Armstrong climate

Armstrong Frost Dates and Growing Season

NOAA 1991-2020 normals for Swea City 5N, 15.3 km from Armstrong, elevation 1180 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Swea City 5N. 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 4 and the first fall freeze around October 1, a growing season of about 150 days.

Median at 36°F
last May 11, first September 23
Median at 32°F
last May 4, first October 1
Median at 28°F
last April 25, first October 10

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

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

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°F151145141137134132128124119
32°F167161157153150146143139133
28°F189182177172168164160156149
24°F210203198194190185181177170
20°F230222217212208203198193187
16°F247239233230225220214208200

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
8,171
Cooling degree days, base 65°F
497
Growing degree days, base 40°F
4,304
Growing degree days, base 45°F
3,306
Growing degree days, base 50°F
2,421
Growing degree days, base 55°F
1,653
Growing degree days, base 60°F
1,006

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,60600
February1,33000
March1,06807
April633249
May28728244
June50120521
July14178629
August34123555
September18342325
October564485
November99007
December1,41400

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 15.3 km from Armstrong. 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 USC00138026 (Swea City 5N). Public domain.