New River Frost Dates and Growing Season

NOAA 1991-2020 normals for Castle Hot Springs, 22.2 km from New River, elevation 1990 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Castle Hot Springs. 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 January 15 and the first fall freeze around December 22, a growing season of about 341 days.

Median at 36°F: last February 14, first December 7
Median at 32°F: last January 15, first December 22
Median at 28°F: last January 9, first January 6

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	Mar 10	Mar 3	Feb 26	Feb 20	Feb 14	Feb 8	Feb 1	Jan 24	Jan 12
32°F	Feb 21	Feb 11	Feb 2	Jan 26	Jan 15	Jan 8	Jan 3	Dec 28	Dec 19
28°F	Feb 15	Feb 6	Jan 27	Jan 17	Jan 9	Jan 5	Jan 1	Dec 28	Dec 18
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	Nov 19	Nov 25	Nov 30	Dec 3	Dec 7	Dec 11	Dec 15	Dec 19	Dec 25
32°F	Nov 29	Dec 6	Dec 12	Dec 17	Dec 22	Dec 27	Jan 1	Jan 9	Jan 26
28°F	Dec 15	Dec 23	Dec 28	Jan 1	Jan 6	Jan 10	Jan 22	Jan 29	Feb 12
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	334	322	312	304	296	289	281	273	262
32°F	365	365	358	351	341	333	325	316	303
28°F	365	365	365	365	365	365	365	356	337
24°F	365	365	365	365	365	365	365	365	365
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: 83% of years
28°F or colder: 30% of years
24°F or colder: 2% 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: 1,046
Cooling degree days, base 65°F: 3,842
Growing degree days, base 40°F: 11,922
Growing degree days, base 45°F: 10,103
Growing degree days, base 50°F: 8,314
Growing degree days, base 55°F: 6,632
Growing degree days, base 60°F: 5,131

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	274	3	208
February	207	10	230
March	111	57	413
April	33	153	570
May	3	382	843
June	0	642	1,092
July	0	812	1,277
August	0	792	1,257
September	0	606	1,056
October	8	311	769
November	108	72	416
December	302	3	184

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 22.2 km from New River. 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 USC00021353 (Castle Hot Springs). Public domain.