Frost Dates for 99559 — Bethel, Alaska

Half of springs at Bethel Airport, AK see their last 32°F night by May 21; only one in ten is past it by May 8, and one in ten still has one after June 2. Planting tender crops after May 28 leaves a 30% risk of a frost on them, after May 31 20%, and after June 2 10%. The 12 days between the even-odds date and the one-in-ten date is narrower than at 65% of the 37 Alaska stations behind this site's ZIP pages.

In fall the first 32°F night comes by September 22 in half of years, as early as September 7 one year in ten, and holds off past October 4 one year in ten. A hard freeze of 28°F follows 9 days later, around October 1; in spring the one-in-ten date for 28°F is May 26, 7 days before the 32°F one. The freeze-free season lasts 122 days in a typical year, shorter than at 68% of the 37 Alaska stations; it reaches 140 days one year in ten and falls short of 104 one year in ten. On average 214 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 12 days longer, to about June 2, and return around September 8. Severe cold of 16°F or below comes every winter, usually between October 25 and April 21.

July is the warmest month at Bethel Airport, AK, averaging 64°F by day and 49°F at night; January is the coldest at 13°F and 1°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 16% of years.

Warmth for growing adds up to 501 growing degree days a year (base 50°F), fewer than at 78% of the 37 Alaska stations. 29% of them (145) have built up by the end of June, and July adds the most, 200. Precipitation totals 19.8 inches a year, most in August (3.4") and least in March (0.7"); May to September bring 60% of it.

The first measurable snow (0.1 inch or more) in Bethel usually arrives around October 13, and the last of the season around May 12. Those are the middle dates across the winters of 1991–2020 at Bethel Airport, AK, 2.1 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 9 days after the last snow.

ZIP 99559 is 2 miles from Bethel Airport, AK, which stands at 102 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 99559 in zone 4a, the band where the coldest night of the year averaged -30 to -25°F (-34.4 to -31.7°C) over 1991–2020; a zone describes winter lows, not frost dates.