Shelby County climate

Shelby County Frost Dates and Growing Season

NOAA 1991-2020 normals for Shelbyville Dam, 3.3 km from Shelby County, elevation 650 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Shelbyville Dam. 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 10 and the first fall freeze around October 24, a growing season of about 195 days.

Median at 36°F
last April 22, first October 14
Median at 32°F
last April 10, first October 24
Median at 28°F
last March 31, first November 3

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 9May 3Apr 29Apr 25Apr 22Apr 19Apr 16Apr 13Apr 9
32°FApr 28Apr 21Apr 17Apr 14Apr 10Apr 8Apr 5Apr 3Mar 30
28°FApr 14Apr 8Apr 5Apr 2Mar 31Mar 28Mar 25Mar 20Mar 15
24°FApr 4Mar 31Mar 28Mar 24Mar 21Mar 18Mar 14Mar 10Mar 4
20°FMar 28Mar 22Mar 18Mar 15Mar 11Mar 8Mar 5Mar 1Feb 23
16°FMar 20Mar 15Mar 10Mar 7Mar 3Feb 28Feb 26Feb 19Feb 10

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 30Oct 5Oct 8Oct 12Oct 14Oct 17Oct 20Oct 23Oct 27
32°FOct 8Oct 14Oct 18Oct 21Oct 24Oct 26Oct 29Nov 1Nov 5
28°FOct 20Oct 26Oct 29Nov 1Nov 3Nov 6Nov 10Nov 15Nov 21
24°FNov 1Nov 4Nov 7Nov 11Nov 15Nov 19Nov 23Nov 28Dec 4
20°FNov 8Nov 13Nov 18Nov 22Nov 26Nov 30Dec 4Dec 7Dec 14
16°FNov 16Nov 23Nov 28Dec 1Dec 5Dec 9Dec 14Dec 20Dec 25

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°F192186181177173169165159153
32°F214207204199195191186181175
28°F238231226221217213209203197
24°F263254249243238234230223217
20°F282274269263258255250244235
16°F303295288283278272267260252

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
5,321
Cooling degree days, base 65°F
1,186
Growing degree days, base 40°F
6,131
Growing degree days, base 45°F
4,883
Growing degree days, base 50°F
3,769
Growing degree days, base 55°F
2,781
Growing degree days, base 60°F
1,919

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,16102
February92407
March716350
April35613179
May12291438
June11250688
July1351815
August2304767
September54147544
October30827231
November664145
December1,00305

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 3.3 km from Shelby 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 USC00117876 (Shelbyville Dam). Public domain.