U.S. Frost Dates by Weather Station

NOAA station USW00014940 · Iowa

Frost Dates for Mason City (Airport), IA

NOAA weather station Mason City Municipal Airport (USW00014940), Iowa · elevation 1225 ft (373 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 15only 1 spring in 10 still has a frost later
  • Typical last frostMay 350% chance of a frost after it
  • Typical first fall frostOctober 150% chance of a frost before it
  • Frost-free season150 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Mason City (Airport), IASpring: a frost after April 19 in 9 years out of 10, after May 3 in half of the years, after May 15 in 1 year out of 10. Fall: the first frost comes before September 18 in 1 year out of 10, before October 1 in half of the years, before October 15 in 9 years out of 10. Typical frost-free season: 150 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~May 3First frost ~Oct 1Frost season at Mason City (Airport), IASpring: a frost after April 19 in 9 years out of 10, after May 3 in half of the years, after May 15 in 1 year out of 10. Fall: the first frost comes before September 18 in 1 year out of 10, before October 1 in half of the years, before October 15 in 9 years out of 10. Typical frost-free season: 150 days.JFMAMJJASONDLast frost ~May 3First frost ~Oct 1
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 15September 18
50% (typical)May 3October 1
90% (likely)April 19October 15
Frost-free season (32°F, 50% chance)
150 days
Hard frost (28°F), last in spring, 50% chance after
April 24
Hard frost (28°F), first in fall, 50% chance before
October 11

The last 15 years (2006–2020 normals)

Typical last frost May 3 (same as the 30-year normal), typical first fall frost October 6 (5 days later). Frost-free season 153 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 (Airport): chance of a frost after this dateSpring, chance of a frost after the date: 10% May 15, 20% May 10, 30% May 7, 40% May 5, 50% May 3, 60% April 30, 70% April 28, 80% April 24, 90% April 19. Observed last spring frost in recent years: 2021 May 29, 2022 May 23, 2023 May 3, 2024 April 24, 2025 April 26, 2026 May 11.Spring: last frostChance of a frost after this date10%50%90%May 1Jun 1May 15May 3Apr 192021: May 292022: May 232023: May 32024: April 242025: April 262026: May 11Fall at Mason City (Airport): chance of a frost before this dateFall, chance of a frost before the date: 10% September 18, 20% September 23, 30% September 26, 40% September 29, 50% October 1, 60% October 4, 70% October 7, 80% October 10, 90% October 15. Observed first fall frost in recent years: 2021 October 21, 2022 September 27, 2023 October 9, 2024 October 14, 2025 October 8.Fall: first frostChance of a frost before this date10%50%90%Oct 1Sep 18Oct 1Oct 152021: October 212022: September 272023: October 92024: October 142025: October 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 29October 21
2022May 23September 27
2023May 3October 9
2024April 24October 14
2025April 26October 8
2026May 11season not over yet

What these dates mean

The cautious fall date to finish harvesting tender crops is September 18, when there is only a 10 % chance that a light frost has arrived earlier. In spring, the cautious date for setting out tender crops is May 15, providing a similar 10 % chance that a light frost will occur later.

The 50 % dates—May 3 for spring and October 1 for fall—mark the median point at which a light frost is equally likely to have happened or not; they serve as a reference for the overall frost risk but are not frost dates themselves. A hard frost (28 °F) becomes a 50 % chance on April 24 in spring and October 11 in fall, and it matters more than a light frost because the lower temperature can cause more severe damage to plant tissue.

How to read the probabilities

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

  • Spring: "10% after May 15" 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 18" 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 (Airport), IANumbered nearest stations: 1 — Mason City, IA — 7 mi; 2 — Forest City, IA — 18 mi; 3 — Northwood, IA — 20 mi; 4 — Osage, IA — 27 mi; 5 — Hampton, IA — 28 mi.MN25 mi · 40 km1. Mason City, IA — 7 mi12. Forest City, IA — 18 mi23. Northwood, IA — 20 mi34. Osage, IA — 27 mi45. Hampton, IA — 28 mi5
  1. Mason City, IA 7 mi (11 km)
  2. Forest City, IA 18 mi (29 km)
  3. Northwood, IA 20 mi (33 km)
  4. Osage, IA 27 mi (44 km)
  5. Hampton, IA 28 mi (45 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: 43.1544, -93.3269.