Boone County climate

Boone County Frost Dates and Growing Season

NOAA 1991-2020 normals for Marengo, 18.9 km from Boone County, elevation 815 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Marengo. 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 28 and the first fall freeze around October 12, a growing season of about 166 days.

Median at 36°F
last May 10, first October 3
Median at 32°F
last April 28, first October 12
Median at 28°F
last April 18, first October 25

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

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 21Sep 25Sep 28Oct 1Oct 3Oct 5Oct 7Oct 11Oct 16
32°FSep 28Oct 2Oct 6Oct 9Oct 12Oct 16Oct 19Oct 23Oct 27
28°FOct 7Oct 15Oct 19Oct 21Oct 25Oct 28Oct 31Nov 2Nov 6
24°FOct 21Oct 26Oct 31Nov 1Nov 4Nov 7Nov 11Nov 14Nov 19
20°FNov 1Nov 5Nov 8Nov 12Nov 15Nov 18Nov 21Nov 25Nov 29
16°FNov 8Nov 14Nov 18Nov 21Nov 24Nov 27Nov 30Dec 3Dec 9

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°F165158153149145142138132125
32°F187180174170166162157152146
28°F209203198193188184178173165
24°F233226221216211207203198188
20°F256249243239235232226221213
16°F275267262258254249244239234

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
6,822
Cooling degree days, base 65°F
726
Growing degree days, base 40°F
4,913
Growing degree days, base 45°F
3,819
Growing degree days, base 50°F
2,850
Growing degree days, base 55°F
2,013
Growing degree days, base 60°F
1,302

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,36500
February1,13401
March919119
April532481
May23943290
June45151557
July7243700
August17193642
September12482414
October4389130
November810016
December1,19301

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 18.9 km from Boone County. 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 USC00115326 (Marengo). Public domain.