Mount Ayr climate

Mount Ayr Frost Dates and Growing Season

NOAA 1991-2020 normals for Mt Ayr, 1.3 km from Mount Ayr, elevation 1140 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Mt Ayr. 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 25 and the first fall freeze around October 12, a growing season of about 169 days.

Median at 36°F
last May 4, first October 1
Median at 32°F
last April 25, first October 12
Median at 28°F
last April 14, first October 23

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 20May 14May 10May 8May 4May 2Apr 29Apr 26Apr 21
32°FMay 9May 4May 1Apr 27Apr 25Apr 21Apr 18Apr 15Apr 10
28°FApr 29Apr 24Apr 21Apr 17Apr 14Apr 12Apr 8Apr 5Apr 1
24°FApr 20Apr 15Apr 11Apr 8Apr 6Apr 3Mar 31Mar 27Mar 20
20°FApr 12Apr 7Apr 3Mar 31Mar 28Mar 23Mar 19Mar 14Mar 8
16°FApr 3Mar 29Mar 24Mar 19Mar 16Mar 13Mar 9Mar 5Feb 28

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 18Sep 22Sep 26Sep 29Oct 1Oct 4Oct 7Oct 10Oct 14
32°FSep 27Oct 1Oct 6Oct 9Oct 12Oct 15Oct 18Oct 21Oct 25
28°FOct 9Oct 14Oct 18Oct 21Oct 23Oct 25Oct 28Oct 31Nov 3
24°FOct 18Oct 23Oct 26Oct 29Nov 1Nov 2Nov 5Nov 9Nov 14
20°FOct 27Nov 1Nov 3Nov 5Nov 8Nov 11Nov 14Nov 19Nov 26
16°FNov 3Nov 7Nov 11Nov 14Nov 18Nov 21Nov 26Dec 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°F168162157154150146142136130
32°F188182178173169166161156149
28°F210202198194191187183176169
24°F228221216212209205201196189
20°F250240235231226221217212206
16°F271261256251245240235229222

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
6,422
Cooling degree days, base 65°F
865
Growing degree days, base 40°F
5,314
Growing degree days, base 45°F
4,175
Growing degree days, base 50°F
3,158
Growing degree days, base 55°F
2,262
Growing degree days, base 60°F
1,495

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,33900
February1,07902
March824132
April4645115
May19346332
June26175599
July4291752
August9243699
September10794441
October41012161
November797024
December1,17001

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 1.3 km from Mount Ayr. 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 USC00135769 (Mt Ayr). Public domain.