Arlee climate

Arlee Frost Dates and Growing Season

NOAA 1991-2020 normals for Saint Ignatius, 17 km from Arlee, elevation 2915 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Saint Ignatius. 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 142 days.

Median at 36°F
last June 1, first September 22
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 24Jun 15Jun 10Jun 5Jun 1May 29May 25May 20May 12
32°FJun 1May 24May 19May 15May 12May 9May 6May 1Apr 28
28°FMay 17May 10May 5May 2Apr 30Apr 27Apr 24Apr 19Apr 9
24°FMay 1Apr 26Apr 19Apr 13Apr 7Apr 2Mar 29Mar 23Mar 14
20°FApr 13Apr 1Mar 27Mar 22Mar 17Mar 13Mar 9Mar 5Mar 1
16°FMar 24Mar 17Mar 12Mar 7Mar 5Mar 2Feb 26Feb 20Feb 11

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 4Sep 10Sep 14Sep 19Sep 22Sep 25Sep 27Oct 1Oct 4
32°FSep 19Sep 24Sep 28Sep 30Oct 2Oct 4Oct 6Oct 10Oct 14
28°FSep 29Oct 2Oct 6Oct 9Oct 12Oct 15Oct 19Oct 22Oct 27
24°FOct 7Oct 13Oct 18Oct 21Oct 24Oct 27Oct 31Nov 3Nov 10
20°FOct 19Oct 25Oct 30Nov 1Nov 4Nov 8Nov 13Nov 19Nov 25
16°FOct 28Nov 2Nov 6Nov 10Nov 15Nov 20Nov 25Nov 29Dec 6

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°F134126121115110105989282
32°F162154150146142137133128119
28°F193183175170166162158153145
24°F225216209204198193187180170
20°F257248241234230224218211201
16°F286274266260254248242234224

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
6,655
Cooling degree days, base 65°F
361
Growing degree days, base 40°F
4,085
Growing degree days, base 45°F
2,981
Growing degree days, base 50°F
2,070
Growing degree days, base 55°F
1,326
Growing degree days, base 60°F
756

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,09400
February94901
March79707
April551038
May3237181
June15431330
July35162592
August38139566
September18822292
October560059
November86606
December1,10000

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 17 km from Arlee. 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 USC00247286 (Saint Ignatius). Public domain.