Teton County climate

Teton County Frost Dates and Growing Season

NOAA 1991-2020 normals for Choteau, 4.1 km from Teton County, elevation 3845 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Choteau. 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 28 and the first fall freeze around September 12, a growing season of about 104 days.

Median at 32°F
last May 28, first September 12
Median at 28°F
last May 14, first September 25

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°F–––––––––
32°FJun 20Jun 10Jun 4Jun 1May 28May 25May 22May 19May 15
28°FMay 27May 23May 19May 16May 14May 11May 8May 5May 1
24°FMay 14May 9May 5May 3Apr 30Apr 28Apr 25Apr 20Apr 15
20°FMay 4Apr 29Apr 25Apr 22Apr 18Apr 14Apr 11Apr 7Apr 3
16°FApr 26Apr 20Apr 16Apr 12Apr 9Apr 6Apr 2Mar 30Mar 21

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°F–––––––––
32°FAug 30Sep 3Sep 7Sep 10Sep 12Sep 15Sep 18Sep 21Sep 27
28°FSep 9Sep 16Sep 19Sep 22Sep 25Sep 28Oct 1Oct 3Oct 7
24°FSep 24Sep 29Oct 1Oct 3Oct 5Oct 8Oct 11Oct 16Oct 21
20°FOct 1Oct 5Oct 9Oct 12Oct 16Oct 19Oct 22Oct 25Oct 30
16°FOct 6Oct 13Oct 17Oct 21Oct 24Oct 27Oct 30Nov 2Nov 8

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°F928478736963585243
32°F12411811310810499948778
28°F151145141137134130125120112
24°F177170165161157154149145138
20°F201194188184179175170164158
16°F221212206201197192186181172

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
7,845
Cooling degree days, base 65°F
155
Growing degree days, base 40°F
3,412
Growing degree days, base 45°F
2,393
Growing degree days, base 50°F
1,568
Growing degree days, base 55°F
917
Growing degree days, base 60°F
443

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,22800
February1,07702
March96105
April680022
May4352114
June21713254
July6772470
August9057435
September28910206
October644254
November95107
December1,20900

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

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Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00241737 (Choteau). Public domain.