Alton climate

Alton Frost Dates and Growing Season

NOAA 1991-2020 normals for Sioux Ctr 2 Se, 13.8 km from Alton, elevation 1360 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Sioux Ctr 2 Se. 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 30 and the first fall freeze around October 3, a growing season of about 154 days.

Median at 36°F
last May 9, first September 24
Median at 32°F
last April 30, first October 3
Median at 28°F
last April 21, first October 14

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

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 19Sep 22Sep 24Sep 28Oct 1Oct 4Oct 7
32°FSep 19Sep 24Sep 28Oct 1Oct 3Oct 5Oct 9Oct 13Oct 18
28°FSep 29Oct 3Oct 7Oct 11Oct 14Oct 17Oct 20Oct 23Oct 27
24°FOct 8Oct 14Oct 19Oct 22Oct 24Oct 27Oct 30Nov 1Nov 6
20°FOct 20Oct 25Oct 29Oct 31Nov 2Nov 5Nov 7Nov 11Nov 16
16°FOct 29Nov 1Nov 4Nov 7Nov 10Nov 13Nov 17Nov 21Nov 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°F156150145141138134129124119
32°F174167163158154151146142135
28°F196189183179175171167161154
24°F216209204200196192187183175
20°F236228223218214210206201194
16°F252245239235230225221215207

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
6,993
Cooling degree days, base 65°F
825
Growing degree days, base 40°F
5,134
Growing degree days, base 45°F
4,041
Growing degree days, base 50°F
3,058
Growing degree days, base 55°F
2,189
Growing degree days, base 60°F
1,442

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,44400
February1,16501
March908121
April4977107
May19261350
June25194619
July6264722
August16199648
September11489430
October44211147
November882014
December1,30200

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 13.8 km from Alton. 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 USC00137700 (Sioux Ctr 2 Se). Public domain.