Frost Dates for 99685 — Unalaska, Alaska

Half of springs at Dutch Harbor, AK see their last 32°F night by May 6; only one in ten is past it by April 23, and one in ten still has one after May 22. Planting tender crops after May 12 leaves a 30% risk of a frost on them, after May 16 20%, and after May 22 10%. The 16 days between the even-odds date and the one-in-ten date is wider than at 81% of the 37 Alaska 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 September 20 one year in ten, and holds off past November 5 one year in ten. A hard freeze of 28°F follows 17 days later, around November 4; in spring the one-in-ten date for 28°F is May 5, 17 days before the 32°F one. The freeze-free season lasts 164 days in a typical year, longer than at 84% of the 37 Alaska stations; it reaches 188 days one year in ten and falls short of 133 one year in ten. On average 112 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 20 days longer, to about May 26, and return around October 1. Nights of 24°F, cold enough to damage hardier plants, come in 100% of winters and 16°F in 73%.

August is the warmest month at Dutch Harbor, AK, averaging 59°F by day and 48°F at night; January is the coldest at 37°F and 29°F. A freezing night is all but certain in every month from November 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 73% of years.

Warmth for growing adds up to 250 growing degree days a year (base 50°F), fewer than at 97% of the 37 Alaska stations. 4% of them (10) have built up by the end of June, and August adds the most, 121. Precipitation totals 60.3 inches a year, most in October (7.9") and least in July (2.3"); May to September bring 29% of it.

The first measurable snow (0.1 inch or more) in Unalaska usually arrives around November 11, and the last of the season around April 19. Those are the middle dates across the winters of 1991–2020 at Dutch Harbor, AK, 1.8 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 17 days after the last snow.

ZIP 99685 is 2 miles from Dutch Harbor, AK, which stands at 10 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 99685 in zone 8a, the band where the coldest night of the year averaged 10 to 15°F (-12.2 to -9.4°C) over 1991–2020; a zone describes winter lows, not frost dates.