Glacier County climate

Glacier County Frost Dates and Growing Season

NOAA 1991-2020 normals for St. Mary 1 Ssw, 32.4 km from Glacier County, elevation 4555 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for St. Mary 1 Ssw. 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 3 and the first fall freeze around September 8, a growing season of about 94 days.

Median at 32°F
last June 3, first September 8
Median at 28°F
last May 14, first September 25

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°FJun 23Jun 16Jun 10Jun 6Jun 3May 31May 28May 24May 19
28°FMay 28May 24May 20May 17May 14May 12May 9May 6May 2
24°FMay 15May 9May 6May 3May 1Apr 29Apr 25Apr 21Apr 14
20°FMay 4Apr 29Apr 25Apr 21Apr 17Apr 14Apr 10Apr 6Apr 2
16°FApr 26Apr 20Apr 16Apr 12Apr 8Apr 5Apr 2Mar 29Mar 21

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 22Aug 29Sep 1Sep 5Sep 8Sep 12Sep 16Sep 20Sep 26
28°FSep 7Sep 14Sep 19Sep 22Sep 25Sep 28Oct 1Oct 3Oct 9
24°FSep 22Sep 28Oct 1Oct 3Oct 5Oct 9Oct 13Oct 18Oct 24
20°FOct 1Oct 4Oct 8Oct 13Oct 17Oct 22Oct 26Oct 31Nov 5
16°FOct 6Oct 13Oct 19Oct 24Oct 28Nov 1Nov 4Nov 8Nov 16

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°F756762565147433830
32°F117110104989489837768
28°F152145140136131127124119111
24°F182173168162158154150144137
20°F209199192188183179173167157
16°F228218212206201195190184177

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
9,282
Cooling degree days, base 65°F
21
Growing degree days, base 40°F
2,340
Growing degree days, base 45°F
1,509
Growing degree days, base 50°F
867
Growing degree days, base 55°F
406
Growing degree days, base 60°F
132

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,29400
February1,17700
March1,12400
April84803
May602036
June3831106
July17511303
August1968286
September4241110
October764021
November1,02002
December1,27600

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 32.4 km from Glacier 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 USW00004130 (St. Mary 1 Ssw). Public domain.