Dexter climate

Dexter Frost Dates and Growing Season

NOAA 1991-2020 normals for Winterset 1S, 28.5 km from Dexter, elevation 1100 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Winterset 1S. 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 30 and the first fall freeze around October 3, a growing season of about 157 days.

Median at 36°F
last May 9, first September 26
Median at 32°F
last April 30, first October 3
Median at 28°F
last April 20, first October 15

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 18May 14May 11May 9May 7May 4May 1Apr 27
32°FMay 11May 7May 4May 2Apr 30Apr 28Apr 24Apr 21Apr 17
28°FMay 3Apr 30Apr 26Apr 23Apr 20Apr 17Apr 15Apr 11Apr 6
24°FApr 24Apr 20Apr 17Apr 13Apr 10Apr 8Apr 5Apr 1Mar 27
20°FApr 17Apr 11Apr 7Apr 4Apr 2Mar 31Mar 26Mar 21Mar 14
16°FApr 8Apr 2Mar 31Mar 27Mar 23Mar 19Mar 14Mar 10Mar 4

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 12Sep 17Sep 20Sep 23Sep 26Sep 29Oct 1Oct 4Oct 9
32°FSep 20Sep 25Sep 28Oct 1Oct 3Oct 6Oct 11Oct 14Oct 20
28°FSep 29Oct 3Oct 8Oct 12Oct 15Oct 18Oct 22Oct 25Oct 28
24°FOct 10Oct 16Oct 20Oct 24Oct 26Oct 29Oct 31Nov 2Nov 7
20°FOct 20Oct 26Oct 30Nov 1Nov 3Nov 5Nov 9Nov 13Nov 19
16°FOct 30Nov 2Nov 6Nov 9Nov 12Nov 15Nov 19Nov 24Dec 1

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°F158152148144140136132127121
32°F178171165161157153149144138
28°F197192187183178174169164156
24°F218211206202197194190185177
20°F238231227221216211207201194
16°F259251245239234230225219210

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
6,870
Cooling degree days, base 65°F
738
Growing degree days, base 40°F
5,008
Growing degree days, base 45°F
3,903
Growing degree days, base 50°F
2,919
Growing degree days, base 55°F
2,058
Growing degree days, base 60°F
1,329

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,40000
February1,14801
March893127
April515390
May21340309
June32157575
July7255713
August18199646
September13275399
October45010141
November839019
December1,22401

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 28.5 km from Dexter. 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 USC00139132 (Winterset 1S). Public domain.