Ottumwa climate

Ottumwa Frost Dates and Growing Season

NOAA 1991-2020 normals for Ottumwa Industrial AP, 10.2 km from Ottumwa, elevation 842 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Ottumwa Industrial 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 23 and the first fall freeze around October 14, a growing season of about 172 days.

Median at 36°F
last May 3, first October 3
Median at 32°F
last April 23, first October 14
Median at 28°F
last April 13, first October 24

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 16May 12May 8May 5May 3Apr 30Apr 27Apr 23Apr 18
32°FMay 7May 3Apr 29Apr 26Apr 23Apr 20Apr 17Apr 13Apr 9
28°FApr 28Apr 23Apr 20Apr 16Apr 13Apr 10Apr 8Apr 4Apr 1
24°FApr 20Apr 14Apr 9Apr 6Apr 4Apr 1Mar 29Mar 24Mar 15
20°FApr 9Apr 3Mar 31Mar 27Mar 23Mar 19Mar 15Mar 11Mar 6
16°FMar 31Mar 23Mar 19Mar 15Mar 12Mar 10Mar 6Mar 3Feb 27

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 19Sep 24Sep 28Sep 30Oct 3Oct 5Oct 9Oct 12Oct 17
32°FSep 29Oct 3Oct 7Oct 11Oct 14Oct 17Oct 20Oct 23Oct 26
28°FOct 10Oct 16Oct 19Oct 22Oct 24Oct 26Oct 29Nov 1Nov 5
24°FOct 21Oct 25Oct 28Oct 31Nov 2Nov 4Nov 6Nov 10Nov 15
20°FNov 1Nov 3Nov 6Nov 8Nov 11Nov 14Nov 17Nov 22Nov 29
16°FNov 6Nov 10Nov 13Nov 16Nov 20Nov 23Nov 27Dec 1Dec 6

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°F172165161156152148144140133
32°F192185181177172168165159152
28°F211205200196192189185180173
24°F235227221216212207203198191
20°F257249243238232228223217210
16°F276267261255251247242236227

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
6,145
Cooling degree days, base 65°F
985
Growing degree days, base 40°F
5,572
Growing degree days, base 45°F
4,403
Growing degree days, base 50°F
3,359
Growing degree days, base 55°F
2,440
Growing degree days, base 60°F
1,645

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,30500
February1,05303
March791238
April4269134
May16267379
June17215648
July2315778
August7255713
September98107462
October39116173
November762029
December1,13302

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 10.2 km from Ottumwa. 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 USW00014950 (Ottumwa Industrial AP). Public domain.