Westport climate

Westport Frost Dates and Growing Season

NOAA 1991-2020 normals for Westport Island, 1.2 km from Westport, elevation 90 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Westport Island. 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 April 28 and the first fall freeze around October 21, a growing season of about 175 days.

Median at 36°F
last May 10, first October 7
Median at 32°F
last April 28, first October 21
Median at 28°F
last April 13, first November 4

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 23May 18May 15May 13May 10May 8May 5May 3Apr 30
32°FMay 9May 4May 2Apr 30Apr 28Apr 26Apr 24Apr 21Apr 16
28°FApr 26Apr 22Apr 19Apr 16Apr 13Apr 10Apr 8Apr 6Apr 3
24°FApr 15Apr 10Apr 7Apr 5Apr 3Apr 1Mar 31Mar 26Mar 21
20°FApr 5Apr 2Mar 31Mar 29Mar 25Mar 22Mar 20Mar 16Mar 13
16°FMar 31Mar 26Mar 23Mar 20Mar 17Mar 15Mar 12Mar 10Mar 6

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 25Sep 29Oct 2Oct 4Oct 7Oct 10Oct 12Oct 15Oct 20
32°FOct 5Oct 10Oct 15Oct 18Oct 21Oct 25Oct 28Oct 31Nov 4
28°FOct 22Oct 28Oct 31Nov 2Nov 4Nov 7Nov 9Nov 13Nov 17
24°FNov 4Nov 9Nov 12Nov 15Nov 17Nov 20Nov 22Nov 24Nov 28
20°FNov 14Nov 19Nov 22Nov 24Nov 27Nov 30Dec 2Dec 5Dec 10
16°FNov 24Nov 28Dec 1Dec 3Dec 5Dec 8Dec 10Dec 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°F165159154151147144140135129
32°F195188183179175172167162156
28°F221214210207203200197192186
24°F244238234230226223220215210
20°F265258253249246242238234227
16°F283277272267263260256251245

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
7,165
Cooling degree days, base 65°F
265
Growing degree days, base 40°F
3,940
Growing degree days, base 45°F
2,889
Growing degree days, base 50°F
1,990
Growing degree days, base 55°F
1,250
Growing degree days, base 60°F
670

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,31300
February1,13100
March1,01102
April665016
May3674137
June13133354
July24109550
August2796535
September16321312
October473176
November75609
December1,10500

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 1.2 km from Westport. 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 USC00179547 (Westport Island). Public domain.