Challis climate

Challis Frost Dates and Growing Season

NOAA 1991-2020 normals for Challis AP, 2.4 km from Challis, elevation 5033 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Challis AP. 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 25, a growing season of about 131 days.

Median at 36°F
last June 4, first September 14
Median at 32°F
last May 14, first September 25
Median at 28°F
last May 1, first October 7

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 21Jun 15Jun 12Jun 8Jun 4May 31May 27May 22May 17
32°FJun 2May 26May 20May 17May 14May 11May 8May 5May 2
28°FMay 12May 9May 6May 3May 1Apr 29Apr 27Apr 23Apr 19
24°FMay 3Apr 29Apr 26Apr 23Apr 20Apr 17Apr 14Apr 10Apr 6
20°FApr 21Apr 15Apr 10Apr 6Apr 3Mar 31Mar 27Mar 20Mar 13
16°FApr 5Mar 30Mar 23Mar 16Mar 13Mar 10Mar 7Mar 4Feb 28

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 31Sep 5Sep 8Sep 11Sep 14Sep 17Sep 19Sep 22Sep 26
32°FSep 10Sep 16Sep 20Sep 23Sep 25Sep 27Sep 29Oct 3Oct 7
28°FSep 24Sep 28Oct 1Oct 4Oct 7Oct 10Oct 14Oct 17Oct 21
24°FOct 5Oct 9Oct 13Oct 16Oct 19Oct 22Oct 25Oct 28Nov 1
20°FOct 14Oct 20Oct 24Oct 28Oct 30Nov 1Nov 4Nov 7Nov 12
16°FOct 24Oct 30Nov 1Nov 5Nov 8Nov 11Nov 14Nov 17Nov 23

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°F12211511010510096918780
32°F151144140135131127123116109
28°F175170165161158154151146140
24°F202194189185181177173168162
20°F232225218213208204198192184
16°F257250245240237231226219209

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
7,795
Cooling degree days, base 65°F
312
Growing degree days, base 40°F
3,785
Growing degree days, base 45°F
2,793
Growing degree days, base 50°F
1,959
Growing degree days, base 55°F
1,272
Growing degree days, base 60°F
719

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,39700
February1,06800
March85702
April630021
May3634156
June15532333
July19152598
August36110540
September21715266
October618043
November1,03401
December1,40100

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 2.4 km from Challis. 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 USW00004114 (Challis AP). Public domain.