Frost Dates for 62664 — Mason City, Illinois

Half of springs at Mason City 4SE, IL see their last 32°F night by April 17; only one in ten is past it by April 4, and one in ten still has one after May 4. Planting tender crops after April 23 leaves a 30% risk of a frost on them, after April 28 20%, and after May 4 10%. The 17 days between the even-odds date and the one-in-ten date is wider than at 79% of the 158 Illinois stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 18 in half of years, as early as October 4 one year in ten, and holds off past October 30 one year in ten. A hard freeze of 28°F follows 11 days later, around October 29; in spring the one-in-ten date for 28°F is April 20, 14 days before the 32°F one. The freeze-free season lasts 182 days in a typical year, shorter than at 56% of the 158 Illinois stations; it reaches 202 days one year in ten and falls short of 161 one year in ten. On average 122 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 April 28, and return around October 7. Severe cold of 16°F or below comes every winter, usually between November 24 and March 10.

July is the warmest month at Mason City 4SE, IL, averaging 85°F by day and 65°F at night; January is the coldest at 33°F and 16°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to September the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 15% of years.

Warmth for growing adds up to 3,481 growing degree days a year (base 50°F), fewer than at 53% of the 158 Illinois stations. 37% of them (1,275) have built up by the end of June, and July adds the most, 772. Precipitation totals 39.3 inches a year, most in July (5.1") and least in December (1.7"); May to September bring 53% of it.

The nearest other station, Petersburg 1E, IL, 14 miles southwest of Mason City, has a last frost 4 days earlier (April 13), a first frost 3 days later (October 21), a season 7 days longer.

The first measurable snow (0.1 inch or more) in Mason City usually arrives around December 6, and the last of the season around March 18. Those are the middle dates across the winters of 1991–2020 at Mason City 4SE, IL, 3.8 miles away: half of years saw snow earlier, half later. Snow fell in 95% of those winters. The last 32°F night of spring typically comes 30 days after the last snow.

ZIP 62664 is 4 miles from Mason City 4SE, IL, which stands at 600 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 1 other ZIP code shares the station. The 2023 USDA Plant Hardiness Zone Map lists ZIP 62664 in zone 6a, the band where the coldest night of the year averaged -10 to -5°F (-23.3 to -20.6°C) over 1991–2020; a zone describes winter lows, not frost dates.