Phoenix climate

Phoenix Frost Dates and Growing Season

NOAA 1991-2020 normals for Ashland, 10.9 km from Phoenix, elevation 1729 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Ashland. 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 May 6 and the first fall freeze around October 8, a growing season of about 154 days.

Median at 36°F
last May 29, first September 23
Median at 32°F
last May 6, first October 8
Median at 28°F
last April 15, first October 26

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°FJun 17Jun 10Jun 6Jun 2May 29May 25May 21May 18May 12
32°FMay 27May 19May 14May 10May 6May 3Apr 30Apr 27Apr 20
28°FMay 4Apr 29Apr 24Apr 20Apr 15Apr 11Apr 6Apr 1Mar 27
24°FApr 10Apr 2Mar 29Mar 24Mar 19Mar 14Mar 8Mar 1Feb 21
20°FMar 10Mar 1Feb 25Feb 18Feb 12Feb 4Jan 26Jan 12Dec 28
16°FFeb 16Jan 31Jan 22Jan 11Dec 31Dec 26Dec 20Dec 14Dec 4

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°FSep 6Sep 12Sep 15Sep 19Sep 23Sep 26Sep 30Oct 4Oct 9
32°FSep 21Sep 28Oct 1Oct 5Oct 8Oct 12Oct 15Oct 20Oct 26
28°FOct 10Oct 16Oct 20Oct 23Oct 26Oct 30Nov 2Nov 7Nov 16
24°FOct 22Oct 29Nov 2Nov 7Nov 12Nov 18Nov 24Dec 1Dec 8
20°FNov 7Nov 15Nov 21Nov 29Dec 4Dec 10Dec 17Dec 27Jan 11
16°FNov 20Dec 1Dec 5Dec 11Dec 16Dec 21Dec 28Jan 5Jan 25

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°F1381301241181141091059991
32°F177170164159154149143137128
28°F220211205198192187182176168
24°F274262252244238231225218207
20°F340328317309300290280270256
16°F365365365365355346338329312

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
100% of years
24°F or colder
100% of years
20°F or colder
96% of years
16°F or colder
59% 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
5,023
Cooling degree days, base 65°F
381
Growing degree days, base 40°F
4,783
Growing degree days, base 45°F
3,427
Growing degree days, base 50°F
2,333
Growing degree days, base 55°F
1,486
Growing degree days, base 60°F
843

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January79704
February65206
March623018
April480062
May28611211
June12742366
July22157600
August24130570
September12840364
October4031113
November650015
December83104

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 10.9 km from Phoenix. 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 USC00350304 (Ashland). Public domain.