Beaverhead County Frost Dates and Growing Season

NOAA 1991-2020 normals for Dillon 18 Wsw, 8.8 km from Beaverhead County, elevation 5971 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Dillon 18 Wsw. 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 May 30, first September 14

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 17	Jun 12	Jun 7	Jun 2	May 30	May 25	May 21	May 17	May 12
24°F	May 30	May 22	May 18	May 13	May 10	May 8	May 5	May 2	Apr 28
20°F	May 13	May 8	May 4	May 1	Apr 29	Apr 26	Apr 22	Apr 17	Apr 13
16°F	May 1	Apr 26	Apr 21	Apr 16	Apr 13	Apr 9	Apr 6	Apr 2	Mar 28

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	Sep 1	Sep 4	Sep 8	Sep 11	Sep 14	Sep 17	Sep 20	Sep 23	Sep 28
24°F	Sep 11	Sep 17	Sep 21	Sep 24	Sep 28	Sep 30	Oct 2	Oct 6	Oct 12
20°F	Sep 24	Sep 30	Oct 3	Oct 5	Oct 9	Oct 12	Oct 15	Oct 19	Oct 23
16°F	Oct 4	Oct 11	Oct 15	Oct 18	Oct 21	Oct 24	Oct 27	Oct 30	Nov 3

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	62	55	50	45	41	37	33	29	23
32°F	94	86	81	77	72	68	62	56	48
28°F	128	120	114	110	105	100	95	89	83
24°F	157	151	145	140	136	132	126	121	112
20°F	184	177	172	167	163	158	154	149	142
16°F	213	204	199	194	190	185	180	174	166

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: 9,590
Cooling degree days, base 65°F: 35
Growing degree days, base 40°F: 2,511
Growing degree days, base 45°F: 1,705
Growing degree days, base 50°F: 1,053
Growing degree days, base 55°F: 550
Growing degree days, base 60°F: 204

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	1,459	0	0
February	1,247	0	0
March	1,104	0	0
April	815	0	3
May	564	0	42
June	332	2	155
July	100	21	387
August	149	13	333
September	398	1	118
October	795	0	15
November	1,154	0	0
December	1,474	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 8.8 km from Beaverhead County. 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 USW00004137 (Dillon 18 Wsw). Public domain.