Mason City climate

Mason City Frost Dates and Growing Season

NOAA 1991-2020 normals for Mason City, 1.1 km from Mason City, elevation 1105 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Mason City. 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 29 and the first fall freeze around October 7, a growing season of about 161 days.

Median at 36°F
last May 7, first September 30
Median at 32°F
last April 29, first October 7
Median at 28°F
last April 18, first October 18

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

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 17Sep 22Sep 25Sep 28Sep 30Oct 1Oct 4Oct 7Oct 12
32°FSep 25Sep 29Oct 1Oct 4Oct 7Oct 10Oct 13Oct 17Oct 21
28°FOct 1Oct 8Oct 12Oct 15Oct 18Oct 20Oct 23Oct 27Oct 31
24°FOct 13Oct 19Oct 23Oct 26Oct 29Oct 31Nov 2Nov 5Nov 11
20°FOct 25Oct 30Nov 1Nov 3Nov 6Nov 9Nov 12Nov 16Nov 22
16°FNov 2Nov 6Nov 9Nov 12Nov 15Nov 18Nov 22Nov 26Dec 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°F161154150146144140137132125
32°F180173169165161157152148143
28°F202194189185181178173169162
24°F224216211206202198194190183
20°F243235231226221218213208201
16°F262254248242238234230225218

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
7,654
Cooling degree days, base 65°F
607
Growing degree days, base 40°F
4,587
Growing degree days, base 45°F
3,557
Growing degree days, base 50°F
2,638
Growing degree days, base 55°F
1,834
Growing degree days, base 60°F
1,151

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,53400
February1,26700
March1,019011
April586365
May25732267
June43137545
July10218673
August23153595
September15558364
October5077108
November923010
December1,33100

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 Mason City. 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 USC00135230 (Mason City). Public domain.