Jefferson County climate

Jefferson County Frost Dates and Growing Season

NOAA 1991-2020 normals for Watertown Intl AP, 2.5 km from Jefferson County, elevation 318 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Watertown Intl AP. 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 11 and the first fall freeze around October 1, a growing season of about 140 days.

Median at 36°F
last May 25, first September 19
Median at 32°F
last May 11, first October 1
Median at 28°F
last April 30, first October 11

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 8Jun 2May 31May 28May 25May 22May 19May 15May 10
32°FMay 26May 21May 17May 14May 11May 8May 6May 2Apr 29
28°FMay 14May 8May 5May 2Apr 30Apr 27Apr 25Apr 22Apr 18
24°FMay 2Apr 27Apr 24Apr 22Apr 19Apr 17Apr 15Apr 12Apr 7
20°FApr 21Apr 17Apr 14Apr 11Apr 8Apr 6Apr 3Apr 1Mar 29
16°FApr 12Apr 7Apr 4Apr 1Mar 31Mar 29Mar 26Mar 22Mar 16

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 3Sep 9Sep 12Sep 16Sep 19Sep 22Sep 25Sep 28Oct 1
32°FSep 16Sep 21Sep 24Sep 27Oct 1Oct 2Oct 5Oct 9Oct 13
28°FSep 25Oct 1Oct 4Oct 7Oct 11Oct 14Oct 18Oct 22Oct 27
24°FOct 5Oct 11Oct 16Oct 20Oct 24Oct 28Oct 31Nov 3Nov 8
20°FOct 19Oct 25Oct 29Nov 1Nov 4Nov 7Nov 10Nov 14Nov 20
16°FNov 1Nov 5Nov 9Nov 13Nov 16Nov 19Nov 23Nov 28Dec 3

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°F13512812311911511210710396
32°F160153148144140136132126119
28°F182175171166163158155150143
24°F207200196191187182178172165
20°F229224218214211207202197189
16°F249243238234230226223218213

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
7,524
Cooling degree days, base 65°F
386
Growing degree days, base 40°F
4,135
Growing degree days, base 45°F
3,100
Growing degree days, base 50°F
2,207
Growing degree days, base 55°F
1,448
Growing degree days, base 60°F
831

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,39801
February1,23100
March1,07405
April658139
May32614193
June10164415
July22146589
August40116541
September19040312
October506493
November809018
December1,17001

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 2.5 km from Jefferson 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 USW00094790 (Watertown Intl AP). Public domain.