Frost Dates for 51537 — Harlan, Iowa

Half of springs at Harlan, IA see their last 32°F night by April 27; only one in ten is past it by April 12, 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 5 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 9 in half of years, as early as September 24 one year in ten, and holds off past October 22 one year in ten. A hard freeze of 28°F follows 11 days later, around October 20; 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 165 days in a typical year, shorter than at 45% of the 132 Iowa stations; it reaches 183 days one year in ten and falls short of 145 one year in ten. On average 148 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 September 30. Severe cold of 16°F or below comes every winter, usually between November 14 and March 21.

July is the warmest month at Harlan, IA, averaging 86°F by day and 64°F at night; January is the coldest at 31°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 35% of years.

Warmth for growing adds up to 3,280 growing degree days a year (base 50°F), more than at 83% of the 132 Iowa stations. 36% of them (1,176) have built up by the end of June, and July adds the most, 767. Precipitation totals 34.3 inches a year, most in June (5.2") and least in January (0.9"); May to September bring 64% of it.

The nearest other station, Audubon, IA, 20 miles east of Harlan, has a last frost 5 days later (May 2), a first frost 6 days earlier (October 3), a season 11 days shorter.

The first measurable snow (0.1 inch or more) in Harlan usually arrives around November 12, and the last of the season around March 25. Those are the middle dates across the winters of 1991–2020 at Harlan, IA, 2 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 33 days after the last snow.

ZIP 51537 is 2 miles from Harlan, IA, which stands at 1,270 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 51537 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.