Ajo Frost Dates and Growing Season
Frost and the growing season
NOAA's 1991-2020 freeze probabilities for Ajo. 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 10 and the first fall freeze around December 21, a growing season of about 347 days.
- Median at 36°F
- last February 5, first December 8
- Median at 32°F
- last January 10, first December 21
- Median at 28°F
- last January 8, first January 3
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 13 | Mar 4 | Feb 25 | Feb 13 | Feb 5 | Jan 29 | Jan 22 | Jan 12 | Jan 3 |
| 32°F | Feb 15 | Feb 3 | Jan 27 | Jan 17 | Jan 10 | Jan 3 | Dec 31 | Dec 23 | Dec 11 |
| 28°F | Feb 10 | Feb 1 | Jan 20 | Jan 13 | Jan 8 | Jan 3 | Dec 29 | Dec 16 | Nov 30 |
| 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 5 | Dec 8 | Dec 12 | Dec 16 | Dec 21 | Dec 30 |
| 32°F | Nov 25 | Dec 5 | Dec 10 | Dec 16 | Dec 21 | Dec 27 | Dec 31 | Jan 7 | Jan 24 |
| 28°F | Nov 27 | Dec 8 | Dec 23 | Dec 30 | Jan 3 | Jan 9 | Jan 14 | Jan 24 | Feb 5 |
| 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 | 341 | 331 | 322 | 315 | 306 | 297 | 288 | 277 | 263 |
| 32°F | 365 | 365 | 360 | 354 | 347 | 338 | 331 | 321 | 306 |
| 28°F | 365 | 365 | 365 | 365 | 365 | 365 | 365 | 358 | 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
- 99% of years
- 32°F or colder
- 81% of years
- 28°F or colder
- 29% 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
- 1,041
- Cooling degree days, base 65°F
- 4,179
- Growing degree days, base 40°F
- 12,265
- Growing degree days, base 45°F
- 10,451
- Growing degree days, base 50°F
- 8,674
- Growing degree days, base 55°F
- 6,997
- Growing degree days, base 60°F
- 5,491
Growing degree days by month, base 50°F
Heating and cooling by month
| Month | Heating DD | Cooling DD | Growing DD 50 |
|---|---|---|---|
| January | 278 | 5 | 211 |
| February | 183 | 20 | 265 |
| March | 104 | 87 | 450 |
| April | 31 | 201 | 620 |
| May | 4 | 431 | 891 |
| June | 0 | 676 | 1,126 |
| July | 0 | 829 | 1,294 |
| August | 0 | 809 | 1,274 |
| September | 0 | 652 | 1,102 |
| October | 9 | 371 | 828 |
| November | 117 | 93 | 430 |
| December | 314 | 5 | 182 |
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 0.3 km from Ajo. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.
More
Sources
NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00020080 (Ajo). Public domain.