Boston climate

Boston Frost Dates and Growing Season

NOAA 1991-2020 normals for Colden 1W, 2.9 km from Boston, elevation 1535 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Colden 1W. 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 4 and the first fall freeze around October 17, a growing season of about 166 days.

Median at 36°F
last May 13, first October 5
Median at 32°F
last May 4, first October 17
Median at 28°F
last April 24, first November 1

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 27May 23May 20May 17May 13May 10May 7May 3Apr 29
32°FMay 19May 13May 9May 6May 4May 1Apr 29Apr 25Apr 21
28°FMay 9May 3Apr 30Apr 27Apr 24Apr 21Apr 19Apr 15Apr 10
24°FApr 27Apr 22Apr 18Apr 16Apr 13Apr 9Apr 7Apr 4Mar 31
20°FApr 16Apr 11Apr 8Apr 5Apr 3Apr 1Mar 31Mar 26Mar 20
16°FApr 7Apr 3Apr 1Mar 30Mar 26Mar 22Mar 19Mar 16Mar 10

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 21Sep 26Sep 30Oct 3Oct 5Oct 8Oct 11Oct 15Oct 19
32°FOct 1Oct 6Oct 10Oct 14Oct 17Oct 21Oct 24Oct 28Nov 3
28°FOct 16Oct 22Oct 27Oct 31Nov 1Nov 4Nov 7Nov 11Nov 16
24°FNov 1Nov 4Nov 7Nov 10Nov 13Nov 16Nov 19Nov 24Nov 30
20°FNov 8Nov 12Nov 16Nov 19Nov 23Nov 26Nov 30Dec 3Dec 9
16°FNov 14Nov 20Nov 25Nov 28Dec 1Dec 5Dec 9Dec 14Dec 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°F165158153149145141137132125
32°F189181175170166162157152143
28°F213206200196192188183176168
24°F235229223218214210205199192
20°F256247242237233229224219214
16°F276268262257252247242237230

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
7,268
Cooling degree days, base 65°F
359
Growing degree days, base 40°F
4,190
Growing degree days, base 45°F
3,151
Growing degree days, base 50°F
2,243
Growing degree days, base 55°F
1,465
Growing degree days, base 60°F
825

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,31301
February1,17201
March1,059111
April655151
May31820220
June9469427
July28130567
August4194518
September16941329
October4974100
November798017
December1,12501

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 Boston. 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 USC00301625 (Colden 1W). Public domain.