Sweet Grass County climate

Sweet Grass County Frost Dates and Growing Season

NOAA 1991-2020 normals for Big Timber, 2.2 km from Sweet Grass County, elevation 4100 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Big Timber. 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 14 and the first fall freeze around September 26, a growing season of about 133 days.

Median at 36°F
last May 28, first September 13
Median at 32°F
last May 14, first September 26
Median at 28°F
last May 2, first October 5

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 12Jun 6Jun 3May 31May 28May 26May 23May 20May 16
32°FMay 27May 23May 20May 17May 14May 12May 9May 6May 2
28°FMay 16May 10May 7May 4May 2Apr 30Apr 28Apr 24Apr 18
24°FMay 4Apr 30Apr 26Apr 22Apr 19Apr 16Apr 12Apr 7Apr 1
20°FApr 24Apr 18Apr 14Apr 10Apr 6Apr 3Mar 31Mar 25Mar 18
16°FApr 14Apr 8Apr 3Mar 31Mar 27Mar 23Mar 19Mar 13Mar 6

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 28Sep 3Sep 6Sep 10Sep 13Sep 16Sep 19Sep 22Sep 28
32°FSep 10Sep 16Sep 21Sep 24Sep 26Sep 29Oct 1Oct 3Oct 8
28°FSep 22Sep 27Oct 1Oct 3Oct 5Oct 8Oct 12Oct 16Oct 21
24°FOct 1Oct 5Oct 9Oct 13Oct 16Oct 19Oct 23Oct 26Nov 1
20°FOct 7Oct 13Oct 18Oct 22Oct 26Oct 29Nov 2Nov 6Nov 11
16°FOct 15Oct 22Oct 27Oct 31Nov 3Nov 6Nov 10Nov 15Nov 21

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°F127119115110106102989385
32°F150145140137133129125120114
28°F176169164159156152149144136
24°F203195190185181176171165158
20°F226218211206201197191186178
16°F247237231226221215209203194

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
6,910
Cooling degree days, base 65°F
374
Growing degree days, base 40°F
4,134
Growing degree days, base 45°F
3,030
Growing degree days, base 50°F
2,118
Growing degree days, base 55°F
1,379
Growing degree days, base 60°F
796

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,12201
February98503
March848012
April612038
May3645163
June13439360
July21171615
August38134562
September21023281
October569274
November872010
December1,13601

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 2.2 km from Sweet Grass County. 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 USC00240780 (Big Timber). Public domain.