Hawaii County Frost Dates and Growing Season

NOAA 1991-2020 normals for Mauna Loa 5 Nne, 9.3 km from Hawaii County, elevation 11180 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Mauna Loa 5 Nne. 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 12 and the first fall freeze around December 1, a growing season of about 235 days.

Median at 32°F: last April 12, first December 1
Median at 28°F: last February 13, first January 22

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.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	–	–	–	–	–	–	–	–	–
32°F	May 18	May 4	Apr 27	Apr 19	Apr 12	Apr 5	Mar 30	Mar 23	Mar 13
28°F	Mar 13	Mar 4	Feb 26	Feb 20	Feb 13	Feb 10	Feb 4	Jan 30	Jan 15
24°F	–	–	–	–	–	–	–	–	–
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.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	–	–	–	–	–	–	–	–	–
32°F	Nov 9	Nov 16	Nov 21	Nov 26	Dec 1	Dec 7	Dec 16	Dec 25	Jan 7
28°F	Dec 17	Dec 29	Jan 7	Jan 15	Jan 22	Jan 31	Feb 6	Feb 13	Feb 23
24°F	–	–	–	–	–	–	–	–	–
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.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	153	129	114	101	91	82	75	66	56
32°F	276	262	252	243	235	227	217	207	187
28°F	365	365	351	336	327	319	311	304	293
24°F	365	365	365	365	365	365	365	365	352
20°F	365	365	365	365	365	365	365	365	365
16°F	365	365	365	365	365	365	365	365	365

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: 73% of years
24°F or colder: 10% of years
20°F or colder: 0% of years
16°F or colder: 0% 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: 6,919
Cooling degree days, base 65°F: 0
Growing degree days, base 40°F: 2,289
Growing degree days, base 45°F: 874
Growing degree days, base 50°F: 138
Growing degree days, base 55°F: 4
Growing degree days, base 60°F: 0

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Growing DD 50
January	674	3
February	638	2
March	693	2
April	606	4
May	546	18
June	470	31
July	493	28
August	496	24
September	494	15
October	536	10
November	606	1
December	668	1

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 9.3 km from Hawaii County. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USW00021514 (Mauna Loa 5 Nne). Public domain.