Affton climate

Affton Frost Dates and Growing Season

NOAA 1991-2020 normals for Cahokia, 12.2 km from Affton, elevation 400 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Cahokia. 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 6 and the first fall freeze around October 27, a growing season of about 203 days.

Median at 36°F
last April 18, first October 18
Median at 32°F
last April 6, first October 27
Median at 28°F
last March 27, first November 5

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 3Apr 28Apr 24Apr 21Apr 18Apr 15Apr 12Apr 8Apr 4
32°FApr 20Apr 16Apr 12Apr 9Apr 6Apr 3Mar 31Mar 28Mar 22
28°FApr 10Apr 5Apr 2Mar 30Mar 27Mar 24Mar 20Mar 15Mar 11
24°FMar 31Mar 26Mar 22Mar 19Mar 15Mar 13Mar 9Mar 5Feb 28
20°FMar 21Mar 16Mar 12Mar 9Mar 6Mar 3Feb 28Feb 24Feb 16
16°FMar 14Mar 9Mar 6Mar 2Feb 28Feb 23Feb 18Feb 11Feb 1

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°FOct 5Oct 10Oct 13Oct 16Oct 18Oct 20Oct 22Oct 25Oct 28
32°FOct 14Oct 20Oct 22Oct 25Oct 27Oct 29Oct 31Nov 2Nov 7
28°FOct 25Oct 29Nov 1Nov 3Nov 5Nov 8Nov 12Nov 16Nov 23
24°FNov 3Nov 6Nov 10Nov 14Nov 18Nov 22Nov 26Dec 1Dec 6
20°FNov 9Nov 16Nov 21Nov 25Nov 29Dec 2Dec 6Dec 11Dec 18
16°FNov 20Nov 27Dec 1Dec 5Dec 10Dec 14Dec 18Dec 23Dec 29

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°F199193189185181177173169162
32°F223215211207203199195190184
28°F247240234230225221217212205
24°F271262257251247243238234228
20°F291283277271267262257251244
16°F316305298292287280274269259

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
4,631
Cooling degree days, base 65°F
1,459
Growing degree days, base 40°F
6,773
Growing degree days, base 45°F
5,443
Growing degree days, base 50°F
4,249
Growing degree days, base 55°F
3,185
Growing degree days, base 60°F
2,253

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,04305
February818014
March613581
April28529240
May84117499
June6301745
July0425890
August2373837
September41175584
October25732273
November579270
December904013

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 12.2 km from Affton. 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 USC00111160 (Cahokia). Public domain.