Helena climate

Helena Frost Dates and Growing Season

NOAA 1991-2020 normals for Helena 1 Sse, 0.9 km from Helena, elevation 1350 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Helena 1 Sse. 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 April 9 and the first fall freeze around October 31, a growing season of about 205 days.

Median at 36°F
last April 19, first October 23
Median at 32°F
last April 9, first October 31
Median at 28°F
last March 31, first November 8

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°FMay 2Apr 29Apr 25Apr 22Apr 19Apr 16Apr 13Apr 9Apr 4
32°FApr 23Apr 19Apr 15Apr 11Apr 9Apr 6Apr 3Mar 31Mar 25
28°FApr 14Apr 9Apr 5Apr 2Mar 31Mar 27Mar 22Mar 15Mar 9
24°FApr 4Mar 31Mar 25Mar 20Mar 15Mar 11Mar 7Mar 3Feb 27
20°FMar 22Mar 14Mar 9Mar 5Mar 2Feb 28Feb 25Feb 20Feb 13
16°FMar 11Mar 4Mar 1Feb 26Feb 22Feb 18Feb 12Feb 4Jan 26

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°FOct 11Oct 15Oct 18Oct 21Oct 23Oct 26Oct 28Oct 31Nov 4
32°FOct 18Oct 22Oct 26Oct 28Oct 31Nov 2Nov 5Nov 8Nov 13
28°FOct 25Oct 30Nov 2Nov 5Nov 8Nov 11Nov 14Nov 18Nov 23
24°FNov 1Nov 7Nov 11Nov 14Nov 18Nov 22Nov 26Nov 30Dec 3
20°FNov 10Nov 17Nov 22Nov 26Nov 29Dec 1Dec 4Dec 9Dec 18
16°FNov 22Nov 27Dec 1Dec 4Dec 8Dec 13Dec 19Dec 24Jan 2

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°F206199195190186183179174166
32°F227220213209205201197192184
28°F250241234229225221215210203
24°F271264258252247242236230221
20°F295285280274270264259252242
16°F323313304298292286280273263

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
4,162
Cooling degree days, base 65°F
1,932
Growing degree days, base 40°F
7,510
Growing degree days, base 45°F
6,105
Growing degree days, base 50°F
4,848
Growing degree days, base 55°F
3,738
Growing degree days, base 60°F
2,769

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January92806
February735020
March5244103
April26127253
May71152547
June3385832
July05471,012
August0495959
September25263689
October20755338
November541380
December86609

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 0.9 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 USC00344019 (Helena 1 Sse). Public domain.