U.S. Frost Dates by Weather Station

NOAA station USC00130200 · Iowa

Frost Dates for Ames, IA

NOAA weather station Ames 8 WSW (USC00130200), Iowa · elevation 1099 ft (335 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 5only 1 spring in 10 still has a frost later
  • Typical last frostApril 2350% chance of a frost after it
  • Typical first fall frostOctober 1450% chance of a frost before it
  • Frost-free season173 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Ames, IASpring: a frost after April 7 in 9 years out of 10, after April 23 in half of the years, after May 5 in 1 year out of 10. Fall: the first frost comes before September 29 in 1 year out of 10, before October 14 in half of the years, before October 27 in 9 years out of 10. Typical frost-free season: 173 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 23First frost ~Oct 14Frost season at Ames, IASpring: a frost after April 7 in 9 years out of 10, after April 23 in half of the years, after May 5 in 1 year out of 10. Fall: the first frost comes before September 29 in 1 year out of 10, before October 14 in half of the years, before October 27 in 9 years out of 10. Typical frost-free season: 173 days.JFMAMJJASONDLast frost ~Apr 23First frost ~Oct 14
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 5September 29
50% (typical)April 23October 14
90% (likely)April 7October 27
Frost-free season (32°F, 50% chance)
173 days
Hard frost (28°F), last in spring, 50% chance after
April 11
Hard frost (28°F), first in fall, 50% chance before
October 24

The last 15 years (2006–2020 normals)

Typical last frost April 23 (same as the 30-year normal), typical first fall frost October 17 (3 days later). Frost-free season 177 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 Ames: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 5, 20% May 2, 30% April 30, 40% April 26, 50% April 23, 60% April 20, 70% April 16, 80% April 12, 90% April 7. Observed last spring frost in recent years: 2021 April 22, 2022 April 26, 2023 April 24, 2024 April 21, 2025 April 8, 2026 April 20.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 5Apr 23Apr 72021: April 222022: April 262023: April 242024: April 212025: April 82026: April 20Fall at Ames: chance of a frost before this dateFall, chance of a frost before the date: 10% September 29, 20% October 4, 30% October 7, 40% October 11, 50% October 14, 60% October 17, 70% October 19, 80% October 22, 90% October 27. Observed first fall frost in recent years: 2021 October 23, 2022 October 8, 2023 October 28, 2024 October 15, 2025 November 2.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 29Oct 14Oct 272021: October 232022: October 82023: October 282024: October 152025: November 2
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
2021April 22October 23
2022April 26October 8
2023April 24October 28
2024April 21October 15
2025April 8November 2
2026April 20season not over yet

What these dates mean

May 5 is the cautious spring date for light frost, meaning only a 10 % chance of a frost later and allowing a gardener to set out tender crops with confidence.

The 50 % dates are April 23 in spring and October 14 in fall; on each of these days the probability of a light frost is even, so a gardener can treat them as the midpoint of the frost risk window. The 50 % hard-frost dates are April 11 and October 24, marking the median moments when a temperature of 28 °F becomes as likely as not.

September 29 is the cautious fall date for light frost, with a 10 % chance of a frost earlier and giving a gardener a reliable point to finish harvesting tender crops. A hard frost matters more than a light frost because the lower 28 °F threshold can damage plant tissue that survives the milder 32 °F conditions.

How to read the probabilities

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

  • Spring: "10% after May 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 September 29" 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 Ames, IANumbered nearest stations: 1 — Boone, IA — 6 mi; 2 — Ankeny, IA — 21 mi; 3 — Perry, IA — 21 mi; 4 — Webster City, IA — 31 mi; 5 — Jefferson, IA — 32 mi.25 mi · 40 km1. Boone, IA — 6 mi12. Ankeny, IA — 21 mi23. Perry, IA — 21 mi34. Webster City, IA — 31 mi45. Jefferson, IA — 32 mi5
  1. Boone, IA 6 mi (10 km)
  2. Ankeny, IA 21 mi (34 km)
  3. Perry, IA 21 mi (34 km)
  4. Webster City, IA 31 mi (50 km)
  5. Jefferson, IA 32 mi (51 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: 42.0208, -93.7742.