USDA Hardiness Zone 9b
Frost Dates for 85321 — Ajo, Arizona
NOAA 1991–2020 normals from Ajo, AZ (1,751 ft), 5 miles northwest.
Updated
A freeze (32°F) reaches Ajo in 81% of winters. When it does, the first fall frost comes around December 21 and the last spring frost around January 10; after February 15 the chance of another is under 8%.
- Last spring frost
- January 10
- First fall frost
- December 21
- Winters with a freeze
- 81%
- Hardiness zone
- 9b
50%, in freezing years
50%, in freezing years
32°F or below, 1991–2020
25 to 30°F (-3.9 to -1.1°C) coldest night
- First snow
- Rare
- Last snow
- Rare
- Winters with snow
- 5%
- Snow per year
- 0.1"
median, 1991–2020
median, 1991–2020
1 of 20 observed
NOAA normal
Frost risk by date in Ajo
In the 81% of winters that bring a 32°F night, half of springs at Ajo, AZ see their last 32°F night by January 10; only one in ten is past it by December 11, and one in ten still has one after February 15. Counting the winters with no freeze at all, the risk of a 32°F night after February 15 is under 8% and after January 10 41%. The 36 days between the even-odds date and the one-in-ten date is wider than at 95% of the 98 Arizona stations behind this site's ZIP pages, so a fixed calendar date is a gamble here.
In fall the first 32°F night comes by December 21 in half of years, as early as November 25 one year in ten, and holds off past January 24 one year in ten. A hard freeze of 28°F is less common: it came in 29% of winters. The freeze-free season lasts 347 days in a typical year, longer than at 92% of the 98 Arizona stations; it reaches 365 days one year in ten and falls short of 306 one year in ten. On average 2 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 26 days longer, to about February 5, and return around December 8. A 24°F night is unusual: 2% of winters had one.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F99% of yrs | Jan 3 | Jan 12 | Jan 22 | Jan 29 | Feb 5 | Feb 13 | Feb 25 | Mar 4 | Mar 13 |
| 32°F81% of yrs | Dec 11 | Dec 23 | Dec 31 | Jan 3 | Jan 10 | Jan 17 | Jan 27 | Feb 3 | Feb 15 |
| 28°F29% of yrs | too rare for a date | ||||||||
| 24°F2% of yrs | too rare for a date | ||||||||
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F99% of yrs | Nov 19 | Nov 25 | Nov 30 | Dec 5 | Dec 8 | Dec 12 | Dec 16 | Dec 21 | Dec 30 |
| 32°F81% of yrs | Nov 25 | Dec 5 | Dec 10 | Dec 16 | Dec 21 | Dec 27 | Dec 31 | Jan 7 | Jan 24 |
| 28°F29% of yrs | too rare for a date | ||||||||
| 24°F2% of yrs | too rare for a date | ||||||||
Freeze probabilities 1991–2020 at Ajo, AZ, from NOAA NCEI U.S. Climate Normals. Where a temperature is reached in fewer than 99.5% of years, the dates describe only the years that reach it. Not a forecast.
Frost risk on any date in 85321 →Compare Ajo with another place →
Weather by month in Ajo
July is the warmest month at Ajo, AZ, averaging 104°F by day and 80°F at night; December is the coldest at 67°F and 43°F. From March to October the chance of a 32°F night stays under 10% each month. In spring, February still brings one in 17% of years.
Warmth for growing adds up to 8,674 growing degree days a year (base 50°F), more than at 89% of the 102 Arizona stations. 41% of them (3,564) have built up by the end of June, and July adds the most, 1,294. Precipitation totals 7.0 inches a year, most in August (1.4") and least in May (0.1"); May to September bring 43% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 68 | 45 | 0.7 | 43% | 1 | 211 |
| Feb | 71 | 48 | 0.8 | 17% | <1 | 265 |
| Mar | 77 | 52 | 0.8 | 2% | 0 | 450 |
| Apr | 84 | 57 | 0.2 | 0% | 0 | 620 |
| May | 92 | 66 | 0.1 | 0% | 0 | 891 |
| Jun | 101 | 74 | 0.1 | 0% | 0 | 1126 |
| Jul | 104 | 80 | 0.6 | 0% | 0 | 1294 |
| Aug | 103 | 79 | 1.4 | 0% | 0 | 1274 |
| Sep | 99 | 74 | 0.8 | 0% | 0 | 1102 |
| Oct | 89 | 64 | 0.4 | 0% | 0 | 828 |
| Nov | 77 | 51 | 0.5 | 12% | <1 | 430 |
| Dec | 67 | 43 | 0.6 | 53% | 1 | 182 |
Monthly normals 1991–2020 at Ajo, AZ (1,751 ft), NOAA NCEI. "32°F night": chance of at least one in the month. GDD: growing degree days, base 50°F.
When it snows in Ajo, Arizona
Measurable snow is rare here: only 1 of 20 winters from 1991 to 2020 recorded 0.1 inch or more at Ajo, AZ, 5 miles away, so there is no typical first-snow date to plan around.
Where these figures come from
ZIP 85321 is 5 miles from Ajo, AZ, which stands at 1,751 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 85321 in zone 9b, the band where the coldest night of the year averaged 25 to 30°F (-3.9 to -1.1°C) over 1991–2020; a zone describes winter lows, not frost dates.
Hardiness zone and temperature range: USDA Plant Hardiness Zone Map 2023 (USDA-ARS, Oregon State University), ZIP-code listing, unchanged. °C converted from °F by this site.
Thirty-year averages, not a forecast: check the forecast before planting out. How stations are matched and which pages are offered to search engines: methodology.