Helena climate

Helena Frost Dates and Growing Season

NOAA 1991-2020 normals for Helena Rgnl AP, 5.7 km from Helena, elevation 3828 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Helena Rgnl AP. 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 9 and the first fall freeze around September 30, a growing season of about 142 days.

Median at 36°F
last May 25, first September 19
Median at 32°F
last May 9, first September 30
Median at 28°F
last April 29, first October 7

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 6Jun 1May 28May 25May 21May 18May 15May 10
32°FMay 25May 19May 15May 12May 9May 7May 4May 2Apr 29
28°FMay 11May 6May 3May 1Apr 29Apr 26Apr 22Apr 19Apr 15
24°FMay 1Apr 28Apr 24Apr 20Apr 16Apr 13Apr 10Apr 5Apr 1
20°FApr 21Apr 15Apr 10Apr 6Apr 4Mar 31Mar 28Mar 21Mar 13
16°FApr 11Apr 4Mar 31Mar 27Mar 22Mar 17Mar 12Mar 8Mar 3

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°FSep 4Sep 8Sep 12Sep 16Sep 19Sep 22Sep 25Sep 28Oct 1
32°FSep 17Sep 22Sep 25Sep 27Sep 30Oct 1Oct 4Oct 7Oct 11
28°FSep 25Sep 30Oct 2Oct 5Oct 7Oct 10Oct 14Oct 17Oct 22
24°FOct 1Oct 6Oct 11Oct 15Oct 18Oct 21Oct 25Oct 28Nov 2
20°FOct 11Oct 19Oct 24Oct 27Oct 30Nov 1Nov 5Nov 9Nov 13
16°FOct 22Oct 28Nov 1Nov 4Nov 7Nov 10Nov 14Nov 17Nov 23

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°F13512812311911411010510093
32°F159153149145142138134129121
28°F182175169166162159155151145
24°F207199194189184179175169163
20°F233224218212208203197191184
16°F254245239233228223218212201

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
7,499
Cooling degree days, base 65°F
434
Growing degree days, base 40°F
4,029
Growing degree days, base 45°F
2,998
Growing degree days, base 50°F
2,130
Growing degree days, base 55°F
1,418
Growing degree days, base 60°F
853

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,30400
February1,05800
March89805
April614033
May3507168
June14445355
July23197638
August40156583
September21028287
October603157
November96603
December1,29000

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 5.7 km from Helena. 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 USW00024144 (Helena Rgnl AP). Public domain.