Frost Dates for 50060 — Corydon, Iowa

Half of springs at Allerton, IA see their last 32°F night by April 21; only one in ten is past it by April 7, and one in ten still has one after May 6. Planting tender crops after April 27 leaves a 30% risk of a frost on them, after May 1 20%, and after May 6 10%. The 15 days between the even-odds date and the one-in-ten date is wider than at 82% of the 132 Iowa stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 19 in half of years, as early as October 3 one year in ten, and holds off past October 29 one year in ten. A hard freeze of 28°F follows 8 days later, around October 27; in spring the one-in-ten date for 28°F is April 25, 11 days before the 32°F one. The freeze-free season lasts 178 days in a typical year, longer than at 84% of the 132 Iowa stations; it reaches 196 days one year in ten and falls short of 157 one year in ten. On average 141 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 11 days longer, to about May 2, and return around October 7. Severe cold of 16°F or below comes every winter, usually between November 20 and March 14.

July is the warmest month at Allerton, IA, averaging 85°F by day and 64°F at night; January is the coldest at 32°F and 13°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to September the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 21% of years.

Warmth for growing adds up to 3,188 growing degree days a year (base 50°F), more than at 71% of the 132 Iowa stations. 34% of them (1,097) have built up by the end of June, and July adds the most, 761. Precipitation totals 40.5 inches a year, most in May (5.7") and least in January (1.1"); May to September bring 61% of it.

The nearest other station, Leon 6 ESE, IA, 17 miles west of Corydon, has a last frost 9 days later (April 30), a first frost 11 days earlier (October 8), a season 17 days shorter.

The first measurable snow (0.1 inch or more) in Corydon usually arrives around December 1, and the last of the season around March 18. Those are the middle dates across the winters of 1991–2020 at Allerton, IA, 4.6 miles away: half of years saw snow earlier, half later. Every one of those winters saw snow. The last 32°F night of spring typically comes 34 days after the last snow.

ZIP 50060 is 5 miles from Allerton, IA, which stands at 1,090 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 50060 in zone 5b, the band where the coldest night of the year averaged -15 to -10°F (-26.1 to -23.3°C) over 1991–2020; a zone describes winter lows, not frost dates.