Frost Dates for 19539 — Mertztown, Pennsylvania

Half of springs at Rodale Rsch Ctr, PA see their last 32°F night by April 26; only one in ten is past it by April 10, and one in ten still has one after May 13. Planting tender crops after May 3 leaves a 30% risk of a frost on them, after May 7 20%, and after May 13 10%. The 17 days between the even-odds date and the one-in-ten date is wider than at 50% of the 176 Pennsylvania stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 14 in half of years, as early as October 1 one year in ten, and holds off past October 31 one year in ten. A hard freeze of 28°F follows 12 days later, around October 26; in spring the one-in-ten date for 28°F is April 29, 14 days before the 32°F one. The freeze-free season lasts 171 days in a typical year, shorter than at 57% of the 176 Pennsylvania stations; it reaches 194 days one year in ten and falls short of 150 one year in ten. On average 131 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 8, and return around October 5. Severe cold of 16°F or below comes every winter, usually between December 6 and March 9.

July is the warmest month at Rodale Rsch Ctr, PA, averaging 85°F by day and 62°F at night; January is the coldest at 38°F and 20°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 37% of years.

Warmth for growing adds up to 3,067 growing degree days a year (base 50°F), more than at 57% of the 176 Pennsylvania stations. 33% of them (1,020) have built up by the end of June, and July adds the most, 733. Precipitation totals 48.8 inches a year, most in July (5.4") and least in February (2.8"); May to September bring 47% of it.

The nearest other station, Reading 4 NNW, PA, 14 miles west of Mertztown and 190 feet lower, has a last frost 16 days earlier (April 10), a first frost 17 days later (October 31), a season 31 days longer.

Measurable snow is rare here: only 7 of 19 winters from 1991 to 2020 recorded 0.1 inch or more at Bechtelsville 1ENE, PA, 9 miles away, so there is no typical first-snow date to plan around.

ZIP 19539 is 4 miles from Rodale Rsch Ctr, PA, which stands at 550 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 11 other ZIP codes share the station; its figures are presented in full for 18062 (Macungie). The 2023 USDA Plant Hardiness Zone Map lists ZIP 19539 in zone 6b, the band where the coldest night of the year averaged -5 to 0°F (-20.6 to -17.8°C) over 1991–2020; a zone describes winter lows, not frost dates.