U.S. Frost Dates by Weather Station

NOAA station USC00045933 · California

Frost Dates for Mount Hamilton, CA

NOAA weather station Mount Hamilton (USC00045933), California · elevation 4206 ft (1282 m) · hardiness zone 9a*

  • Lower-risk planting dateafter June 5only 1 spring in 10 still has a frost later
  • Typical last frostMay 350% chance of a frost after it
  • Typical first fall frostNovember 1850% chance of a frost before it
  • Frost-free season197 daystypical year, 32°F
  • Hardiness zone9acalculated*

The year at a glance

Frost season at Mount Hamilton, CASpring: a frost after April 7 in 9 years out of 10, after May 3 in half of the years, after June 5 in 1 year out of 10. Fall: the first frost comes before October 22 in 1 year out of 10, before November 18 in half of the years, before December 12 in 9 years out of 10. Typical frost-free season: 197 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 3First frost ~Nov 18Frost season at Mount Hamilton, CASpring: a frost after April 7 in 9 years out of 10, after May 3 in half of the years, after June 5 in 1 year out of 10. Fall: the first frost comes before October 22 in 1 year out of 10, before November 18 in half of the years, before December 12 in 9 years out of 10. Typical frost-free season: 197 days.JFMAMJJASONDLast frost ~May 3First frost ~Nov 18
frost likely chance fading 90% → 10% frost-free in a typical yearThe year at a glance. Blue: frost is likely; the gradient fades as the chance drops from 90% to 10%; green: the frost-free part of a typical year.

Frost dates (32°F)

Probability of frost, based on 30 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)June 5October 22
50% (typical)May 3November 18
90% (likely)April 7December 12
Frost-free season (32°F, 50% chance)
197 days
Hard frost (28°F), last in spring, 50% chance after
April 10
Hard frost (28°F), first in fall, 50% chance before
December 7

The last 15 years (2006–2020 normals)

Typical last frost May 3 (same as the 30-year normal), typical first fall frost November 19 (1 day later). Frost-free season 195 days.

A shorter, more recent NOAA period. The 30-year normals above remain the main reference: 15 years carry more year-to-year noise.

Add these dates to your calendar Yearly reminders for the typical last frost, the lower-risk planting date and the typical first fall frost (.ics).

How the chance of frost changes with the date

Spring at Mount Hamilton: chance of a frost after this dateSpring, chance of a frost after the date: 10% June 5, 20% May 26, 30% May 16, 40% May 9, 50% May 3, 60% April 29, 70% April 24, 80% April 17, 90% April 7. Observed last spring frost in recent years: 2021 May 20, 2022 May 11, 2023 May 2, 2024 May 6, 2025 April 26, 2026 April 2.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1Jun 1Jun 5May 3Apr 72021: May 202022: May 112023: May 22024: May 62025: April 262026: April 2Fall at Mount Hamilton: chance of a frost before this dateFall, chance of a frost before the date: 10% October 22, 20% November 1, 30% November 7, 40% November 13, 50% November 18, 60% November 25, 70% December 1, 80% December 4, 90% December 12. Observed first fall frost in recent years: 2021 December 9, 2022 November 3, 2023 December 8, 2024 November 15, 2025 January 8.Fall: first frostChance of a frost before this date10%50%90%Nov 1Dec 1Jan 1Oct 22Nov 18Dec 122021: December 92022: November 32023: December 82024: November 152025: January 8
Two charts, because spring and fall answer different questions: in spring, the chance that a frost still comes after a date; in fall, the chance that the first frost has already come before it. NOAA publishes these odds from 10% to 90%, so the lines stop there. ◆ marks the actual date in each recent year.

What actually happened in recent years

Observed dates of the last spring and first fall frost (32°F) at this station; ◆ marks them on the charts above
YearLast spring frostFirst fall frost
2021May 20December 9
2022May 11November 3
2023May 2December 8
2024May 6November 15
2025April 26January 8
2026April 2season not over yet

What these dates mean

June 5 is the cautious spring date for Mount Hamilton; with only a 10% chance of a light frost after that day, a gardener can plan to set out tender crops with relatively low risk. The equally cautious fall date is October 22, when there is just a 10% chance that a light frost has arrived earlier, marking a reasonable deadline for finishing harvest of tender crops.

The 50% dates, May 3 in spring and November 18 in fall, indicate the point at which a light frost is equally likely to occur later (spring) or earlier (fall), giving a balanced reference for risk. Hard-frost thresholds are reached around April 10 in spring and December 7 in fall; because 28°F can cause far more severe damage than a 32°F light frost, these dates are especially important for protecting vulnerable plants.

How to read the probabilities

The dates are probabilities from 30 years of weather, not a forecast for this year.

  • Spring: "10% after June 5" means that in about one spring out of ten a frost still happens after that date. The 90% date is the earliest: a frost after it happens in most years.
  • Fall: "10% before October 22" means that in about one fall out of ten the first frost arrives before that date.
  • 50% is the typical date: half of the years are earlier, half are later.
  • A light frost (32°F) damages tender plants; a hard frost (28°F) also stops most hardy vegetables.

Nearby weather stations

Map of weather stations near Mount Hamilton, CANumbered nearest stations: 1 — Sonora, CA — 81 mi; 2 — King City, CA — 83 mi; 3 — Calaveras Big Trees, CA — 97 mi; 4 — Graton, CA — 100 mi; 5 — Cherry Valley Dam, CA — 104 mi.CA50 mi · 80 km1. Sonora, CA — 81 mi12. King City, CA — 83 mi23. Calaveras Big Trees, CA — 97 mi34. Graton, CA — 100 mi45. Cherry Valley Dam, CA — 104 mi5
  1. Sonora, CA 81 mi (130 km)
  2. King City, CA 83 mi (134 km)
  3. Calaveras Big Trees, CA 97 mi (156 km)
  4. Graton, CA 100 mi (162 km)
  5. Cherry Valley Dam, CA 104 mi (167 km)

Sources

* Hardiness zone 9a: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 37.3436, -121.6425.