Groton climate

Groton Frost Dates and Growing Season

NOAA 1991-2020 normals for Andover #2, 15.5 km from Groton, elevation 1478 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Andover #2. 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 6 and the first fall freeze around October 4, a growing season of about 150 days.

Median at 36°F
last May 16, first September 26
Median at 32°F
last May 6, first October 4
Median at 28°F
last April 28, first October 14

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 25May 22May 19May 16May 13May 9May 6Apr 30
32°FMay 21May 16May 12May 8May 6May 3Apr 30Apr 28Apr 22
28°FMay 12May 7May 3May 1Apr 28Apr 25Apr 22Apr 17Apr 12
24°FMay 3Apr 30Apr 26Apr 22Apr 18Apr 15Apr 11Apr 8Apr 3
20°FApr 26Apr 20Apr 15Apr 12Apr 9Apr 6Apr 3Mar 31Mar 24
16°FApr 17Apr 11Apr 7Apr 4Apr 1Mar 29Mar 25Mar 21Mar 15

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 12Sep 17Sep 20Sep 23Sep 26Sep 28Sep 30Oct 3Oct 7
32°FSep 21Sep 27Sep 30Oct 2Oct 4Oct 6Oct 10Oct 13Oct 19
28°FOct 1Oct 4Oct 7Oct 11Oct 14Oct 17Oct 20Oct 23Oct 27
24°FOct 8Oct 13Oct 17Oct 20Oct 24Oct 27Oct 30Nov 2Nov 6
20°FOct 17Oct 23Oct 27Oct 30Nov 1Nov 4Nov 6Nov 10Nov 15
16°FOct 25Oct 31Nov 2Nov 5Nov 8Nov 10Nov 14Nov 17Nov 23

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°F152146141137133129125121114
32°F169163159154150147143138132
28°F187180176172168164160156149
24°F210201195191187183179173166
20°F224217213208204200196191183
16°F241233228223219215210204198

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
8,121
Cooling degree days, base 65°F
593
Growing degree days, base 40°F
4,466
Growing degree days, base 45°F
3,461
Growing degree days, base 50°F
2,564
Growing degree days, base 55°F
1,781
Growing degree days, base 60°F
1,119

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,61000
February1,32000
March1,059012
April620161
May27628255
June58110504
July15216666
August24172614
September16860355
October560592
November98706
December1,42400

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 15.5 km from Groton. 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 USC00390120 (Andover #2). Public domain.