Frost Dates for 59840 — Hamilton, Montana

Half of springs at Hamilton, MT see their last 32°F night by May 21; only one in ten is past it by May 7, and one in ten still has one after June 6. Planting tender crops after May 28 leaves a 30% risk of a frost on them, after June 1 20%, and after June 6 10%. The 16 days between the even-odds date and the one-in-ten date is narrower than at 40% of the 68 Montana stations behind this site's ZIP pages.

In fall the first 32°F night comes by September 22 in half of years, as early as September 6 one year in ten, and holds off past October 5 one year in ten. A hard freeze of 28°F follows 10 days later, around October 2; in spring the one-in-ten date for 28°F is May 19, 18 days before the 32°F one. The freeze-free season lasts 122 days in a typical year, shorter than at 57% of the 68 Montana stations; it reaches 143 days one year in ten and falls short of 100 one year in ten. On average 173 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 20 days longer, to about June 10, and return around September 9. Severe cold of 16°F or below comes every winter, usually between November 7 and March 14.

July is the warmest month at Hamilton, MT, averaging 84°F by day and 50°F at night; December is the coldest at 34°F and 18°F. A freezing night is all but certain in every month from October to May (a 90% chance or more). From July to August the chance of a 32°F night stays under 10% each month. In spring, June still brings one in 21% of years.

Warmth for growing adds up to 1,654 growing degree days a year (base 50°F), fewer than at 76% of the 68 Montana stations. 25% of them (419) have built up by the end of June, and July adds the most, 522. Precipitation totals 13.7 inches a year, most in June (1.7") and least in August (0.8"); May to September bring 42% of it.

The nearest other station, Western Ag Rsch Ctr, MT, 8 miles north of Hamilton, has a last frost 2 days later (May 23), a first frost 1 day earlier (September 21), a season 4 days shorter.

The first measurable snow (0.1 inch or more) in Hamilton usually arrives around November 21, and the last of the season around March 29. Those are the middle dates across the winters of 1991–2020 at Hamilton, MT, 2.4 miles away: half of years saw snow earlier, half later. Snow fell in 90% of those winters. The last 32°F night of spring typically comes 53 days after the last snow.

ZIP 59840 is 2 miles from Hamilton, MT, which stands at 3,585 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 59840 in zone 5b, the band where the coldest night of the year averaged -15 to -10°F (-26.1 to -23.3°C) over 1991–2020; a zone describes winter lows, not frost dates.