Frost Dates for 82501 — Riverton, Wyoming

Half of springs at Riverton, WY see their last 32°F night by May 12; only one in ten is past it by May 1, and one in ten still has one after May 26. Planting tender crops after May 18 leaves a 30% risk of a frost on them, after May 21 20%, and after May 26 10%. The 14 days between the even-odds date and the one-in-ten date is narrower than at 58% of the 33 Wyoming 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 11 one year in ten, and holds off past October 7 one year in ten. A hard freeze of 28°F follows 10 days later, around October 4; in spring the one-in-ten date for 28°F is May 16, 10 days before the 32°F one. The freeze-free season lasts 132 days in a typical year, longer than at 64% of the 33 Wyoming stations; it reaches 151 days one year in ten and falls short of 114 one year in ten. On average 193 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 16 days longer, to about May 28, and return around September 14. Severe cold of 16°F or below comes every winter, usually between November 1 and March 27.

July is the warmest month at Riverton, WY, averaging 88°F by day and 55°F at night; January is the coldest at 29°F and 4°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 89% of years.

Warmth for growing adds up to 2,218 growing degree days a year (base 50°F), more than at 67% of the 33 Wyoming stations. 29% of them (638) have built up by the end of June, and July adds the most, 662. Precipitation totals 9.5 inches a year, most in May (2.0") and least in December (0.3"); May to September bring 54% of it.

The nearest other station, Lander 11 SSE, WY, 28 miles southwest of Riverton and 818 feet higher, has a last frost 1 day later (May 13), a first frost 8 days later (October 2), a season 8 days longer.

The first measurable snow (0.1 inch or more) in Riverton usually arrives around October 14, and the last of the season around May 1. Those are the middle dates across the winters of 1991–2020 at Riverton, WY, 1.7 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 11 days after the last snow.

ZIP 82501 is 2 miles from Riverton, WY, which stands at 4,955 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 82501 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.