Frost Dates for 50171 — Montezuma, Iowa

Half of springs at Grinnell 3 SW, IA see their last 32°F night by May 3; only one in ten is past it by April 19, and one in ten still has one after May 17. Planting tender crops after May 8 leaves a 30% risk of a frost on them, after May 13 20%, and after May 17 10%. The 14 days between the even-odds date and the one-in-ten date is wider than at 58% of the 132 Iowa stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 6 in half of years, as early as September 22 one year in ten, and holds off past October 20 one year in ten. A hard freeze of 28°F follows 11 days later, around October 17; in spring the one-in-ten date for 28°F is May 9, 8 days before the 32°F one. The freeze-free season lasts 155 days in a typical year, shorter than at 82% of the 132 Iowa stations; it reaches 175 days one year in ten and falls short of 136 one year in ten. On average 157 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 10 days longer, to about May 13, and return around September 27. Severe cold of 16°F or below comes every winter, usually between November 14 and March 22.

July is the warmest month at Grinnell 3 SW, IA, averaging 83°F by day and 62°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 60% of years.

Warmth for growing adds up to 2,803 growing degree days a year (base 50°F), fewer than at 74% of the 132 Iowa stations. 34% of them (947) have built up by the end of June, and July adds the most, 699. Precipitation totals 38.3 inches a year, most in June (5.5") and least in January (1.1"); May to September bring 59% of it.

The nearest other station, Oskaloosa, IA, 19 miles south of Montezuma, has a last frost 12 days earlier (April 21), a first frost 10 days later (October 16), a season 21 days longer.

The first measurable snow (0.1 inch or more) in Montezuma usually arrives around November 30, and the last of the season around March 24. Those are the middle dates across the winters of 1991–2020 at Montezuma 1 W, IA, 0.5 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 40 days after the last snow.

ZIP 50171 is 14 miles from Grinnell 3 SW, IA, which stands at 905 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 1 other ZIP code shares the station; its figures are presented in full for 50112 (Grinnell). The 2023 USDA Plant Hardiness Zone Map lists ZIP 50171 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.