Frost Dates for 51442 — Denison, Iowa

Half of springs at Denison, IA see their last 32°F night by April 27; only one in ten is past it by April 13, and one in ten still has one after May 9. Planting tender crops after May 2 leaves a 30% risk of a frost on them, after May 4 20%, and after May 9 10%. The 12 days between the even-odds date and the one-in-ten date is narrower than at 67% of the 132 Iowa stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 11 in half of years, as early as September 27 one year in ten, and holds off past October 25 one year in ten. A hard freeze of 28°F follows 11 days later, around October 22; in spring the one-in-ten date for 28°F is April 30, 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 186 days one year in ten and falls short of 147 one year in ten. On average 149 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 8 days longer, to about May 5, and return around October 1. Severe cold of 16°F or below comes every winter, usually between November 17 and March 22.

July is the warmest month at Denison, IA, averaging 83°F by day and 63°F at night; January is the coldest at 27°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 2,908 growing degree days a year (base 50°F), fewer than at 61% of the 132 Iowa stations. 34% of them (991) have built up by the end of June, and July adds the most, 715. Precipitation totals 33.6 inches a year, most in June (4.9") and least in January (0.7"); May to September bring 63% of it.

The nearest other station, Harlan, IA, 26 miles south of Denison, has the same 50% last-frost date, a first frost 2 days earlier (October 9), a season 1 day shorter.

The first measurable snow (0.1 inch or more) in Denison usually arrives around November 16, and the last of the season around April 2. Those are the middle dates across the winters of 1991–2020 at Denison, IA, 1.7 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 25 days after the last snow.

ZIP 51442 is 2 miles from Denison, IA, which stands at 1,401 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 51442 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.