Butte climate

Butte Frost Dates and Growing Season

NOAA 1991-2020 normals for Butte Bert Mooney AP, 5.1 km from Butte, elevation 5506 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Butte Bert Mooney 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 June 13 and the first fall freeze around September 5, a growing season of about 83 days.

Median at 32°F
last June 13, first September 5
Median at 28°F
last May 21, first September 17

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°FJul 1Jun 25Jun 21Jun 17Jun 13Jun 10Jun 5Jun 1May 27
28°FJun 10Jun 2May 28May 24May 21May 18May 15May 12May 8
24°FMay 19May 15May 12May 9May 7May 4May 2Apr 30Apr 25
20°FMay 8May 4May 1Apr 29Apr 26Apr 23Apr 20Apr 15Apr 10
16°FApr 30Apr 24Apr 20Apr 16Apr 12Apr 8Apr 4Apr 1Mar 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°F–––––––––
32°FAug 24Aug 28Aug 31Sep 3Sep 5Sep 8Sep 11Sep 14Sep 19
28°FSep 2Sep 7Sep 11Sep 14Sep 17Sep 20Sep 23Sep 26Oct 1
24°FSep 15Sep 21Sep 24Sep 28Oct 1Oct 3Oct 6Oct 10Oct 15
20°FSep 27Oct 1Oct 6Oct 9Oct 12Oct 15Oct 18Oct 22Oct 26
16°FOct 7Oct 11Oct 16Oct 19Oct 23Oct 26Oct 28Oct 31Nov 4

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°F756660544944403529
32°F1059892888378746960
28°F13813212712211711310810294
24°F164158154150146142139134127
20°F188182177173169165160156149
16°F215207201197192188183178169

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
9,129
Cooling degree days, base 65°F
83
Growing degree days, base 40°F
2,742
Growing degree days, base 45°F
1,892
Growing degree days, base 50°F
1,210
Growing degree days, base 55°F
678
Growing degree days, base 60°F
299

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,39500
February1,19800
March1,03700
April78706
May538062
June2906192
July9148422
August12427372
September3682141
October758016
November1,11500
December1,42700

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 5.1 km from Butte. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

More

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USW00024135 (Butte Bert Mooney AP). Public domain.