Portsmouth climate

Portsmouth Frost Dates and Growing Season

NOAA 1991-2020 normals for Norfolk S, 7.3 km from Portsmouth, elevation 10 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Norfolk S. 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 23 and the first fall freeze around November 16, a growing season of about 239 days.

Median at 36°F
last April 5, first November 6
Median at 32°F
last March 23, first November 16
Median at 28°F
last March 10, first December 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°FApr 18Apr 13Apr 10Apr 7Apr 5Apr 2Mar 31Mar 27Mar 22
32°FApr 6Apr 2Mar 30Mar 27Mar 23Mar 21Mar 17Mar 14Mar 7
28°FMar 26Mar 20Mar 17Mar 14Mar 10Mar 7Mar 3Feb 26Feb 19
24°FMar 13Mar 7Mar 3Feb 27Feb 24Feb 18Feb 13Feb 6Jan 30
20°FMar 1Feb 23Feb 16Feb 10Feb 4Jan 31Jan 25Jan 19Jan 10
16°FFeb 16Feb 7Feb 1Jan 29Jan 24Jan 19Jan 14Jan 8Jan 1

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 27Oct 31Nov 1Nov 4Nov 6Nov 8Nov 11Nov 14Nov 19
32°FNov 4Nov 7Nov 11Nov 14Nov 16Nov 20Nov 22Nov 26Dec 1
28°FNov 14Nov 19Nov 24Nov 27Dec 1Dec 4Dec 7Dec 15Dec 25
24°FDec 1Dec 5Dec 10Dec 17Dec 22Dec 28Jan 1Jan 6Jan 13
20°FDec 13Dec 22Dec 29Jan 1Jan 6Jan 10Jan 15Jan 21Jan 31
16°FDec 24Dec 31Jan 4Jan 9Jan 13Jan 17Jan 22Jan 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°F236229225221217213210205200
32°F262254248244239235231227220
28°F295286279273269264259252243
24°F327318310305300294289282274
20°F364351341334329323316308301
16°F365365365365356347339332320

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
99% of years
20°F or colder
91% of years
16°F or colder
61% 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
3,378
Cooling degree days, base 65°F
1,608
Growing degree days, base 40°F
7,622
Growing degree days, base 45°F
6,097
Growing degree days, base 50°F
4,735
Growing degree days, base 55°F
3,529
Growing degree days, base 60°F
2,486

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January759029
February618143
March5017109
April21842291
May63125527
June4307753
July0451916
August0392857
September9222663
October15055376
November4145121
December641151

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 7.3 km from Portsmouth. 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 USC00446147 (Norfolk S). Public domain.