Chinook climate

Chinook Frost Dates and Growing Season

NOAA 1991-2020 normals for Chinook, 0.4 km from Chinook, elevation 2420 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Chinook. 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 23, a growing season of about 130 days.

Median at 36°F
last May 25, first September 12
Median at 32°F
last May 14, first September 23
Median at 28°F
last May 3, first October 1

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 8Jun 2May 31May 28May 25May 23May 21May 17May 13
32°FMay 27May 23May 20May 17May 14May 11May 9May 6May 1
28°FMay 16May 12May 8May 5May 3May 1Apr 29Apr 25Apr 19
24°FMay 6May 2Apr 30Apr 26Apr 23Apr 21Apr 17Apr 15Apr 9
20°FApr 28Apr 22Apr 19Apr 16Apr 13Apr 10Apr 5Apr 2Mar 29
16°FApr 18Apr 12Apr 8Apr 4Apr 2Mar 31Mar 26Mar 20Mar 14

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 1Sep 5Sep 7Sep 10Sep 12Sep 15Sep 18Sep 20Sep 24
32°FSep 11Sep 15Sep 19Sep 21Sep 23Sep 25Sep 27Sep 30Oct 3
28°FSep 20Sep 24Sep 26Sep 29Oct 1Oct 2Oct 5Oct 7Oct 11
24°FSep 25Sep 30Oct 2Oct 5Oct 8Oct 11Oct 14Oct 17Oct 21
20°FOct 1Oct 5Oct 10Oct 13Oct 16Oct 19Oct 22Oct 26Oct 31
16°FOct 8Oct 15Oct 18Oct 22Oct 26Oct 29Nov 1Nov 4Nov 8

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°F1261211171131091061029791
32°F148142137134130126121118112
28°F167162157153149146143138131
24°F186180175171167162159154148
20°F208200194189185181177172164
16°F229220215210205201196190182

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
8,571
Cooling degree days, base 65°F
351
Growing degree days, base 40°F
3,765
Growing degree days, base 45°F
2,803
Growing degree days, base 50°F
1,979
Growing degree days, base 55°F
1,290
Growing degree days, base 60°F
742

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,53000
February1,28400
March1,07403
April656030
May3648168
June12343374
July28151587
August54132544
September26517232
October692039
November1,07603
December1,42600

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 0.4 km from Chinook. 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 USC00241722 (Chinook). Public domain.