Weather in Boston in December

The month to avoid. Of the twelve, December is the one to avoid. Highs sit at 4.1 °C and the day is short; this is an indoors-with-excursions month, not an outdoors-all-day one.

December ranks 11 of 12 — near the bottom of Boston's year. September is the month that works, and the swing between the two is 60 points, which is a different holiday rather than a slightly better one.

Rain on 11 days, 4 of them heavy. Enough to need a plan B, not enough to need a plan around it. Layers, and a genuinely waterproof outer — at 4.1 °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 20 °C and the coldest night fell to -19.5 °C, and the wettest single day dropped 62 mm — roughly 53% 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 185 mm against 58 mm in the driest. In a month this wet, even a tame spread is a lot of rain in absolute terms — the 116 mm average is a reasonable planning figure, not a promise.

Wind is light at 2.8 km/h, so the temperature on the page is close to the temperature on your skin, and humidity runs at 86%, which at 4.1 °C means damp rather than muggy — the kind of cold that gets into a coat and stays there.

How December sits in the year

The month is on the way down: 6.1 °C cooler than November, with January dropping another 3.3 °C. The first week and the last are not the same trip.

Daylight in December

December sits at the dark end of Boston's year — 9.1 hours of daylight against 15.3 at midsummer, holding steady all month. Sightseeing days are genuinely shorter, and an itinerary built for summer will not fit.

Sea temperature

The sea sits at about 8.2 °C in December — too cold to swim without a wetsuit. The sea peaks at 19.7 °C in August and bottoms out at 4.6 °C — water lags the air by a month or two, which is why the best swimming often comes after the best weather.

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 Boston versus nearby

If Boston is one option among several, the nearby comparison is worth making. New York City is a better bet in December — 2.8 °C warmer, 1 day less rain. Bar Harbor is close enough to be a coin toss — within a degree on temperature, 3 days more rain.

So: December is a compromise. September is 59 points better and is what Boston 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 Boston

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