Irwin climate

Irwin Frost Dates and Growing Season

NOAA 1991-2020 normals for Winthrop Univ, 33.4 km from Irwin, elevation 650 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Winthrop Univ. 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 March 27 and the first fall freeze around November 5, a growing season of about 225 days.

Median at 36°F
last April 9, first October 27
Median at 32°F
last March 27, first November 5
Median at 28°F
last March 15, first November 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°FApr 23Apr 18Apr 15Apr 11Apr 9Apr 6Apr 4Mar 31Mar 24
32°FApr 11Apr 5Apr 2Mar 30Mar 27Mar 24Mar 20Mar 17Mar 10
28°FMar 31Mar 27Mar 22Mar 18Mar 15Mar 11Mar 6Mar 1Feb 23
24°FMar 22Mar 16Mar 11Mar 5Mar 1Feb 25Feb 20Feb 15Feb 5
20°FMar 12Mar 3Feb 26Feb 19Feb 13Feb 5Feb 1Jan 25Jan 12
16°FFeb 28Feb 19Feb 10Feb 3Jan 31Jan 25Jan 19Jan 10Dec 31

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 16Oct 20Oct 23Oct 25Oct 27Oct 30Nov 1Nov 5Nov 9
32°FOct 24Oct 27Oct 31Nov 2Nov 5Nov 8Nov 11Nov 16Nov 23
28°FNov 2Nov 7Nov 10Nov 13Nov 18Nov 22Nov 27Dec 2Dec 9
24°FNov 15Nov 22Nov 27Dec 1Dec 5Dec 10Dec 18Dec 27Jan 5
20°FDec 1Dec 6Dec 12Dec 22Dec 28Jan 1Jan 7Jan 14Jan 26
16°FDec 12Dec 24Dec 30Jan 3Jan 8Jan 14Jan 20Jan 29Feb 9

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°F223215211206203199194189183
32°F248240234229225220216212205
28°F277267260254249244238232226
24°F313302293287280273267258247
20°F347335328319312305298288277
16°F365365361351341334327317305

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
97% of years
16°F or colder
74% 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
2,872
Cooling degree days, base 65°F
1,839
Growing degree days, base 40°F
8,249
Growing degree days, base 45°F
6,651
Growing degree days, base 50°F
5,205
Growing degree days, base 55°F
3,920
Growing degree days, base 60°F
2,795

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January681039
February522158
March36611179
April13455377
May29181617
June2363811
July0471936
August0432897
September9257697
October13459396
November3867142
December609155

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 33.4 km from Irwin. 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 USC00389350 (Winthrop Univ). Public domain.