Johnson County climate

Johnson County Frost Dates and Growing Season

NOAA 1991-2020 normals for Iowa City Muni AP, 5.7 km from Johnson County, elevation 650 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Iowa 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 April 21 and the first fall freeze around October 18, a growing season of about 180 days.

Median at 36°F
last May 1, first October 9
Median at 32°F
last April 21, first October 18
Median at 28°F
last April 9, first October 28

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°FMay 15May 10May 6May 3May 1Apr 28Apr 25Apr 22Apr 17
32°FMay 6May 1Apr 28Apr 24Apr 21Apr 18Apr 14Apr 10Apr 5
28°FApr 27Apr 21Apr 16Apr 12Apr 9Apr 6Apr 3Mar 31Mar 25
24°FApr 15Apr 9Apr 5Apr 2Mar 31Mar 27Mar 22Mar 18Mar 11
20°FApr 3Mar 30Mar 25Mar 21Mar 18Mar 14Mar 10Mar 7Mar 2
16°FMar 24Mar 19Mar 15Mar 12Mar 9Mar 6Mar 3Feb 28Feb 24

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 26Oct 1Oct 4Oct 7Oct 9Oct 11Oct 13Oct 17Oct 21
32°FOct 4Oct 10Oct 13Oct 16Oct 18Oct 20Oct 23Oct 26Oct 31
28°FOct 15Oct 19Oct 23Oct 25Oct 28Oct 30Nov 1Nov 4Nov 9
24°FOct 26Oct 30Nov 1Nov 3Nov 6Nov 9Nov 12Nov 16Nov 20
20°FNov 3Nov 7Nov 10Nov 14Nov 17Nov 21Nov 24Nov 27Dec 2
16°FNov 9Nov 15Nov 20Nov 24Nov 27Nov 30Dec 3Dec 6Dec 13

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°F178173168164161156153147141
32°F201193188184180176172166159
28°F221214209205202197192187180
24°F245236231226222218214210203
20°F264258252248243238233228221
16°F280273269264261256251245239

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
6,114
Cooling degree days, base 65°F
1,045
Growing degree days, base 40°F
5,652
Growing degree days, base 45°F
4,485
Growing degree days, base 50°F
3,441
Growing degree days, base 55°F
2,517
Growing degree days, base 60°F
1,717

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,31000
February1,06402
March792234
April4166134
May14973395
June13233670
July2336800
August4269730
September85110477
October38415172
November756025
December1,14102

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 5.7 km from Johnson 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 USW00014937 (Iowa City Muni AP). Public domain.