Rock County climate

Rock County Frost Dates and Growing Season

NOAA 1991-2020 normals for Afton - Janesville Wwtp, 3.8 km from Rock County, elevation 762 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Afton - Janesville 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 15, a growing season of about 170 days.

Median at 36°F
last May 6, first October 4
Median at 32°F
last April 26, first October 15
Median at 28°F
last April 15, first October 26

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 23May 17May 13May 9May 6May 4May 1Apr 27Apr 22
32°FMay 11May 5May 1Apr 29Apr 26Apr 23Apr 20Apr 17Apr 13
28°FApr 29Apr 25Apr 21Apr 18Apr 15Apr 12Apr 8Apr 5Apr 2
24°FApr 17Apr 12Apr 8Apr 5Apr 3Apr 1Mar 29Mar 24Mar 18
20°FApr 6Apr 3Mar 31Mar 28Mar 24Mar 21Mar 18Mar 14Mar 10
16°FMar 30Mar 25Mar 21Mar 19Mar 16Mar 14Mar 10Mar 7Mar 2

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 22Sep 26Sep 29Oct 1Oct 4Oct 6Oct 9Oct 12Oct 16
32°FOct 1Oct 6Oct 10Oct 12Oct 15Oct 17Oct 20Oct 23Oct 27
28°FOct 12Oct 16Oct 20Oct 23Oct 26Oct 28Oct 31Nov 3Nov 7
24°FOct 24Oct 30Nov 1Nov 3Nov 5Nov 8Nov 11Nov 16Nov 20
20°FNov 2Nov 6Nov 9Nov 13Nov 15Nov 19Nov 22Nov 25Dec 1
16°FNov 10Nov 15Nov 19Nov 21Nov 24Nov 28Dec 1Dec 5Dec 12

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°F167161157153149146142137130
32°F190182178174170166162158151
28°F212205201198193189184179172
24°F235228224220216212209204198
20°F257250245241237232228224217
16°F273266262257253249244240234

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
6,947
Cooling degree days, base 65°F
710
Growing degree days, base 40°F
4,867
Growing degree days, base 45°F
3,787
Growing degree days, base 50°F
2,831
Growing degree days, base 55°F
2,000
Growing degree days, base 60°F
1,289

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,39600
February1,16101
March931115
April543374
May23143297
June38156569
July6240699
August15188639
September13071397
October4547124
November828015
December1,21501

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 3.8 km from Rock 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 USC00470045 (Afton - Janesville Wwtp). Public domain.