Weather in Boise in December

The month to avoid. December is the month most people should skip in Boise. Highs sit at 4.2 °C and the day is short; this is an indoors-with-excursions month, not an outdoors-all-day one.

December ranks 12 of 12 — near the bottom of Boise's year. September is the month that works, and the swing between the two is 67 points, which is a different holiday rather than a slightly better one.

Rain on 12 days, 1 of them heavy. Enough to need a plan B, not enough to need a plan around it. Layers, and a genuinely waterproof outer — at 4.2 °C the difference between a good day and a miserable one is entirely down to what you are wearing.

What the averages hide

Averages flatten a month, and this one hides a few things: 24 nights drop below freezing, so anything left outside stays frozen. Across the twenty years measured, the hottest December day reached 16.1 °C and the coldest night fell to -19.2 °C, and the wettest single day dropped 28 mm — roughly 43% of the month's rain in one day.

December is statistically steady but not small: the year-to-year variation is modest in relative terms, yet the wettest December on record still delivered 122 mm against 22 mm in the driest. In a month this wet, even a tame spread is a lot of rain in absolute terms — the 65 mm average is a reasonable planning figure, not a promise.

Wind is light at 7.8 km/h, so the temperature on the page is close to the temperature on your skin, and humidity runs at 78%, which at 4.2 °C means damp rather than muggy — the kind of cold that gets into a coat and stays there.

How December sits in the year

Temperatures hold steady either side: 10.5 °C in November, 4.2 here, 4.2 in January. Nothing about the timing within the month matters much.

Daylight in December

December sits at the dark end of Boise's year — 9 hours of daylight against 15.4 at midsummer, holding steady all month. Sightseeing days are genuinely shorter, and an itinerary built for summer will not fit.

An inland climate

With no coast nearby, the year swings a fair way — 38.7 °C between the warmest afternoons and the coldest nights, wider than any coastal city at this latitude would manage. At 823 m the air is thin enough to matter — sun that burns faster than the temperature suggests, and nights that cool off quickly.

How busy it gets

Off season. The genuine off season: cheapest by a distance, with the caveat that some tours and restaurants close entirely.

Crowd levels are estimated from climate desirability and school-holiday calendars, not from visitor numbers.

December in Boise versus nearby

If Boise is one option among several, the nearby comparison is worth making. Tri-Cities is a better bet in December — 2.5 °C warmer, 2 days less rain. Spokane is close enough to be a coin toss — 2.7 °C cooler, 2 days more rain.

So: December is a compromise. September is 67 points better and is what Boise is actually like at its best — worth moving the trip for if that is possible, and worth adjusting expectations for if it is not.

Every month in Boise

See all twelve months in Boise, or where else to go in December.