Johnson County climate

Johnson County Frost Dates and Growing Season

NOAA 1991-2020 normals for Mtn City 2, 4.8 km from Johnson County, elevation 2419 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Mtn City 2. 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 27 and the first fall freeze around October 14, a growing season of about 170 days.

Median at 36°F
last May 9, first October 6
Median at 32°F
last April 27, first October 14
Median at 28°F
last April 12, first October 24

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 25May 20May 15May 11May 9May 6May 3Apr 30Apr 24
32°FMay 15May 8May 4Apr 30Apr 27Apr 23Apr 20Apr 17Apr 12
28°FApr 28Apr 22Apr 19Apr 16Apr 12Apr 9Apr 6Apr 2Mar 29
24°FApr 15Apr 9Apr 6Apr 3Apr 1Mar 28Mar 25Mar 21Mar 15
20°FApr 3Mar 29Mar 25Mar 22Mar 18Mar 15Mar 11Mar 7Feb 28
16°FMar 24Mar 17Mar 13Mar 9Mar 5Mar 1Feb 24Feb 16Feb 8

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 23Sep 28Oct 1Oct 4Oct 6Oct 8Oct 10Oct 14Oct 20
32°FOct 3Oct 6Oct 9Oct 12Oct 14Oct 17Oct 21Oct 24Oct 30
28°FOct 10Oct 15Oct 18Oct 21Oct 24Oct 27Oct 30Nov 2Nov 6
24°FOct 20Oct 25Oct 28Nov 1Nov 3Nov 5Nov 8Nov 13Nov 19
20°FOct 31Nov 4Nov 8Nov 12Nov 15Nov 19Nov 23Nov 28Dec 4
16°FNov 10Nov 17Nov 22Nov 27Nov 30Dec 3Dec 6Dec 12Dec 22

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°F169162157153149145141136129
32°F192184179175170166161156149
28°F214208202198194190186181173
24°F240231226221217212208203195
20°F267258252246241236232226218
16°F299291282276270265259252245

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
4,837
Cooling degree days, base 65°F
632
Growing degree days, base 40°F
5,630
Growing degree days, base 45°F
4,288
Growing degree days, base 50°F
3,119
Growing degree days, base 55°F
2,125
Growing degree days, base 60°F
1,288

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January95307
February781010
March651047
April3655163
May15740359
June32126545
July7200657
August10175629
September8374443
October33511193
November613151
December849014

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 4.8 km from Johnson 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 USC00406292 (Mtn City 2). Public domain.