Frost Dates for 80301 — Boulder, Colorado

Half of springs at Boulder, CO see their last 32°F night by May 6; only one in ten is past it by April 24, and one in ten still has one after May 18. Planting tender crops after May 11 leaves a 30% risk of a frost on them, after May 14 20%, and after May 18 10%. The 12 days between the even-odds date and the one-in-ten date is narrower than at 81% of the 110 Colorado stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 5 in half of years, as early as September 21 one year in ten, and holds off past October 18 one year in ten. A hard freeze of 28°F follows 10 days later, around October 15; in spring the one-in-ten date for 28°F is May 10, 8 days before the 32°F one. The freeze-free season lasts 151 days in a typical year, longer than at 62% of the 110 Colorado stations; it reaches 170 days one year in ten and falls short of 131 one year in ten. On average 138 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 17, and return around September 28. Severe cold of 16°F or below comes every winter, usually between November 12 and March 21.

July is the warmest month at Boulder, CO, averaging 88°F by day and 57°F at night; December is the coldest at 46°F and 21°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to August the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 73% of years.

Warmth for growing adds up to 2,812 growing degree days a year (base 50°F), more than at 69% of the 110 Colorado stations. 31% of them (885) have built up by the end of June, and July adds the most, 697. Precipitation totals 21.3 inches a year, most in May (3.2") and least in January (0.8"); May to September bring 51% of it.

The nearest other station, Longmont 2 ESE, CO, 11 miles northeast of Boulder and 534 feet lower, has a last frost 1 day later (May 7), the same first-frost date.

The first measurable snow (0.1 inch or more) in Boulder usually arrives around October 10, and the last of the season around May 2. Those are the middle dates across the winters of 1991–2020 at Boulder, CO, 4.4 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 4 days after the last snow.

ZIP 80301 is 4 miles from Boulder, CO, which stands at 5,484 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 6 other ZIP codes share the station; its figures are presented in full for 80027 (Louisville). The 2023 USDA Plant Hardiness Zone Map lists ZIP 80301 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.