Williamson County climate

Williamson County Frost Dates and Growing Season

NOAA 1991-2020 normals for Carbondale Sewage Plt, 20.7 km from Williamson County, elevation 390 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Carbondale Sewage Plt. 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 9 and the first fall freeze around October 22, a growing season of about 195 days.

Median at 36°F
last April 22, first October 13
Median at 32°F
last April 9, first October 22
Median at 28°F
last March 29, first November 1

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 5May 2Apr 29Apr 25Apr 22Apr 19Apr 15Apr 11Apr 5
32°FApr 25Apr 20Apr 16Apr 12Apr 9Apr 5Apr 2Mar 30Mar 25
28°FApr 14Apr 8Apr 4Mar 31Mar 29Mar 26Mar 22Mar 18Mar 11
24°FApr 2Mar 29Mar 25Mar 21Mar 17Mar 13Mar 10Mar 5Feb 28
20°FMar 22Mar 17Mar 14Mar 10Mar 6Mar 3Feb 28Feb 22Feb 14
16°FMar 15Mar 9Mar 5Mar 1Feb 25Feb 20Feb 14Feb 7Feb 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 30Oct 4Oct 8Oct 10Oct 13Oct 15Oct 18Oct 21Oct 25
32°FOct 10Oct 14Oct 18Oct 20Oct 22Oct 24Oct 27Oct 30Nov 2
28°FOct 19Oct 24Oct 27Oct 29Nov 1Nov 3Nov 6Nov 11Nov 18
24°FOct 30Nov 2Nov 5Nov 9Nov 13Nov 17Nov 22Nov 27Dec 3
20°FNov 8Nov 13Nov 18Nov 23Nov 27Dec 1Dec 4Dec 10Dec 18
16°FNov 18Nov 26Dec 1Dec 5Dec 10Dec 15Dec 21Dec 26Jan 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°F193186181176173169165160154
32°F214207203198195191186181175
28°F240232226220216212208202193
24°F267257250245241236231225217
20°F293284276269265259254248241
16°F318309300295289284277269257

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
4,624
Cooling degree days, base 65°F
1,362
Growing degree days, base 40°F
6,642
Growing degree days, base 45°F
5,304
Growing degree days, base 50°F
4,111
Growing degree days, base 55°F
3,057
Growing degree days, base 60°F
2,140

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,01209
February802015
March614373
April29424229
May87113493
June6288732
July0398863
August2342806
September52159559
October27830252
November587268
December890014

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 20.7 km from Williamson 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 USC00111265 (Carbondale Sewage Plt). Public domain.