Frost Dates for 20817 — Bethesda, Maryland

Half of springs at Dalecarlia Rsvr, MD see their last 32°F night by April 9; only one in ten is past it by March 27, and one in ten still has one after April 26. Planting tender crops after April 16 leaves a 30% risk of a frost on them, after April 20 20%, and after April 26 10%. The 17 days between the even-odds date and the one-in-ten date is wider than at 64% of the 50 Maryland stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 31 in half of years, as early as October 16 one year in ten, and holds off past November 13 one year in ten. A hard freeze of 28°F follows 9 days later, around November 9; in spring the one-in-ten date for 28°F is April 12, 14 days before the 32°F one. The freeze-free season lasts 202 days in a typical year, shorter than at 50% of the 50 Maryland stations; it reaches 223 days one year in ten and falls short of 181 one year in ten. On average 96 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 14 days longer, to about April 23, and return around October 20. Severe cold of 16°F or below comes in 99% of winters, usually between December 27 and February 23.

July is the warmest month at Dalecarlia Rsvr, MD, averaging 88°F by day and 68°F at night; January is the coldest at 43°F and 26°F. A freezing night is all but certain in every month from November to March (a 90% chance or more). From May to September the chance of a 32°F night stays under 10% each month. In spring, April still brings one in 78% of years.

Warmth for growing adds up to 4,035 growing degree days a year (base 50°F), more than at 64% of the 50 Maryland stations. 36% of them (1,443) have built up by the end of June, and July adds the most, 871. Precipitation totals 48.3 inches a year, most in July (5.1") and least in February (3.2"); May to September bring 47% of it.

The nearest other station, Rockville 1 NE, MD, 7 miles north of Bethesda and 290 feet higher, has a last frost 8 days earlier (April 1), a first frost 6 days later (November 6), a season 17 days longer.

The first measurable snow (0.1 inch or more) in Bethesda usually arrives around January 8, and the last of the season around February 16. Those are the middle dates across the winters of 1991–2020 at Dalecarlia Rsvr, MD, 4.2 miles away: half of years saw snow earlier, half later. Snow fell in 87% of those winters. The last 32°F night of spring typically comes 52 days after the last snow.

ZIP 20817 is 4 miles from Dalecarlia Rsvr, MD, which stands at 150 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 17 other ZIP codes share the station; its figures are presented in full for 22201 (Arlington). The 2023 USDA Plant Hardiness Zone Map lists ZIP 20817 in zone 7b, the band where the coldest night of the year averaged 5 to 10°F (-15.0 to -12.2°C) over 1991–2020; a zone describes winter lows, not frost dates.