Frost Dates for 18017 — Bethlehem, Pennsylvania

Half of springs at Allentown International Airport, PA see their last 32°F night by April 20; only one in ten is past it by April 6, and one in ten still has one after May 6. Planting tender crops after April 26 leaves a 30% risk of a frost on them, after April 30 20%, and after May 6 10%. The 16 days between the even-odds date and the one-in-ten date is narrower than at 51% of the 176 Pennsylvania stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 24 in half of years, as early as October 8 one year in ten, and holds off past November 6 one year in ten. A hard freeze of 28°F follows 11 days later, around November 4; in spring the one-in-ten date for 28°F is April 18, 18 days before the 32°F one. The freeze-free season lasts 184 days in a typical year, longer than at 68% of the 176 Pennsylvania stations; it reaches 204 days one year in ten and falls short of 163 one year in ten. On average 118 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 May 2, and return around October 12. Severe cold of 16°F or below comes every winter, usually between December 12 and March 4.

July is the warmest month at Allentown International Airport, PA, averaging 86°F by day and 65°F at night; January is the coldest at 38°F and 22°F. A freezing night is all but certain in every month from November 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 20% of years.

Warmth for growing adds up to 3,405 growing degree days a year (base 50°F), more than at 86% of the 176 Pennsylvania stations. 34% of them (1,165) have built up by the end of June, and July adds the most, 792. Precipitation totals 47.4 inches a year, most in July (5.3") and least in February (2.8"); May to September bring 48% of it.

The nearest other station, Springtown 1 NNE, PA, 7 miles southeast of Bethlehem and 470 feet higher, has a last frost 1 day later (April 21), a first frost 2 days earlier (October 22).

The first measurable snow (0.1 inch or more) in Bethlehem usually arrives around December 6, and the last of the season around March 22. Those are the middle dates across the winters of 1991–2020 at Allentown International Airport, PA, 4.2 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 29 days after the last snow.

ZIP 18017 is 4 miles from Allentown International Airport, PA, which stands at 390 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 16 other ZIP codes share the station; its figures are presented in full for 18102 (Allentown). The 2023 USDA Plant Hardiness Zone Map lists ZIP 18017 in zone 7a, the band where the coldest night of the year averaged 0 to 5°F (-17.8 to -15.0°C) over 1991–2020; a zone describes winter lows, not frost dates.