Frost Dates for 21140 — Riva, Maryland

Half of springs at Annapolis Police Brks, MD see their last 32°F night by March 27; only one in ten is past it by March 9, and one in ten still has one after April 9. Planting tender crops after March 31 leaves a 30% risk of a frost on them, after April 4 20%, and after April 9 10%. The 13 days between the even-odds date and the one-in-ten date is narrower than at 80% of the 50 Maryland stations behind this site's ZIP pages.

In fall the first 32°F night comes by November 13 in half of years, as early as November 1 one year in ten, and holds off past December 1 one year in ten. A hard freeze of 28°F follows 15 days later, around November 28; in spring the one-in-ten date for 28°F is March 29, 11 days before the 32°F one. The freeze-free season lasts 234 days in a typical year, longer than at 94% of the 50 Maryland stations; it reaches 257 days one year in ten and falls short of 213 one year in ten. On average 81 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 April 7, and return around November 2. Nights of 24°F, cold enough to damage hardier plants, come in 100% of winters and 16°F in 87%.

July is the warmest month at Annapolis Police Brks, MD, averaging 89°F by day and 71°F at night; January is the coldest at 44°F and 29°F. A freezing night is all but certain in every month from December to March (a 90% chance or more). From May to October the chance of a 32°F night stays under 10% each month. In spring, April still brings one in 30% of years.

Warmth for growing adds up to 4,561 growing degree days a year (base 50°F), more than at 96% of the 50 Maryland stations. 36% of them (1,638) have built up by the end of June, and July adds the most, 936.

The nearest other station, Annapolis Naf, MD, 7 miles east of Riva, has a last frost 4 days earlier (March 23), a first frost 7 days later (November 20), a season 7 days longer.

The first measurable snow (0.1 inch or more) in Riva usually arrives around December 20, and the last of the season around February 18. Those are the middle dates across the winters of 1991–2020 at Upper Marlboro 3 NNW, MD, 12.2 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 37 days after the last snow.

ZIP 21140 is 5 miles from Annapolis Police Brks, MD, which stands at 25 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 7 other ZIP codes share the station; its figures are presented in full for 21122 (Pasadena). The 2023 USDA Plant Hardiness Zone Map lists ZIP 21140 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.