U.S. Frost Dates by Weather Station

NOAA station USC00170480 · Maine

Frost Dates for Belfast, ME

NOAA weather station Belfast (USC00170480), Maine · elevation 20 ft (6 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 21only 1 spring in 10 still has a frost later
  • Typical last frostMay 750% chance of a frost after it
  • Typical first fall frostOctober 550% chance of a frost before it
  • Frost-free season151 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Belfast, MESpring: a frost after April 28 in 9 years out of 10, after May 7 in half of the years, after May 21 in 1 year out of 10. Fall: the first frost comes before September 24 in 1 year out of 10, before October 5 in half of the years, before October 20 in 9 years out of 10. Typical frost-free season: 151 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 7First frost ~Oct 5Frost season at Belfast, MESpring: a frost after April 28 in 9 years out of 10, after May 7 in half of the years, after May 21 in 1 year out of 10. Fall: the first frost comes before September 24 in 1 year out of 10, before October 5 in half of the years, before October 20 in 9 years out of 10. Typical frost-free season: 151 days.JFMAMJJASONDLast frost ~May 7First frost ~Oct 5
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)May 21September 24
50% (typical)May 7October 5
90% (likely)April 28October 20
Frost-free season (32°F, 50% chance)
151 days
Hard frost (28°F), last in spring, 50% chance after
April 27
Hard frost (28°F), first in fall, 50% chance before
October 19

The last 15 years (2006–2020 normals)

Typical last frost May 3 (4 days earlier than the 30-year normal), typical first fall frost October 13 (8 days later). Frost-free season 159 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 Belfast: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 21, 20% May 16, 30% May 13, 40% May 10, 50% May 7, 60% May 5, 70% May 3, 80% April 30, 90% April 28. Observed last spring frost in recent years: 2021 May 7, 2022 May 9.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 21May 7Apr 282021: May 72022: May 9Fall at Belfast: chance of a frost before this dateFall, chance of a frost before the date: 10% September 24, 20% September 29, 30% October 1, 40% October 3, 50% October 5, 60% October 8, 70% October 11, 80% October 15, 90% October 20. Observed first fall frost in recent years: 2021 October 24, 2022 October 3.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 24Oct 5Oct 202021: October 242022: October 3
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 7October 24
2022May 9October 3
2023incomplete recordno data
2024no datano data
2025no datano data
2026no dataseason not over yet

What these dates mean

September 24 is the cautious fall date in Belfast for finishing the harvest, because it carries only a 10 % chance that a light frost has arrived earlier.

For tender crops the conservative spring date is May 21, when a light frost later than that date occurs with a 10 % chance; planting earlier on April 28 bears a 90 % chance and is a gamble. The 50 % dates—May 7 in spring and October 5 in fall—mark the median point where a light frost is equally likely to occur later (spring) or earlier (fall). The hard-frost median dates are April 27 and October 19; a 28 °F event can cause more severe injury than a 32 °F light frost, so it is a more critical threshold for growers. The record spans 30 years, and the interval between the two 32 °F 50 % dates—May 7 and October 5—is 151 days.

How to read the probabilities

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

  • Spring: "10% after May 21" 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 September 24" 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 Belfast, MENumbered nearest stations: 1 — West Rockport, ME — 16 mi; 2 — Bangor, ME — 29 mi; 3 — Tenants Harbor, ME — 30 mi; 4 — near Waterville, ME — 34 mi; 5 — Newcastle, ME — 36 mi.25 mi · 40 km1. West Rockport, ME — 16 mi12. Bangor, ME — 29 mi23. Tenants Harbor, ME — 30 mi34. near Waterville, ME — 34 mi45. Newcastle, ME — 36 mi5
  1. West Rockport, ME 16 mi (26 km)
  2. Bangor, ME 29 mi (47 km)
  3. Tenants Harbor, ME 30 mi (48 km)
  4. near Waterville, ME 34 mi (55 km)
  5. Newcastle, ME 36 mi (59 km)

Sources

* Hardiness zone 5b: zone derived from PRISM 2023 grid at the station location (not the official USDA map). Source data: PRISM Group, Oregon State University.

Station coordinates: 44.395, -68.9892.