USDA Hardiness Zone 10b
Frost Dates for 91001 — Altadena, California
NOAA 1991–2020 normals from Pasadena, CA (864 ft), 3 miles south.
Updated
Frost is not a yearly event in Altadena: a night at or below 32°F came in 13% of the winters of 1991–2020 at Pasadena, CA, and a 36°F night, cold enough for frost on exposed leaves, in 75%. There is no reliable "last frost date" here; most years have none.
- Winters with a freeze
- 13%
- Winters with 36°F
- 75%
- Coldest month low
- 45°F
- Hardiness zone
- 10b
32°F or below, 1991–2020
frost possible on leaves
average, December
35 to 40°F (1.7 to 4.4°C) coldest night
- First snow
- Rare
- Last snow
- Rare
- Winters with snow
- 0%
- Snow per year
- 0"
median, 1991–2020
median, 1991–2020
0 of 20 observed
NOAA normal
Frost risk by date in Altadena
At Pasadena, CA, 3 miles south of Altadena, 13% of the winters of 1991–2020 had at least one night at or below 32°F, 0% reached 28°F. Nights of 36°F, enough for frost on exposed leaves, are more common (75% of winters) and, when they come, fall between about December 28 and January 17. January is the month most likely to bring one: a 8% chance in a given year. 62 of the 283 other California stations behind this site's ZIP pages freeze in fewer than half of winters too.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F75% of yrs | Dec 18 | Dec 31 | Jan 5 | Jan 11 | Jan 17 | Jan 27 | Feb 6 | Feb 17 | Feb 27 |
| 32°F13% of yrs | too rare for a date | ||||||||
| 28°F0% of yrs | too rare for a date | ||||||||
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F75% of yrs | Dec 5 | Dec 12 | Dec 18 | Dec 22 | Dec 28 | Jan 1 | Jan 7 | Jan 20 | Feb 7 |
| 32°F13% of yrs | too rare for a date | ||||||||
| 28°F0% of yrs | too rare for a date | ||||||||
Freeze probabilities 1991–2020 at Pasadena, CA, 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 91001 →Compare Altadena with another place →
Weather by month in Altadena
August is the warmest month at Pasadena, CA, averaging 91°F by day and 63°F at night; December is the coldest at 67°F and 45°F. Every month stays under a 10% chance of a 32°F night; January is the likeliest at 8%.
Warmth for growing adds up to 5,867 growing degree days a year (base 50°F), more than at 87% of the 283 California stations. 39% of them (2,291) have built up by the end of June, and August adds the most, 845. Precipitation totals 20.0 inches a year, most in February (5.2") and least in August (0.0"); May to September bring 5% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 68 | 46 | 4.5 | 8% | 0 | 219 |
| Feb | 69 | 46 | 5.2 | 2% | 0 | 221 |
| Mar | 73 | 48 | 3.0 | 0% | 0 | 328 |
| Apr | 76 | 51 | 1.1 | 0% | 0 | 402 |
| May | 78 | 54 | 0.5 | 0% | 0 | 499 |
| Jun | 84 | 58 | 0.2 | 0% | 0 | 623 |
| Jul | 89 | 62 | 0.1 | 0% | 0 | 792 |
| Aug | 91 | 63 | 0.0 | 0% | 0 | 845 |
| Sep | 90 | 62 | 0.2 | 0% | 0 | 773 |
| Oct | 83 | 56 | 0.8 | 0% | 0 | 609 |
| Nov | 75 | 50 | 1.1 | 0% | 0 | 365 |
| Dec | 67 | 45 | 3.3 | 3% | 0 | 192 |
Monthly normals 1991–2020 at Pasadena, CA (864 ft), NOAA NCEI. "32°F night": chance of at least one in the month. GDD: growing degree days, base 50°F.
Compared with the next station
The nearest other station, Mt Wilson Cbs, CA, 5 miles northeast of Altadena and 4,846 feet higher, has a 32°F night in 100% of winters against 13% here, and a coolest-month low 8°F colder (37°F).
When it snows in Altadena, California
Measurable snow is rare here: only 0 of 20 winters from 1991 to 2020 recorded 0.1 inch or more at Pasadena, CA, 3 miles away, so there is no typical first-snow date to plan around.
Where these figures come from
ZIP 91001 is 3 miles from Pasadena, CA, which stands at 864 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 16 other ZIP codes share the station; its figures are presented in full for 90042 (Los Angeles). The 2023 USDA Plant Hardiness Zone Map lists ZIP 91001 in zone 10b, the band where the coldest night of the year averaged 35 to 40°F (1.7 to 4.4°C) over 1991–2020; a zone describes winter lows, not frost dates.
Frost dates across California →
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.