Boise County climate

Boise County Frost Dates and Growing Season

NOAA 1991-2020 normals for Idaho City, 18.6 km from Boise County, elevation 3965 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Idaho City. 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 17 and the first fall freeze around September 5, a growing season of about 80 days.

Median at 32°F
last June 17, first September 5
Median at 28°F
last May 22, first September 19

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°FJul 4Jun 28Jun 24Jun 20Jun 17Jun 13Jun 9Jun 4May 30
28°FJun 14Jun 5May 31May 26May 22May 19May 15May 11May 7
24°FMay 18May 13May 9May 6May 3Apr 30Apr 27Apr 23Apr 18
20°FMay 3Apr 27Apr 22Apr 17Apr 14Apr 10Apr 6Apr 2Mar 28
16°FApr 13Apr 6Apr 1Mar 29Mar 25Mar 20Mar 16Mar 11Mar 7

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 23Aug 28Sep 1Sep 3Sep 5Sep 8Sep 12Sep 16Sep 21
28°FSep 3Sep 7Sep 13Sep 16Sep 19Sep 22Sep 25Oct 1Oct 5
24°FSep 16Sep 22Sep 25Sep 29Oct 3Oct 6Oct 9Oct 13Oct 19
20°FOct 1Oct 6Oct 11Oct 15Oct 18Oct 23Oct 26Oct 31Nov 6
16°FOct 14Oct 20Oct 25Oct 30Nov 1Nov 5Nov 10Nov 15Nov 22

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°F726560565247413630
32°F1059690868076716557
28°F1401331281221171121069990
24°F175167160156152148143137128
20°F212203197191186182176169159
16°F249241234229223218212205197

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
7,892
Cooling degree days, base 65°F
202
Growing degree days, base 40°F
3,273
Growing degree days, base 45°F
2,344
Growing degree days, base 50°F
1,586
Growing degree days, base 55°F
976
Growing degree days, base 60°F
513

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,25900
February1,03000
March93300
April697014
May4532104
June23317250
July59100507
August7775465
September2768211
October636034
November97700
December1,26300

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 18.6 km from Boise County. 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 USC00104442 (Idaho City). Public domain.