Iron County climate

Iron County Frost Dates and Growing Season

NOAA 1991-2020 normals for Cedar City Muni AP, 23.9 km from Iron County, elevation 5586 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Cedar City Muni 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 21 and the first fall freeze around September 29, a growing season of about 130 days.

Median at 36°F
last June 3, first September 21
Median at 32°F
last May 21, first September 29
Median at 28°F
last May 6, first October 9

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 18Jun 12Jun 8Jun 5Jun 3May 31May 29May 24May 19
32°FJun 8Jun 2May 30May 24May 21May 17May 13May 9May 4
28°FMay 25May 17May 13May 9May 6May 2Apr 30Apr 26Apr 21
24°FMay 10May 4Apr 30Apr 27Apr 23Apr 19Apr 15Apr 10Apr 4
20°FApr 27Apr 18Apr 12Apr 7Apr 3Mar 31Mar 26Mar 21Mar 13
16°FApr 7Apr 1Mar 25Mar 19Mar 13Mar 9Mar 6Mar 2Feb 23

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 8Sep 13Sep 16Sep 19Sep 21Sep 23Sep 26Sep 28Oct 1
32°FSep 17Sep 22Sep 25Sep 27Sep 29Oct 2Oct 5Oct 9Oct 14
28°FSep 26Sep 30Oct 3Oct 6Oct 9Oct 12Oct 16Oct 19Oct 24
24°FOct 4Oct 10Oct 14Oct 17Oct 20Oct 24Oct 27Nov 1Nov 5
20°FOct 15Oct 20Oct 24Oct 28Nov 1Nov 4Nov 7Nov 10Nov 15
16°FOct 24Oct 31Nov 4Nov 7Nov 10Nov 14Nov 17Nov 21Nov 25

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°F1281211171121091051019689
32°F151144140135130126121115108
28°F176168163159155150145140133
24°F204196190184179174169163155
20°F234225219213208202196188180
16°F264254248242237231226218208

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
6,002
Cooling degree days, base 65°F
678
Growing degree days, base 40°F
4,876
Growing degree days, base 45°F
3,710
Growing degree days, base 50°F
2,730
Growing degree days, base 55°F
1,912
Growing degree days, base 60°F
1,230

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,07300
February85800
March729010
April533053
May27912229
June63111500
July1282746
August4223684
September10850397
October4501105
November79508
December1,11000

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 23.9 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 USW00093129 (Cedar City Muni AP). Public domain.