Boston climate

Boston Frost Dates and Growing Season

NOAA 1991-2020 normals for Monticello 5 Se, 33.3 km from Boston, elevation 98 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Monticello 5 Se. 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 March 7 and the first fall freeze around November 23, a growing season of about 262 days.

Median at 36°F
last March 21, first November 11
Median at 32°F
last March 7, first November 23
Median at 28°F
last February 19, first December 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°FApr 8Apr 2Mar 29Mar 25Mar 21Mar 17Mar 13Mar 7Mar 1
32°FMar 25Mar 19Mar 15Mar 11Mar 7Mar 3Feb 27Feb 20Feb 9
28°FMar 16Mar 8Mar 3Feb 26Feb 19Feb 13Feb 6Jan 31Jan 21
24°FMar 3Feb 23Feb 14Feb 7Feb 1Jan 28Jan 20Jan 11Dec 31
20°FFeb 16Feb 6Feb 1Jan 25Jan 19Jan 11Jan 5Dec 31Dec 24
16°FFeb 7Feb 1Jan 26Jan 17Jan 12Jan 6Dec 31Dec 28Dec 21

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°FOct 26Nov 1Nov 4Nov 7Nov 11Nov 15Nov 19Nov 24Dec 2
32°FNov 3Nov 10Nov 15Nov 19Nov 23Nov 29Dec 2Dec 9Dec 22
28°FNov 16Nov 25Dec 1Dec 5Dec 11Dec 21Jan 1Jan 5Jan 15
24°FDec 2Dec 7Dec 15Dec 26Jan 1Jan 6Jan 12Jan 21Feb 2
20°FDec 8Dec 17Dec 27Jan 1Jan 6Jan 11Jan 18Jan 27Feb 6
16°FDec 11Dec 23Dec 29Jan 1Jan 7Jan 13Jan 19Jan 28Feb 4

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°F263254246240235229224217208
32°F294283275268262256250242233
28°F334321309301294287279271259
24°F365355342335329322312303291
20°F365365365365362352343334323
16°F365365365365365365365365353

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
98% of years
24°F or colder
86% of years
20°F or colder
51% of years
16°F or colder
17% 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
1,581
Cooling degree days, base 65°F
2,406
Growing degree days, base 40°F
9,977
Growing degree days, base 45°F
8,211
Growing degree days, base 50°F
6,533
Growing degree days, base 55°F
4,989
Growing degree days, base 60°F
3,604

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January4268140
February2959177
March17744346
April61106496
May9269725
June0423873
July0491956
August0480945
September1367816
October48159578
November20538301
December36012179

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 33.3 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 USC00085879 (Monticello 5 Se). Public domain.