Frost Dates for 51002 — Alta, Iowa

Half of springs at Storm Lake, IA see their last 32°F night by April 25; only one in ten is past it by April 11, and one in ten still has one after May 7. Planting tender crops after May 1 leaves a 30% risk of a frost on them, after May 3 20%, and after May 7 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 13 in half of years, as early as October 1 one year in ten, and holds off past October 25 one year in ten. A hard freeze of 28°F follows 10 days later, around October 23; in spring the one-in-ten date for 28°F is April 29, 8 days before the 32°F one. The freeze-free season lasts 170 days in a typical year, longer than at 69% of the 132 Iowa stations; it reaches 189 days one year in ten and falls short of 151 one year in ten. On average 151 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 9 days longer, to about May 4, and return around October 3. Severe cold of 16°F or below comes every winter, usually between November 16 and March 20.

July is the warmest month at Storm Lake, IA, averaging 82°F by day and 64°F at night; January is the coldest at 26°F and 9°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 32% of years.

Warmth for growing adds up to 2,908 growing degree days a year (base 50°F), fewer than at 61% of the 132 Iowa stations. 35% of them (1,006) have built up by the end of June, and July adds the most, 713.

The nearest other station, Cherokee, IA, 12 miles northwest of Alta and 232 feet lower, has a last frost 9 days later (May 4), a first frost 9 days earlier (October 4), a season 18 days shorter.

The first measurable snow (0.1 inch or more) in Alta usually arrives around November 18, and the last of the season around March 28. Those are the middle dates across the winters of 1991–2020 at Storm Lake, IA, 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 28 days after the last snow.

ZIP 51002 is 7 miles from Storm Lake, IA, which stands at 1,442 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 1 other ZIP code shares the station; its figures are presented in full for 50588 (Storm Lake). The 2023 USDA Plant Hardiness Zone Map lists ZIP 51002 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.