Tucson Frost Dates and Growing Season

NOAA 1991-2020 normals for Tucson WFO, 2.7 km from Tucson, elevation 2435 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Tucson WFO. 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 20 and the first fall freeze around December 17, a growing season of about 332 days.

Median at 36°F: last February 14, first December 4
Median at 32°F: last January 20, first December 17
Median at 28°F: last January 4, first December 30

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 9	Mar 2	Feb 25	Feb 19	Feb 14	Feb 8	Feb 2	Jan 28	Jan 18
32°F	Feb 18	Feb 8	Feb 2	Jan 28	Jan 20	Jan 12	Jan 5	Dec 29	Dec 19
28°F	Feb 7	Feb 1	Jan 19	Jan 10	Jan 4	Dec 31	Dec 25	Dec 17	Dec 9
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 18	Nov 24	Nov 28	Dec 1	Dec 4	Dec 8	Dec 11	Dec 14	Dec 20
32°F	Nov 28	Dec 4	Dec 9	Dec 14	Dec 17	Dec 21	Dec 26	Dec 31	Jan 12
28°F	Dec 5	Dec 13	Dec 18	Dec 24	Dec 30	Jan 3	Jan 8	Jan 20	Feb 4
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	321	312	304	298	291	285	279	272	263
32°F	364	355	347	340	332	326	317	308	298
28°F	365	365	365	365	365	365	359	348	333
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: 92% of years
28°F or colder: 39% of years
24°F or colder: 5% 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: 1,249
Cooling degree days, base 65°F: 3,538
Growing degree days, base 40°F: 11,415
Growing degree days, base 45°F: 9,602
Growing degree days, base 50°F: 7,843
Growing degree days, base 55°F: 6,211
Growing degree days, base 60°F: 4,773

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	338	0	151
February	230	3	203
March	124	57	401
April	37	165	578
May	3	394	856
June	0	654	1,104
July	0	722	1,187
August	0	685	1,150
September	0	547	997
October	14	262	713
November	136	47	367
December	367	1	136

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 2.7 km from Tucson. 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 USC00028815 (Tucson WFO). Public domain.