Frost Dates for 04102 — Portland, Maine

Half of springs at Portland International Jetport, ME see their last 32°F night by April 26; only one in ten is past it by April 16, 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 11 days between the even-odds date and the one-in-ten date is narrower than at 87% of the 52 Maine stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 15 in half of years, as early as October 2 one year in ten, and holds off past October 30 one year in ten. A hard freeze of 28°F follows 12 days later, around October 27; in spring the one-in-ten date for 28°F is April 25, 12 days before the 32°F one. The freeze-free season lasts 170 days in a typical year, longer than at 83% of the 52 Maine stations; it reaches 188 days one year in ten and falls short of 153 one year in ten. On average 144 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 11 days longer, to about May 7, and return around October 4. Severe cold of 16°F or below comes every winter, usually between December 1 and March 15.

July is the warmest month at Portland International Jetport, ME, averaging 80°F by day and 61°F at night; January is the coldest at 32°F and 16°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 2,327 growing degree days a year (base 50°F), more than at 92% of the 52 Maine stations. 27% of them (639) have built up by the end of June, and July adds the most, 631. Precipitation totals 48.3 inches a year, most in October (5.3") and least in July (3.4"); May to September bring 39% of it.

The nearest other station, West Buxton 2 NNW, ME, 16 miles west of Portland and 175 feet higher, has a last frost 11 days later (May 7), a first frost 11 days earlier (October 4), a season 22 days shorter.

The first measurable snow (0.1 inch or more) in Portland usually arrives around November 26, and the last of the season around April 6. Those are the middle dates across the winters of 1991–2020 at Portland International Jetport, ME, 0.8 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 20 days after the last snow.

ZIP 04102 is under a mile from Portland International Jetport, ME, which stands at 45 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 9 other ZIP codes share the station; its figures are presented in full for 04103 (Portland). The 2023 USDA Plant Hardiness Zone Map lists ZIP 04102 in zone 6a, the band where the coldest night of the year averaged -10 to -5°F (-23.3 to -20.6°C) over 1991–2020; a zone describes winter lows, not frost dates.