Washington County climate

Washington County Frost Dates and Growing Season

NOAA 1991-2020 normals for Potosi 4 Sw, 8 km from Washington County, elevation 1105 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Potosi 4 Sw. 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 11 and the first fall freeze around October 28, a growing season of about 198 days.

Median at 36°F
last April 24, first October 16
Median at 32°F
last April 11, first October 28
Median at 28°F
last March 31, first November 4

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 10May 5May 2Apr 28Apr 24Apr 21Apr 17Apr 13Apr 7
32°FApr 30Apr 24Apr 19Apr 15Apr 11Apr 7Apr 4Mar 31Mar 26
28°FApr 16Apr 9Apr 5Apr 2Mar 31Mar 27Mar 24Mar 20Mar 14
24°FApr 5Mar 31Mar 28Mar 24Mar 21Mar 17Mar 13Mar 9Mar 5
20°FMar 28Mar 22Mar 18Mar 15Mar 11Mar 8Mar 6Mar 1Feb 24
16°FMar 20Mar 15Mar 11Mar 7Mar 5Mar 1Feb 26Feb 20Feb 12

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 29Oct 5Oct 9Oct 13Oct 16Oct 19Oct 21Oct 24Oct 28
32°FOct 14Oct 19Oct 22Oct 25Oct 28Oct 30Nov 1Nov 3Nov 8
28°FOct 24Oct 29Nov 1Nov 2Nov 4Nov 8Nov 11Nov 15Nov 21
24°FNov 1Nov 4Nov 7Nov 11Nov 15Nov 19Nov 24Nov 28Dec 3
20°FNov 7Nov 13Nov 17Nov 22Nov 26Nov 30Dec 2Dec 8Dec 15
16°FNov 15Nov 24Nov 28Dec 1Dec 5Dec 9Dec 14Dec 20Dec 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°F196188183178173168163158150
32°F219213207203198194189183175
28°F243234229224220216213207200
24°F265257251245241236230225218
20°F284276269264259253248242234
16°F303295289284278273267260252

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
4,900
Cooling degree days, base 65°F
1,196
Growing degree days, base 40°F
6,374
Growing degree days, base 45°F
5,055
Growing degree days, base 50°F
3,879
Growing degree days, base 55°F
2,847
Growing degree days, base 60°F
1,948

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,06908
February833017
March646573
April31726220
May11989440
June14234670
July2354817
August4317778
September68138523
October28331256
November611265
December933013

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 8 km from Washington County. 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 USC00236826 (Potosi 4 Sw). Public domain.