	0	1
May	650	0	22
June	408	0	102
July	180	7	297
August	220	3	256
September	428	0	93
October	812	0	7
November	1,163	0	0
December	1,457	0	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 3.4 km from Big Sky. 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 USC00240775 (Big Sky 2WNW). Public domain.