Frost Dates for 51346 — Hartley, Iowa

Half of springs at Sanborn, IA see their last 32°F night by May 1; only one in ten is past it by April 16, and one in ten still has one after May 13. Planting tender crops after May 5 leaves a 30% risk of a frost on them, after May 8 20%, and after May 13 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 7 in half of years, as early as September 23 one year in ten, and holds off past October 20 one year in ten. A hard freeze of 28°F follows 10 days later, around October 17; in spring the one-in-ten date for 28°F is May 4, 9 days before the 32°F one. The freeze-free season lasts 160 days in a typical year, shorter than at 71% of the 132 Iowa stations; it reaches 179 days one year in ten and falls short of 141 one year in ten. On average 166 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 7 days longer, to about May 8, and return around September 28. Severe cold of 16°F or below comes every winter, usually between November 11 and March 26.

July is the warmest month at Sanborn, IA, averaging 83°F by day and 62°F at night; January is the coldest at 24°F and 6°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 52% of years.

Warmth for growing adds up to 2,694 growing degree days a year (base 50°F), fewer than at 86% of the 132 Iowa stations. 34% of them (904) have built up by the end of June, and July adds the most, 690. Precipitation totals 32.2 inches a year, most in June (4.9") and least in January (0.9"); May to September bring 61% of it.

The nearest other station, Spencer 1 N, IA, 16 miles east of Hartley and 225 feet lower, has the same 50% last-frost date, a first frost 4 days earlier (October 3), a season 6 days shorter.

The first measurable snow (0.1 inch or more) in Hartley usually arrives around November 7, and the last of the season around April 10. Those are the middle dates across the winters of 1991–2020 at Sanborn, IA, 9.7 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 21 days after the last snow.

ZIP 51346 is 10 miles from Sanborn, IA, which stands at 1,551 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 51346 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.