Frost Dates for 49801 — Iron Mountain, Michigan

Half of springs at Iron Mountain Ford Airport, MI see their last 32°F night by May 25; only one in ten is past it by May 10, and one in ten still has one after June 6. Planting tender crops after May 30 leaves a 30% risk of a frost on them, after June 2 20%, and after June 6 10%. The 12 days between the even-odds date and the one-in-ten date is narrower than at 99% of the 172 Michigan stations behind this site's ZIP pages.

In fall the first 32°F night comes by September 24 in half of years, as early as September 8 one year in ten, and holds off past October 7 one year in ten. A hard freeze of 28°F follows 11 days later, around October 5; in spring the one-in-ten date for 28°F is May 26, 11 days before the 32°F one. The freeze-free season lasts 121 days in a typical year, shorter than at 94% of the 172 Michigan stations; it reaches 142 days one year in ten and falls short of 102 one year in ten. On average 187 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 11 days longer, to about June 5, and return around September 12. Severe cold of 16°F or below comes every winter, usually between November 11 and April 10.

July is the warmest month at Iron Mountain Ford Airport, MI, averaging 80°F by day and 54°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 May (a 90% chance or more). From July to August the chance of a 32°F night stays under 10% each month. In spring, June still brings one in 24% of years.

Warmth for growing adds up to 1,882 growing degree days a year (base 50°F), fewer than at 85% of the 172 Michigan stations. 30% of them (573) have built up by the end of June, and July adds the most, 535. Precipitation totals 20.8 inches a year, most in June (3.0") and least in January (0.5"); May to September bring 63% of it.

The nearest other station, Iron Mountain-Kingsford Wwtp, MI, 7 miles southwest of Iron Mountain, has a last frost 8 days earlier (May 17), a first frost 7 days later (October 1), a season 14 days longer.

The first measurable snow (0.1 inch or more) in Iron Mountain usually arrives around November 9, and the last of the season around April 15. Those are the middle dates across the winters of 1991–2020 at Iron Mountain-Kingsford Wwtp, MI, 7.3 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 49801 is 7 miles from Iron Mountain Ford Airport, MI, which stands at 1,149 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 49801 in zone 4b, the band where the coldest night of the year averaged -25 to -20°F (-31.7 to -28.9°C) over 1991–2020; a zone describes winter lows, not frost dates.