Rio Verde Frost Dates and Growing Season

NOAA 1991-2020 normals for Fountain Hills, 13.8 km from Rio Verde, elevation 1580 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Fountain Hills. 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 6 and the first fall freeze around December 28, a growing season of about 356 days.

Median at 36°F: last January 31, first December 14
Median at 32°F: last January 6, first December 28
Median at 28°F: last January 9, first January 8

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 1	Feb 20	Feb 14	Feb 5	Jan 31	Jan 23	Jan 16	Jan 7	Dec 31
32°F	Feb 11	Jan 30	Jan 21	Jan 12	Jan 6	Jan 2	Dec 30	Dec 23	Dec 14
28°F	Jan 31	Jan 23	Jan 18	Jan 12	Jan 9	Jan 6	Jan 4	Dec 31	Dec 27
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 25	Dec 1	Dec 6	Dec 11	Dec 14	Dec 18	Dec 22	Dec 27	Jan 3
32°F	Dec 4	Dec 13	Dec 18	Dec 23	Dec 28	Dec 31	Jan 4	Jan 13	Jan 27
28°F	Dec 26	Dec 31	Jan 1	Jan 4	Jan 8	Jan 11	Jan 16	Jan 22	Jan 31
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	352	342	334	324	317	309	301	292	280
32°F	365	365	365	362	356	349	344	334	324
28°F	365	365	365	365	365	365	365	365	355
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: 98% of years
32°F or colder: 63% of years
28°F or colder: 14% of years
24°F or colder: 1% 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: 1,237
Cooling degree days, base 65°F: 3,866
Growing degree days, base 40°F: 11,755
Growing degree days, base 45°F: 9,941
Growing degree days, base 50°F: 8,179
Growing degree days, base 55°F: 6,545
Growing degree days, base 60°F: 5,103

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	335	0	154
February	234	5	204
March	130	46	384
April	33	159	576
May	3	399	860
June	0	651	1,101
July	0	826	1,291
August	0	801	1,266
September	0	612	1,062
October	11	309	763
November	133	58	379
December	359	1	138

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 13.8 km from Rio Verde. 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 USC00023190 (Fountain Hills). Public domain.