Mackinaw climate

Mackinaw Frost Dates and Growing Season

NOAA 1991-2020 normals for Congerville 2NW, 13.6 km from Mackinaw, elevation 640 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Congerville 2NW. 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 21 and the first fall freeze around October 17, a growing season of about 178 days.

Median at 36°F
last May 2, first October 6
Median at 32°F
last April 21, first October 17
Median at 28°F
last April 9, first October 27

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 18May 13May 9May 5May 2Apr 30Apr 26Apr 22Apr 18
32°FMay 7May 2Apr 28Apr 24Apr 21Apr 18Apr 15Apr 12Apr 7
28°FApr 27Apr 19Apr 15Apr 12Apr 9Apr 6Apr 4Apr 1Mar 28
24°FApr 13Apr 7Apr 4Apr 1Mar 30Mar 27Mar 23Mar 17Mar 11
20°FApr 3Mar 30Mar 25Mar 21Mar 17Mar 13Mar 10Mar 5Feb 28
16°FMar 25Mar 18Mar 15Mar 11Mar 8Mar 4Mar 2Feb 26Feb 19

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 25Sep 28Oct 1Oct 4Oct 6Oct 9Oct 12Oct 15Oct 19
32°FOct 3Oct 7Oct 10Oct 14Oct 17Oct 19Oct 22Oct 25Oct 30
28°FOct 11Oct 17Oct 21Oct 24Oct 27Oct 30Nov 1Nov 5Nov 10
24°FOct 24Oct 30Nov 1Nov 4Nov 6Nov 10Nov 13Nov 18Nov 24
20°FNov 4Nov 8Nov 12Nov 15Nov 19Nov 22Nov 25Nov 30Dec 5
16°FNov 11Nov 16Nov 21Nov 26Nov 29Dec 2Dec 6Dec 11Dec 17

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°F176169165161156152148143135
32°F195190186181178173169163155
28°F219211206202198195190185178
24°F244237231225221217212207200
20°F268260254249245241236230222
16°F289281275269265260255250242

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
5,752
Cooling degree days, base 65°F
1,067
Growing degree days, base 40°F
5,807
Growing degree days, base 45°F
4,602
Growing degree days, base 50°F
3,532
Growing degree days, base 55°F
2,583
Growing degree days, base 60°F
1,758

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,23501
February99804
March756342
April40513154
May15686403
June20226657
July3317779
August5265725
September70132514
October34524207
November698141
December1,06303

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 13.6 km from Mackinaw. 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 USC00111836 (Congerville 2NW). Public domain.