U.S. Frost Dates by Weather Station

NOAA station USC00135230 · Iowa

Frost Dates for Mason City, IA

NOAA weather station Mason City (USC00135230), Iowa · elevation 1105 ft (337 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 9only 1 spring in 10 still has a frost later
  • Typical last frostApril 2950% chance of a frost after it
  • Typical first fall frostOctober 750% chance of a frost before it
  • Frost-free season161 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Mason City, IASpring: a frost after April 14 in 9 years out of 10, after April 29 in half of the years, after May 9 in 1 year out of 10. Fall: the first frost comes before September 25 in 1 year out of 10, before October 7 in half of the years, before October 21 in 9 years out of 10. Typical frost-free season: 161 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 29First frost ~Oct 7Frost season at Mason City, IASpring: a frost after April 14 in 9 years out of 10, after April 29 in half of the years, after May 9 in 1 year out of 10. Fall: the first frost comes before September 25 in 1 year out of 10, before October 7 in half of the years, before October 21 in 9 years out of 10. Typical frost-free season: 161 days.JFMAMJJASONDLast frost ~Apr 29First frost ~Oct 7
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 29 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 9September 25
50% (typical)April 29October 7
90% (likely)April 14October 21
Frost-free season (32°F, 50% chance)
161 days
Hard frost (28°F), last in spring, 50% chance after
April 18
Hard frost (28°F), first in fall, 50% chance before
October 18

The last 15 years (2006–2020 normals)

Typical last frost April 30 (1 day later than the 30-year normal), typical first fall frost October 13 (6 days later). Frost-free season 166 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 Mason City: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 9, 20% May 6, 30% May 3, 40% April 30, 50% April 29, 60% April 25, 70% April 22, 80% April 19, 90% April 14. Observed last spring frost in recent years: 2021 May 11, 2022 April 28, 2023 May 4, 2024 April 23, 2025 May 1, 2026 May 2.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 9Apr 29Apr 142021: May 112022: April 282023: May 42024: April 232025: May 12026: May 2Fall at Mason City: chance of a frost before this dateFall, chance of a frost before the date: 10% September 25, 20% September 29, 30% October 1, 40% October 4, 50% October 7, 60% October 10, 70% October 13, 80% October 17, 90% October 21. Observed first fall frost in recent years: 2021 October 22, 2022 September 29, 2023 October 11, 2024 October 15, 2025 October 23.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 25Oct 7Oct 212021: October 222022: September 292023: October 112024: October 152025: October 23
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 11October 22
2022April 28September 29
2023May 4October 11
2024April 23October 15
2025May 1October 23
2026May 2season not over yet

What these dates mean

May 9 is the cautious spring date for light frost at 32 °F in Mason City, offering a gardener a 10 % chance of a later frost and thus a reliable point to set out tender crops. The opposite cautious fall date is September 25, when a 10 % chance of an earlier frost means it is a prudent day to finish harvesting tender crops.

The 50% date, April 29, marks the moment when there is an even odds (50 % chance) of a light frost later in the season, while the fall 50% date, October 7, signals a 50 % chance of a light frost earlier in the season. The median hard-frost dates are April 18 and October 18; hard frosts at 28 °F can damage plants that would survive a light frost at 32 °F. The two 32 °F 50% dates are 161 days apart, based on a 29-year record.

How to read the probabilities

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

  • Spring: "10% after May 9" 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 25" 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 Mason City, IANumbered nearest stations: 1 — Mason City (Airport), IA — 7 mi; 2 — Northwood, IA — 19 mi; 3 — Osage, IA — 21 mi; 4 — Forest City, IA — 23 mi; 5 — Charles City, IA — 27 mi.MN25 mi · 40 km1. Mason City (Airport), IA — 7 mi12. Northwood, IA — 19 mi23. Osage, IA — 21 mi34. Forest City, IA — 23 mi45. Charles City, IA — 27 mi5
  1. Mason City (Airport), IA 7 mi (11 km)
  2. Northwood, IA 19 mi (31 km)
  3. Osage, IA 21 mi (34 km)
  4. Forest City, IA 23 mi (38 km)
  5. Charles City, IA 27 mi (44 km)

Sources

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

Station coordinates: 43.1631, -93.1953.