Harlowton climate

Harlowton Frost Dates and Growing Season

NOAA 1991-2020 normals for Harlowton, 0.7 km from Harlowton, elevation 4222 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Harlowton. 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 23 and the first fall freeze around September 16, a growing season of about 113 days.

Median at 36°F
last June 9, first September 4
Median at 32°F
last May 23, first September 16
Median at 28°F
last May 12, first September 27

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 30Jun 21Jun 16Jun 12Jun 9Jun 4May 31May 29May 24
32°FJun 6May 31May 29May 26May 23May 21May 18May 15May 12
28°FMay 23May 19May 17May 14May 12May 9May 7May 4May 1
24°FMay 15May 10May 7May 5May 3May 1Apr 28Apr 23Apr 17
20°FMay 6May 1Apr 27Apr 24Apr 20Apr 17Apr 13Apr 10Apr 4
16°FApr 26Apr 20Apr 16Apr 12Apr 9Apr 5Apr 2Mar 31Mar 24

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 20Aug 26Aug 30Sep 1Sep 4Sep 6Sep 10Sep 12Sep 16
32°FSep 1Sep 6Sep 10Sep 13Sep 16Sep 18Sep 21Sep 24Sep 28
28°FSep 12Sep 18Sep 21Sep 24Sep 27Sep 30Oct 2Oct 4Oct 8
24°FSep 23Sep 28Oct 1Oct 3Oct 6Oct 8Oct 12Oct 15Oct 19
20°FOct 2Oct 6Oct 10Oct 14Oct 16Oct 19Oct 22Oct 25Oct 29
16°FOct 7Oct 14Oct 18Oct 22Oct 26Oct 29Nov 1Nov 5Nov 9

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°F1059994898580756860
32°F13312612111811310910510093
28°F153147144140137133129124117
24°F176169164159156152148143136
20°F199192186181177172168164157
16°F221213207202197194188182174

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
7,934
Cooling degree days, base 65°F
230
Growing degree days, base 40°F
3,458
Growing degree days, base 45°F
2,494
Growing degree days, base 50°F
1,696
Growing degree days, base 55°F
1,053
Growing degree days, base 60°F
562

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,25600
February1,08801
March97202
April718017
May4452111
June18521293
July46105524
August6590492
September28112215
October668039
November97203
December1,23800

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 0.7 km from Harlowton. 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 USC00243939 (Harlowton). Public domain.