Spearfish climate

Spearfish Frost Dates and Growing Season

NOAA 1991-2020 normals for Spearfish, 1.2 km from Spearfish, elevation 3633 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Spearfish. 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 12 and the first fall freeze around October 2, a growing season of about 143 days.

Median at 36°F
last May 22, first September 25
Median at 32°F
last May 12, first October 2
Median at 28°F
last April 30, first October 12

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 4May 31May 28May 25May 22May 20May 18May 15May 11
32°FMay 25May 20May 17May 15May 12May 9May 6May 3Apr 29
28°FMay 14May 9May 5May 2Apr 30Apr 27Apr 24Apr 20Apr 16
24°FApr 30Apr 27Apr 24Apr 21Apr 18Apr 15Apr 12Apr 7Apr 3
20°FApr 22Apr 18Apr 14Apr 11Apr 8Apr 5Apr 2Mar 29Mar 20
16°FApr 16Apr 11Apr 6Apr 3Mar 31Mar 29Mar 23Mar 17Mar 9

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 9Sep 15Sep 19Sep 22Sep 25Sep 27Sep 30Oct 1Oct 6
32°FSep 21Sep 26Sep 29Oct 1Oct 2Oct 5Oct 9Oct 13Oct 17
28°FSep 29Oct 2Oct 6Oct 9Oct 12Oct 15Oct 18Oct 21Oct 25
24°FOct 5Oct 10Oct 14Oct 17Oct 20Oct 24Oct 27Oct 31Nov 4
20°FOct 12Oct 17Oct 22Oct 25Oct 29Nov 1Nov 5Nov 9Nov 16
16°FOct 20Oct 26Oct 30Nov 2Nov 5Nov 9Nov 13Nov 19Nov 24

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°F140135131127124120116111104
32°F162156150146143139134130123
28°F182176172168164160155151143
24°F206198192188184180175169162
20°F226218212207202198193188182
16°F246236230225220215210203195

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
7,218
Cooling degree days, base 65°F
506
Growing degree days, base 40°F
4,302
Growing degree days, base 45°F
3,238
Growing degree days, base 50°F
2,333
Growing degree days, base 55°F
1,578
Growing degree days, base 60°F
966

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,21502
February1,07503
March930016
April648146
May38010168
June11766404
July22196638
August32172605
September18558344
October569492
November878016
December1,16701

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 1.2 km from Spearfish. 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 USC00397882 (Spearfish). Public domain.