O'Brien County climate

O'Brien County Frost Dates and Growing Season

NOAA 1991-2020 normals for Primghar, 0.4 km from O'Brien County, elevation 1520 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Primghar. 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 29 and the first fall freeze around October 10, a growing season of about 163 days.

Median at 36°F
last May 8, first October 1
Median at 32°F
last April 29, first October 10
Median at 28°F
last April 18, first October 21

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 22May 17May 13May 10May 8May 5May 2Apr 30Apr 24
32°FMay 11May 6May 3May 1Apr 29Apr 26Apr 22Apr 18Apr 13
28°FMay 1Apr 28Apr 24Apr 20Apr 18Apr 15Apr 12Apr 8Apr 3
24°FApr 22Apr 18Apr 14Apr 11Apr 9Apr 5Apr 3Mar 31Mar 25
20°FApr 15Apr 11Apr 7Apr 4Apr 1Mar 30Mar 25Mar 19Mar 13
16°FApr 9Apr 4Mar 31Mar 27Mar 23Mar 19Mar 15Mar 11Mar 6

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 23Sep 26Sep 29Oct 1Oct 3Oct 6Oct 9Oct 14
32°FSep 26Oct 1Oct 4Oct 7Oct 10Oct 13Oct 15Oct 19Oct 23
28°FOct 5Oct 10Oct 14Oct 18Oct 21Oct 24Oct 27Oct 30Nov 4
24°FOct 15Oct 21Oct 24Oct 27Oct 30Nov 1Nov 4Nov 8Nov 13
20°FOct 23Oct 28Nov 1Nov 3Nov 6Nov 9Nov 12Nov 16Nov 21
16°FOct 31Nov 4Nov 7Nov 10Nov 13Nov 16Nov 20Nov 24Nov 30

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°F163158154149146142139135128
32°F184178172167163160156151144
28°F204197193189185180176171162
24°F223216211207202198194190183
20°F241234228223219215211205198
16°F259251244239236231226220212

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
6,989
Cooling degree days, base 65°F
892
Growing degree days, base 40°F
5,224
Growing degree days, base 45°F
4,131
Growing degree days, base 50°F
3,146
Growing degree days, base 55°F
2,273
Growing degree days, base 60°F
1,521

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,45500
February1,18101
March914123
April4967108
May19660346
June28199621
July5291752
August9223679
September10999447
October43512153
November870018
December1,29100

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 0.4 km from O'Brien County. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

More

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00136800 (Primghar). Public domain.