Union County climate

Union County Frost Dates and Growing Season

NOAA 1991-2020 normals for Grenville, 18.2 km from Union County, elevation 6002 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Grenville. 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 5 and the first fall freeze around October 9, a growing season of about 156 days.

Median at 36°F
last May 16, first October 2
Median at 32°F
last May 5, first October 9
Median at 28°F
last April 26, first October 17

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 29May 24May 22May 19May 16May 13May 10May 7May 2
32°FMay 20May 14May 11May 8May 5May 2Apr 30Apr 27Apr 23
28°FMay 9May 5May 1Apr 28Apr 26Apr 23Apr 20Apr 16Apr 11
24°FMay 1Apr 25Apr 21Apr 18Apr 15Apr 11Apr 8Apr 4Mar 29
20°FApr 20Apr 16Apr 11Apr 8Apr 4Mar 31Mar 27Mar 21Mar 14
16°FApr 11Apr 5Apr 1Mar 27Mar 21Mar 16Mar 10Mar 5Feb 27

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 18Sep 24Sep 27Sep 29Oct 2Oct 4Oct 6Oct 8Oct 12
32°FSep 27Oct 1Oct 4Oct 6Oct 9Oct 11Oct 13Oct 16Oct 20
28°FOct 4Oct 8Oct 12Oct 14Oct 17Oct 19Oct 22Oct 25Oct 29
24°FOct 11Oct 17Oct 20Oct 23Oct 27Oct 30Nov 1Nov 3Nov 9
20°FOct 20Oct 26Oct 30Nov 2Nov 4Nov 7Nov 11Nov 14Nov 19
16°FOct 27Nov 2Nov 6Nov 9Nov 13Nov 17Nov 20Nov 24Nov 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°F155150145142138135130126118
32°F173167163159156152148144138
28°F191185181177173170166161154
24°F214206201197193189184179172
20°F239231225219214209205199191
16°F265257249242236230225218209

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
5,364
Cooling degree days, base 65°F
607
Growing degree days, base 40°F
5,229
Growing degree days, base 45°F
3,963
Growing degree days, base 50°F
2,874
Growing degree days, base 55°F
1,957
Growing degree days, base 60°F
1,199

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January97801
February82703
March685028
April463194
May22023287
June34130546
July5223683
August10168623
September9159422
October3783157
November696027
December97802

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 18.2 km from Union County. 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 USC00293706 (Grenville). Public domain.