Cohasset climate

Cohasset Frost Dates and Growing Season

NOAA 1991-2020 normals for Pokegama Dam, 2.9 km from Cohasset, elevation 1280 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Pokegama Dam. 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 16 and the first fall freeze around October 1, a growing season of about 138 days.

Median at 36°F
last May 27, first September 22
Median at 32°F
last May 16, first October 1
Median at 28°F
last May 3, first October 10

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 9Jun 4Jun 2May 30May 27May 24May 22May 19May 15
32°FMay 31May 26May 23May 19May 16May 13May 10May 5May 1
28°FMay 19May 14May 10May 6May 3May 1Apr 28Apr 25Apr 21
24°FMay 6May 1Apr 29Apr 25Apr 23Apr 21Apr 18Apr 14Apr 9
20°FApr 26Apr 22Apr 19Apr 16Apr 13Apr 10Apr 8Apr 5Apr 1
16°FApr 19Apr 14Apr 10Apr 7Apr 5Apr 3Apr 1Mar 28Mar 22

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 6Sep 13Sep 17Sep 19Sep 22Sep 24Sep 26Sep 29Oct 3
32°FSep 18Sep 23Sep 26Sep 28Oct 1Oct 3Oct 5Oct 9Oct 12
28°FSep 27Oct 1Oct 5Oct 7Oct 10Oct 13Oct 16Oct 20Oct 25
24°FOct 8Oct 13Oct 16Oct 19Oct 22Oct 25Oct 28Nov 1Nov 5
20°FOct 17Oct 24Oct 28Oct 31Nov 1Nov 4Nov 7Nov 11Nov 16
16°FOct 30Nov 1Nov 4Nov 7Nov 9Nov 12Nov 15Nov 19Nov 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°F13512912411911511110710295
32°F155150145142138134129124117
28°F178172167162159155151146141
24°F202194190185181177173168162
20°F220214208204201197193189182
16°F236230225221217214209205199

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
9,497
Cooling degree days, base 65°F
275
Growing degree days, base 40°F
3,548
Growing degree days, base 45°F
2,653
Growing degree days, base 50°F
1,865
Growing degree days, base 55°F
1,196
Growing degree days, base 60°F
656

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,78400
February1,48300
March1,22202
April770019
May38311161
June12548377
July40115539
August6381484
September25420242
October685039
November1,10701
December1,58200

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 2.9 km from Cohasset. 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 USC00216612 (Pokegama Dam). Public domain.