U.S. Frost Dates by Weather Station

NOAA station USC00135769 · Iowa

Frost Dates for Mount Ayr, IA

NOAA weather station Mount Ayr (USC00135769), Iowa · elevation 1140 ft (348 m) · hardiness zone 5b*

  • Lower-risk planting dateafter May 9only 1 spring in 10 still has a frost later
  • Typical last frostApril 2550% chance of a frost after it
  • Typical first fall frostOctober 1250% chance of a frost before it
  • Frost-free season169 daystypical year, 32°F
  • Hardiness zone5bcalculated*

The year at a glance

Frost season at Mount Ayr, IASpring: a frost after April 10 in 9 years out of 10, after April 25 in half of the years, after May 9 in 1 year out of 10. Fall: the first frost comes before September 27 in 1 year out of 10, before October 12 in half of the years, before October 25 in 9 years out of 10. Typical frost-free season: 169 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 25First frost ~Oct 12Frost season at Mount Ayr, IASpring: a frost after April 10 in 9 years out of 10, after April 25 in half of the years, after May 9 in 1 year out of 10. Fall: the first frost comes before September 27 in 1 year out of 10, before October 12 in half of the years, before October 25 in 9 years out of 10. Typical frost-free season: 169 days.JFMAMJJASONDLast frost ~Apr 25First frost ~Oct 12
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 26 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 9September 27
50% (typical)April 25October 12
90% (likely)April 10October 25
Frost-free season (32°F, 50% chance)
169 days
Hard frost (28°F), last in spring, 50% chance after
April 14
Hard frost (28°F), first in fall, 50% chance before
October 23

The last 15 years (2006–2020 normals)

Typical last frost April 27 (2 days later than the 30-year normal), typical first fall frost October 15 (3 days later). Frost-free season 170 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 Ayr: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 9, 20% May 4, 30% May 1, 40% April 27, 50% April 25, 60% April 21, 70% April 18, 80% April 15, 90% April 10. Observed last spring frost in recent years: 2021 May 5, 2022 April 27.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 9Apr 25Apr 102021: May 52022: April 27Fall at Mount Ayr: chance of a frost before this dateFall, chance of a frost before the date: 10% September 27, 20% October 1, 30% October 6, 40% October 9, 50% October 12, 60% October 15, 70% October 18, 80% October 21, 90% October 25. Observed first fall frost in recent years: 2021 October 22, 2022 October 8, 2023 October 17.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 27Oct 12Oct 252021: October 222022: October 82023: October 17
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 5October 22
2022April 27October 8
2023incomplete recordOctober 17
2024incomplete recordincomplete record
2025incomplete recordincomplete record
2026incomplete recordseason not over yet

What these dates mean

May 9 is the cautious spring date for Mount Ayr, carrying only a 10 % chance that a light frost will occur later; this lets a gardener schedule the first planting of tender crops with minimal risk. The 50 % spring date, April 25, marks the point where a light frost is equally likely to occur later or not, while the 50 % fall date, October 12, represents an even chance that a light frost has already arrived. For a careful fall schedule the prudent date is September 27, when there is only a 10 % chance that a light frost has come earlier, allowing the final harvest of tender crops. The hard-frost median dates are April 14 in spring and October 23 in fall; because a 28 °F frost can damage plants that survive a 32 °F frost, it is a more critical threshold for garden planning.

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 27" 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 Ayr, IANumbered nearest stations: 1 — Beaconsfield, IA — 12 mi; 2 — Lamoni, IA — 17 mi; 3 — Creston, IA — 24 mi; 4 — Bedford, IA — 25 mi; 5 — Bethany, MO — 33 mi.NE25 mi · 40 km1. Beaconsfield, IA — 12 mi12. Lamoni, IA — 17 mi23. Creston, IA — 24 mi34. Bedford, IA — 25 mi45. Bethany, MO — 33 mi5
  1. Beaconsfield, IA 12 mi (20 km)
  2. Lamoni, IA 17 mi (28 km)
  3. Creston, IA 24 mi (39 km)
  4. Bedford, IA 25 mi (41 km)
  5. Bethany, MO 33 mi (53 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: 40.7053, -94.2428.