Lyon County climate

Lyon County Frost Dates and Growing Season

NOAA 1991-2020 normals for Rock Rapids, 6.4 km from Lyon County, elevation 1350 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Rock Rapids. 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 May 1 and the first fall freeze around October 4, a growing season of about 156 days.

Median at 36°F
last May 10, first September 27
Median at 32°F
last May 1, first October 4
Median at 28°F
last April 21, first October 16

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

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°F156151147143140136132127121
32°F175169164161156153149144137
28°F195188184180176172169163157
24°F215209204200196192187183177
20°F235228222217213209205200193
16°F251244239235230226221216208

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
7,625
Cooling degree days, base 65°F
753
Growing degree days, base 40°F
4,785
Growing degree days, base 45°F
3,750
Growing degree days, base 50°F
2,824
Growing degree days, base 55°F
2,014
Growing degree days, base 60°F
1,322

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,53000
February1,25000
March1,000011
April578473
May24444293
June37172585
July7263721
August18193640
September14872384
October5136107
November94209
December1,35800

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 6.4 km from Lyon 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 USC00137147 (Rock Rapids). Public domain.