Frost Dates for 50428 — Clear Lake, Iowa

Half of springs at Mason City Municipal Airport, 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 15. Planting tender crops after May 7 leaves a 30% risk of a frost on them, after May 10 20%, and after May 15 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 1 in half of years, as early as September 18 one year in ten, and holds off past October 15 one year in ten. A hard freeze of 28°F follows 10 days later, around October 11; in spring the one-in-ten date for 28°F is May 7, 8 days before the 32°F one. The freeze-free season lasts 150 days in a typical year, shorter than at 97% of the 132 Iowa stations; it reaches 168 days one year in ten and falls short of 131 one year in ten. On average 161 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 11, and return around September 24. Severe cold of 16°F or below comes every winter, usually between November 10 and March 27.

July is the warmest month at Mason City Municipal Airport, IA, averaging 83°F by day and 61°F at night; January is the coldest at 24°F and 7°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 59% of years.

Warmth for growing adds up to 2,628 growing degree days a year (base 50°F), fewer than at 91% of the 132 Iowa stations. 35% of them (912) have built up by the end of June, and July adds the most, 666. Precipitation totals 36.4 inches a year, most in June (5.4") and least in January (0.9"); May to September bring 61% of it.

The nearest other station, Mason City, IA, 10 miles east of Clear Lake, has a last frost 4 days earlier (April 29), a first frost 6 days later (October 7), a season 11 days longer.

The first measurable snow (0.1 inch or more) in Clear Lake usually arrives around November 7, and the last of the season around April 9. Those are the middle dates across the winters of 1991–2020 at Mason City Municipal Airport, IA, 3.1 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 24 days after the last snow.

ZIP 50428 is 3 miles from Mason City Municipal Airport, IA, which stands at 1,225 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 50428 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.