Knox climate

Knox Frost Dates and Growing Season

NOAA 1991-2020 normals for Knox Wwtp, 1.1 km from Knox, elevation 690 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Knox Wwtp. 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 26 and the first fall freeze around October 19, a growing season of about 174 days.

Median at 36°F
last May 6, first October 8
Median at 32°F
last April 26, first October 19
Median at 28°F
last April 15, first October 30

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 24May 18May 14May 10May 6May 3Apr 30Apr 27Apr 23
32°FMay 12May 7May 2Apr 29Apr 26Apr 23Apr 21Apr 17Apr 12
28°FApr 30Apr 24Apr 21Apr 18Apr 15Apr 11Apr 8Apr 5Apr 1
24°FApr 18Apr 13Apr 8Apr 5Apr 2Mar 31Mar 28Mar 23Mar 17
20°FApr 6Apr 1Mar 29Mar 25Mar 21Mar 18Mar 14Mar 10Mar 6
16°FMar 27Mar 21Mar 17Mar 13Mar 10Mar 7Mar 4Mar 1Feb 24

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 29Oct 3Oct 6Oct 8Oct 10Oct 13Oct 16Oct 21
32°FOct 5Oct 10Oct 13Oct 16Oct 19Oct 22Oct 24Oct 28Oct 31
28°FOct 15Oct 21Oct 24Oct 27Oct 30Nov 1Nov 4Nov 8Nov 12
24°FOct 30Nov 2Nov 5Nov 8Nov 11Nov 14Nov 17Nov 21Nov 26
20°FNov 8Nov 12Nov 15Nov 19Nov 22Nov 25Nov 29Dec 2Dec 9
16°FNov 14Nov 21Nov 25Nov 28Dec 1Dec 4Dec 8Dec 13Dec 20

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°F172167162157153149144140132
32°F194187183178174170165160154
28°F218211207203198194189184177
24°F243236232227222218214210203
20°F269260255250246240237232224
16°F291282276271266261257251243

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
6,298
Cooling degree days, base 65°F
697
Growing degree days, base 40°F
5,081
Growing degree days, base 45°F
3,934
Growing degree days, base 50°F
2,926
Growing degree days, base 55°F
2,047
Growing degree days, base 60°F
1,301

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,27501
February1,07002
March848123
April4814107
May20651329
June29153575
July8229686
August15181631
September12367397
October40111151
November749022
December1,09302

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 1.1 km from Knox. 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 USC00124657 (Knox Wwtp). Public domain.