Iron County climate

Iron County Frost Dates and Growing Season

NOAA 1991-2020 normals for Amasa 1W, 6.5 km from Iron County, elevation 1470 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Amasa 1W. 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 6 and the first fall freeze around September 9, a growing season of about 94 days.

Median at 32°F
last June 6, first September 9
Median at 28°F
last May 24, 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°FJul 7Jun 21Jun 14Jun 10Jun 6Jun 3May 31May 28May 23
28°FJun 8Jun 3May 31May 27May 24May 21May 17May 13May 6
24°FMay 27May 21May 16May 13May 9May 6May 3Apr 30Apr 26
20°FMay 16May 8May 4Apr 30Apr 28Apr 26Apr 23Apr 19Apr 14
16°FMay 2Apr 28Apr 24Apr 21Apr 18Apr 15Apr 12Apr 8Apr 4

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 20Aug 29Sep 2Sep 5Sep 9Sep 13Sep 17Sep 21Sep 27
28°FSep 7Sep 14Sep 18Sep 22Sep 25Sep 29Oct 1Oct 5Oct 9
24°FSep 22Sep 27Oct 1Oct 4Oct 6Oct 9Oct 13Oct 18Oct 25
20°FOct 5Oct 10Oct 14Oct 18Oct 22Oct 26Oct 30Nov 2Nov 7
16°FOct 17Oct 24Oct 30Nov 1Nov 4Nov 6Nov 10Nov 14Nov 19

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°F897871655851453933
32°F118110105999487807259
28°F14614013412812411911410899
24°F172164158155149145141135127
20°F197189184178173169164159151
16°F219212207202197193188182173

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
9,658
Cooling degree days, base 65°F
175
Growing degree days, base 40°F
3,127
Growing degree days, base 45°F
2,265
Growing degree days, base 50°F
1,526
Growing degree days, base 55°F
918
Growing degree days, base 60°F
461

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,67900
February1,45200
March1,28503
April845014
May4477127
June18132312
July8877455
August12645386
September31313195
October707133
November1,06402
December1,47200

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 6.5 km from Iron 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 USC00200197 (Amasa 1W). Public domain.