Frost Dates for 51401 — Carroll, Iowa

Half of springs at Carroll, IA see their last 32°F night by April 27; only one in ten is past it by April 14, and one in ten still has one after May 8. Planting tender crops after May 2 leaves a 30% risk of a frost on them, after May 5 20%, and after May 8 10%. The 11 days between the even-odds date and the one-in-ten date is narrower than at 88% of the 132 Iowa stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 10 in half of years, as early as September 25 one year in ten, and holds off past October 24 one year in ten. A hard freeze of 28°F follows 11 days later, around October 21; in spring the one-in-ten date for 28°F is April 29, 9 days before the 32°F one. The freeze-free season lasts 166 days in a typical year, longer than at 56% of the 132 Iowa stations; it reaches 185 days one year in ten and falls short of 145 one year in ten. On average 153 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 9 days longer, to about May 6, and return around September 30. Severe cold of 16°F or below comes every winter, usually between November 16 and March 20.

July is the warmest month at Carroll, IA, averaging 84°F by day and 64°F at night; January is the coldest at 28°F and 10°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to August the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 36% of years.

Warmth for growing adds up to 3,081 growing degree days a year (base 50°F), more than at 61% of the 132 Iowa stations. 35% of them (1,069) have built up by the end of June, and July adds the most, 747. Precipitation totals 33.9 inches a year, most in June (5.1") and least in January (0.9"); May to September bring 62% of it.

The nearest other station, Denison, IA, 24 miles west of Carroll and 161 feet higher, has the same 50% last-frost date, a first frost 1 day later (October 11).

The first measurable snow (0.1 inch or more) in Carroll usually arrives around November 22, and the last of the season around March 29. Those are the middle dates across the winters of 1991–2020 at Carroll, IA, 0.9 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 29 days after the last snow.

ZIP 51401 is under a mile from Carroll, IA, which stands at 1,240 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 1 other ZIP code shares the station. The 2023 USDA Plant Hardiness Zone Map lists ZIP 51401 in zone 5a, the band where the coldest night of the year averaged -20 to -15°F (-28.9 to -26.1°C) over 1991–2020; a zone describes winter lows, not frost dates.