Gallatin County climate

Gallatin County Frost Dates and Growing Season

NOAA 1991-2020 normals for Bozeman Montana State Univ, 16.6 km from Gallatin County, elevation 4913 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Bozeman Montana State Univ. 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 May 26 and the first fall freeze around September 22, a growing season of about 116 days.

Median at 36°F
last June 14, first September 8
Median at 32°F
last May 26, first September 22
Median at 28°F
last May 8, first October 3

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.

Threshold10%20%30%40%50%60%70%80%90%
36°FJun 28Jun 23Jun 20Jun 16Jun 14Jun 11Jun 7Jun 3May 28
32°FJun 14Jun 8Jun 3May 31May 26May 23May 19May 15May 9
28°FMay 26May 21May 15May 11May 8May 5May 2Apr 28Apr 23
24°FMay 11May 5May 1Apr 29Apr 25Apr 21Apr 18Apr 15Apr 9
20°FApr 30Apr 24Apr 20Apr 16Apr 13Apr 9Apr 5Apr 1Mar 26
16°FApr 17Apr 13Apr 8Apr 4Apr 1Mar 29Mar 24Mar 19Mar 11

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.

Threshold10%20%30%40%50%60%70%80%90%
36°FAug 26Aug 31Sep 2Sep 5Sep 8Sep 11Sep 14Sep 18Sep 23
32°FSep 5Sep 11Sep 16Sep 19Sep 22Sep 25Sep 28Oct 1Oct 5
28°FSep 18Sep 25Sep 28Oct 1Oct 3Oct 6Oct 9Oct 13Oct 19
24°FSep 29Oct 2Oct 6Oct 9Oct 13Oct 17Oct 20Oct 24Oct 30
20°FOct 5Oct 11Oct 16Oct 20Oct 24Oct 27Oct 31Nov 3Nov 7
16°FOct 16Oct 21Oct 26Oct 30Nov 1Nov 4Nov 7Nov 11Nov 17

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.

Threshold10%20%30%40%50%60%70%80%90%
36°F10810195908783797466
32°F13913212612111611210710193
28°F167160155151147143138133125
24°F192185180175170166161157149
20°F216208203198193189182177168
16°F239231224219214209204196188

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
7,828
Cooling degree days, base 65°F
230
Growing degree days, base 40°F
3,511
Growing degree days, base 45°F
2,531
Growing degree days, base 50°F
1,719
Growing degree days, base 55°F
1,068
Growing degree days, base 60°F
570

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,25100
February1,07200
March93003
April675023
May4362118
June21120272
July48107524
August6585487
September25815236
October626154
November98303
December1,27400

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 16.6 km from Gallatin County. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

More

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00241044 (Bozeman Montana State Univ). Public domain.