Linn County climate

Linn County Frost Dates and Growing Season

NOAA 1991-2020 normals for Mound City, 7.8 km from Linn County, elevation 840 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Mound 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 April 13 and the first fall freeze around October 23, a growing season of about 192 days.

Median at 36°F
last April 25, first October 13
Median at 32°F
last April 13, first October 23
Median at 28°F
last April 2, first November 1

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 8May 4May 1Apr 29Apr 25Apr 22Apr 18Apr 14Apr 8
32°FMay 1Apr 25Apr 21Apr 16Apr 13Apr 9Apr 6Apr 2Mar 29
28°FApr 18Apr 11Apr 7Apr 4Apr 2Mar 31Mar 26Mar 21Mar 15
24°FApr 6Apr 2Mar 30Mar 26Mar 22Mar 18Mar 14Mar 9Mar 3
20°FMar 30Mar 23Mar 18Mar 14Mar 10Mar 6Mar 3Feb 28Feb 22
16°FMar 20Mar 14Mar 8Mar 5Mar 2Feb 27Feb 23Feb 18Feb 10

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 28Oct 3Oct 7Oct 10Oct 13Oct 16Oct 19Oct 22Oct 27
32°FOct 8Oct 14Oct 18Oct 20Oct 23Oct 26Oct 28Oct 31Nov 4
28°FOct 19Oct 23Oct 27Oct 30Nov 1Nov 3Nov 6Nov 11Nov 17
24°FOct 28Nov 1Nov 5Nov 8Nov 12Nov 15Nov 19Nov 24Nov 29
20°FNov 5Nov 10Nov 15Nov 19Nov 23Nov 27Nov 30Dec 3Dec 9
16°FNov 15Nov 22Nov 27Nov 30Dec 2Dec 6Dec 11Dec 16Dec 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°F191184178174169165161155147
32°F215207201197192187182177170
28°F236229222217212209204199190
24°F259250244239234230225219211
20°F280272265260256251245238229
16°F302293287282276270265258250

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
4,941
Cooling degree days, base 65°F
1,355
Growing degree days, base 40°F
6,461
Growing degree days, base 45°F
5,164
Growing degree days, base 50°F
4,013
Growing degree days, base 55°F
2,987
Growing degree days, base 60°F
2,098

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,06903
February84809
March625373
April33519195
May11786437
June9281721
July0422887
August3367829
September63150539
October28927249
November626262
December95808

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 7.8 km from Linn 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 USC00145528 (Mound City). Public domain.