Fort Benton climate

Fort Benton Frost Dates and Growing Season

NOAA 1991-2020 normals for Ft Benton, 1.4 km from Fort Benton, elevation 2649 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Ft Benton. 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 15 and the first fall freeze around September 22, a growing season of about 129 days.

Median at 36°F
last May 28, first September 12
Median at 32°F
last May 15, first September 22
Median at 28°F
last May 4, first October 1

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 13Jun 7Jun 2May 31May 28May 26May 23May 19May 15
32°FMay 30May 24May 20May 17May 15May 12May 10May 7May 3
28°FMay 14May 11May 8May 6May 4May 1Apr 30Apr 26Apr 21
24°FMay 6May 1Apr 29Apr 26Apr 23Apr 20Apr 16Apr 13Apr 7
20°FApr 30Apr 24Apr 19Apr 15Apr 12Apr 8Apr 5Apr 2Mar 29
16°FApr 20Apr 13Apr 9Apr 5Apr 2Mar 31Mar 27Mar 21Mar 15

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 30Sep 4Sep 7Sep 10Sep 12Sep 15Sep 18Sep 21Sep 25
32°FSep 7Sep 14Sep 17Sep 20Sep 22Sep 25Sep 27Oct 1Oct 4
28°FSep 18Sep 23Sep 26Sep 29Oct 1Oct 3Oct 7Oct 10Oct 14
24°FSep 27Oct 1Oct 4Oct 7Oct 10Oct 13Oct 16Oct 19Oct 24
20°FOct 3Oct 8Oct 12Oct 15Oct 19Oct 22Oct 26Oct 29Nov 2
16°FOct 10Oct 17Oct 22Oct 26Oct 29Nov 1Nov 4Nov 8Nov 14

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°F124118113110106102969183
32°F146140135132129125121116109
28°F168161156153149146141137131
24°F189183177174170166161156150
20°F210203197192188184180174166
16°F231223217212208203198192184

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
7,545
Cooling degree days, base 65°F
428
Growing degree days, base 40°F
4,105
Growing degree days, base 45°F
3,050
Growing degree days, base 50°F
2,164
Growing degree days, base 55°F
1,431
Growing degree days, base 60°F
850

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,32201
February1,11002
March95307
April612039
May3358187
June12246376
July24187628
August43161584
September22825272
October615160
November93808
December1,24301

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 1.4 km from Fort Benton. 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 USC00243113 (Ft Benton). Public domain.