Aztalan climate

Aztalan Frost Dates and Growing Season

NOAA 1991-2020 normals for Lake Mills Wwtp, 2.9 km from Aztalan, elevation 811 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Lake Mills 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 29 and the first fall freeze around October 14, a growing season of about 167 days.

Median at 36°F
last May 8, first October 3
Median at 32°F
last April 29, first October 14
Median at 28°F
last April 17, 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 22May 17May 14May 11May 8May 5May 3Apr 30Apr 26
32°FMay 13May 7May 4May 1Apr 29Apr 27Apr 23Apr 20Apr 15
28°FMay 3Apr 29Apr 24Apr 20Apr 17Apr 14Apr 11Apr 7Apr 4
24°FApr 21Apr 14Apr 11Apr 8Apr 5Apr 3Apr 1Mar 29Mar 23
20°FApr 9Apr 4Apr 1Mar 30Mar 27Mar 24Mar 21Mar 17Mar 12
16°FMar 31Mar 27Mar 23Mar 21Mar 18Mar 15Mar 12Mar 9Mar 4

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 20Sep 25Sep 28Oct 1Oct 3Oct 5Oct 8Oct 12Oct 16
32°FSep 29Oct 4Oct 8Oct 11Oct 14Oct 17Oct 20Oct 23Oct 27
28°FOct 8Oct 16Oct 21Oct 24Oct 26Oct 29Oct 31Nov 3Nov 8
24°FOct 25Oct 30Nov 1Nov 4Nov 6Nov 9Nov 13Nov 17Nov 21
20°FNov 2Nov 6Nov 10Nov 14Nov 17Nov 20Nov 22Nov 25Nov 30
16°FNov 8Nov 14Nov 19Nov 21Nov 24Nov 27Nov 30Dec 3Dec 11

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°F165159154150146142138133128
32°F186180176171167163158153146
28°F209202197194190185180175167
24°F233226221217213210206201193
20°F254247241237232228224220213
16°F270262258253250246242237230

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
7,099
Cooling degree days, base 65°F
725
Growing degree days, base 40°F
4,838
Growing degree days, base 45°F
3,773
Growing degree days, base 50°F
2,830
Growing degree days, base 55°F
2,005
Growing degree days, base 60°F
1,301

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,42300
February1,18701
March965114
April559368
May24143292
June37160573
July4251711
August13193645
September13268392
October4577122
November845013
December1,23501

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 2.9 km from Aztalan. 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 USC00474482 (Lake Mills Wwtp). Public domain.