Saint Helena climate

Saint Helena Frost Dates and Growing Season

NOAA 1991-2020 normals for Saint Helena, 0.2 km from Saint Helena, elevation 225 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Saint Helena. 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 February 3 and the first fall freeze around December 4, a growing season of about 306 days.

Median at 36°F
last March 19, first November 18
Median at 32°F
last February 3, first December 4
Median at 28°F
last January 5, first December 17

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 17Apr 9Apr 1Mar 27Mar 19Mar 13Mar 7Feb 27Feb 14
32°FMar 13Feb 28Feb 22Feb 11Feb 3Jan 28Jan 19Jan 7Dec 28
28°FFeb 9Jan 30Jan 19Jan 11Jan 5Dec 31Dec 25Dec 19Dec 11
24°FFeb 2Jan 10Jan 4Dec 31Dec 28Dec 22Dec 16Dec 9Dec 3
20°F–––––––––
16°F–––––––––

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 31Nov 5Nov 9Nov 13Nov 18Nov 21Nov 25Nov 30Dec 5
32°FNov 14Nov 21Nov 27Dec 1Dec 4Dec 8Dec 12Dec 18Jan 1
28°FNov 27Dec 3Dec 7Dec 12Dec 17Dec 22Dec 30Jan 5Jan 18
24°FDec 1Dec 7Dec 12Dec 15Dec 23Dec 28Jan 1Jan 6Jan 25
20°F–––––––––
16°F–––––––––

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°F283269259251243236229223210
32°F341331322314306297288277260
28°F365365365358349344336329317
24°F365365365365365365365365349
20°F365365365365365365365365365
16°F365365365365365365365365365

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
97% of years
28°F or colder
65% of years
24°F or colder
15% of years
20°F or colder
1% of years
16°F or colder
1% 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
2,352
Cooling degree days, base 65°F
922
Growing degree days, base 40°F
7,699
Growing degree days, base 45°F
5,911
Growing degree days, base 50°F
4,262
Growing degree days, base 55°F
2,878
Growing degree days, base 60°F
1,771

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January481051
February359194
March3023182
April19516275
May8765443
June16160594
July1229693
August1226690
September10166606
October9253427
November3113161
December498046

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 0.2 km from Saint Helena. 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 USC00047643 (Saint Helena). Public domain.