U.S. Frost Dates by Weather Station

NOAA station USC00131402 · Iowa

Frost Dates for Charles City, IA

NOAA weather station Charles City (USC00131402), Iowa · elevation 993 ft (303 m) · hardiness zone 5a*

  • Lower-risk planting dateafter May 14only 1 spring in 10 still has a frost later
  • Typical last frostApril 3050% chance of a frost after it
  • Typical first fall frostOctober 650% chance of a frost before it
  • Frost-free season157 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Charles City, IASpring: a frost after April 15 in 9 years out of 10, after April 30 in half of the years, after May 14 in 1 year out of 10. Fall: the first frost comes before September 21 in 1 year out of 10, before October 6 in half of the years, before October 19 in 9 years out of 10. Typical frost-free season: 157 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 30First frost ~Oct 6Frost season at Charles City, IASpring: a frost after April 15 in 9 years out of 10, after April 30 in half of the years, after May 14 in 1 year out of 10. Fall: the first frost comes before September 21 in 1 year out of 10, before October 6 in half of the years, before October 19 in 9 years out of 10. Typical frost-free season: 157 days.JFMAMJJASONDLast frost ~Apr 30First frost ~Oct 6
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 28 years of NOAA records (1991–2020 normals)
ChanceLast frost in spring afterFirst frost in fall before
10% (unlikely)May 14September 21
50% (typical)April 30October 6
90% (likely)April 15October 19
Frost-free season (32°F, 50% chance)
157 days
Hard frost (28°F), last in spring, 50% chance after
April 21
Hard frost (28°F), first in fall, 50% chance before
October 17

The last 15 years (2006–2020 normals)

Typical last frost May 1 (1 day later than the 30-year normal), typical first fall frost October 11 (5 days later). Frost-free season 161 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 Charles City: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 14, 20% May 10, 30% May 6, 40% May 3, 50% April 30, 60% April 28, 70% April 24, 80% April 20, 90% April 15. Observed last spring frost in recent years: 2021 May 11, 2022 May 4, 2023 May 3, 2024 April 24, 2025 April 16, 2026 May 7.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 14Apr 30Apr 152021: May 112022: May 42023: May 32024: April 242025: April 162026: May 7Fall at Charles City: chance of a frost before this dateFall, chance of a frost before the date: 10% September 21, 20% September 26, 30% October 1, 40% October 3, 50% October 6, 60% October 8, 70% October 11, 80% October 15, 90% October 19. Observed first fall frost in recent years: 2021 October 22, 2022 September 28, 2023 October 10, 2024 October 15, 2025 October 23.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 21Oct 6Oct 192021: October 222022: September 282023: October 102024: 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
2022May 4September 28
2023May 3October 10
2024April 24October 15
2025April 16October 23
2026May 7season not over yet

What these dates mean

The 157 days between the 50% dates in Charles City, Iowa – April 30 and October 6 – define the period when a light frost (32 °F) is equally likely to occur before or after each date. For a gardener, the April 30 date marks an even-odds point in spring, while October 6 marks the same balance in fall.

A cautious spring plan uses the May 14 date, which carries a 10 % chance of a light frost later, making it the recommended day to set out tender crops. In autumn, the September 21 date is the cautious choice, with only a 10 % chance that a light frost has arrived earlier, indicating when to finish harvesting tender crops. By contrast, the April 15 (90 % chance later) and October 19 (90 % chance earlier) dates represent gambles. The hard-frost 50 % dates are April 21 and October 17; a hard frost at 28 °F can damage plant tissue more severely than a light frost at 32 °F.

How to read the probabilities

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

  • Spring: "10% after May 14" 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 21" 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 Charles City, IANumbered nearest stations: 1 — Osage, IA — 17 mi; 2 — New Hampton, IA — 18 mi; 3 — Allison, IA — 22 mi; 4 — Tripoli, IA — 27 mi; 5 — Mason City, IA — 27 mi.IAMN25 mi · 40 km1. Osage, IA — 17 mi12. New Hampton, IA — 18 mi23. Allison, IA — 22 mi34. Tripoli, IA — 27 mi45. Mason City, IA — 27 mi5
  1. Osage, IA 17 mi (27 km)
  2. New Hampton, IA 18 mi (29 km)
  3. Allison, IA 22 mi (36 km)
  4. Tripoli, IA 27 mi (44 km)
  5. Mason 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: 43.0603, -92.6717.