Jefferson climate

Jefferson Frost Dates and Growing Season

NOAA 1991-2020 normals for Dorset, 10.6 km from Jefferson, elevation 980 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Dorset. 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 10 and the first fall freeze around October 10, a growing season of about 154 days.

Median at 36°F
last May 22, first September 29
Median at 32°F
last May 10, first October 10
Median at 28°F
last April 28, 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°FJun 6Jun 1May 29May 25May 22May 20May 16May 12May 7
32°FMay 27May 22May 18May 14May 10May 6May 3Apr 30Apr 25
28°FMay 16May 10May 5May 1Apr 28Apr 25Apr 22Apr 19Apr 14
24°FMay 1Apr 26Apr 22Apr 19Apr 16Apr 13Apr 9Apr 5Apr 1
20°FApr 18Apr 13Apr 9Apr 6Apr 3Apr 1Mar 29Mar 25Mar 19
16°FApr 7Apr 2Mar 31Mar 28Mar 24Mar 21Mar 18Mar 14Mar 9

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 13Sep 19Sep 23Sep 26Sep 29Oct 1Oct 4Oct 7Oct 12
32°FSep 26Oct 1Oct 4Oct 6Oct 10Oct 13Oct 16Oct 21Oct 27
28°FOct 8Oct 14Oct 19Oct 22Oct 26Oct 30Nov 2Nov 5Nov 11
24°FOct 22Oct 29Nov 1Nov 4Nov 7Nov 10Nov 13Nov 19Nov 24
20°FNov 5Nov 9Nov 13Nov 16Nov 20Nov 24Nov 28Dec 1Dec 7
16°FNov 12Nov 19Nov 24Nov 28Dec 1Dec 5Dec 8Dec 13Dec 20

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°F149142137132128124119114105
32°F176168162158154149144138131
28°F201193188184179175169164156
24°F227220214209206202196191183
20°F255247240235231226221216209
16°F276267262256252247241236229

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
6,709
Cooling degree days, base 65°F
491
Growing degree days, base 40°F
4,564
Growing degree days, base 45°F
3,457
Growing degree days, base 50°F
2,495
Growing degree days, base 55°F
1,672
Growing degree days, base 60°F
997

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,26001
February1,09901
March952018
April567378
May27831255
June82101472
July26168608
August35131561
September14851358
October4475118
November747021
December1,06803

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 10.6 km from Jefferson. 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 USC00332251 (Dorset). Public domain.