U.S. Frost Dates by Weather Station

NOAA station USC00138339 · Iowa

Frost Dates for Tripoli, IA

NOAA weather station Tripoli (USC00138339), Iowa · elevation 1055 ft (322 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 950% chance of a frost before it
  • Frost-free season162 daystypical year, 32°F
  • Hardiness zone5acalculated*

The year at a glance

Frost season at Tripoli, 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 9 in half of the years, before October 22 in 9 years out of 10. Typical frost-free season: 162 days.JanFebMarAprMayJunJulAugSepOctNovDecLast frost ~Apr 29First frost ~Oct 9Frost season at Tripoli, 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 9 in half of the years, before October 22 in 9 years out of 10. Typical frost-free season: 162 days.JFMAMJJASONDLast frost ~Apr 29First frost ~Oct 9
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.Today

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 25
50% (typical)April 29October 9
90% (likely)April 14October 22
Frost-free season (32°F, 50% chance)
162 days
Hard frost (28°F), last in spring, 50% chance after
April 18
Hard frost (28°F), first in fall, 50% chance before
October 20

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 12 (3 days later). Frost-free season 164 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 Tripoli: chance of a frost after this dateSpring, chance of a frost after the date: 10% May 9, 20% May 6, 30% May 4, 40% May 1, 50% April 29, 60% April 26, 70% April 22, 80% April 18, 90% April 14.Spring: last frostChance of a frost after this date10%50%90%Apr 1May 1May 9Apr 29Apr 14Fall at Tripoli: chance of a frost before this dateFall, chance of a frost before the date: 10% September 25, 20% October 1, 30% October 4, 40% October 6, 50% October 9, 60% October 12, 70% October 14, 80% October 18, 90% October 22.Fall: first frostChance of a frost before this date10%50%90%Oct 1Nov 1Sep 25Oct 9Oct 22
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.

What these dates mean

Setting out tender crops on April 14 carries a 90% chance of a frost later that season, a clear gamble for growers in Tripoli. The cautious spring date is May 9, when only a 10% chance of a frost remains.

The 50% date is April 29, indicating an even odds of a light frost later than this date, while the 50% fall date is October 9, indicating an even odds of a light frost earlier than this date. For a cautious fall plan, September 25 carries only a 10% chance of a frost, whereas October 22 is a gamble with a 90% chance. Hard-frost median dates are April 18 and October 20.

A hard frost at 28 °F can damage plant tissue that survives a light frost at 32 °F, making it a more critical threshold for tender crops.

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 Tripoli, IANumbered nearest stations: 1 — New Hampton, IA — 16 mi; 2 — Waterloo, IA — 19 mi; 3 — Fayette, IA — 22 mi; 4 — Charles City, IA — 27 mi; 5 — Allison, IA — 28 mi.25 mi · 40 km1. New Hampton, IA — 16 mi12. Waterloo, IA — 19 mi23. Fayette, IA — 22 mi34. Charles City, IA — 27 mi45. Allison, IA — 28 mi5
  1. New Hampton, IA 16 mi (26 km)
  2. Waterloo, IA 19 mi (31 km)
  3. Fayette, IA 22 mi (36 km)
  4. Charles City, IA 27 mi (44 km)
  5. Allison, 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: 42.8125, -92.2575.