Weather in Trondheim in September

A hard month. September is a hard month in Trondheim, and worth planning around. You should expect rain on 18 days of the month, which is more days wet than dry.

September ranks 4 of 12 in Trondheim: past the worst of the year but short of July, which is the month to book if the dates are open.

Rain on 18 days, 5 of them heavy. Enough to need a plan B, not enough to need a plan around it. Layers, and a genuinely waterproof outer — at 14.3 °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: the day barely swings — 6 °C between the overnight low and the afternoon high. Across the twenty years measured, the hottest September day reached 23.2 °C and the coldest night fell to 0.6 °C, and the wettest single day dropped 53 mm — roughly 36% of the month's rain in one day.

September is statistically steady but not small: the year-to-year variation is modest in relative terms, yet the wettest September on record still delivered 278 mm against 42 mm in the driest. In a month this wet, even a tame spread is a lot of rain in absolute terms — the 149 mm average is a reasonable planning figure, not a promise.

Wind is light at 2.5 km/h, so the temperature on the page is close to the temperature on your skin, and humidity runs at 88%, which at 14.3 °C means damp rather than muggy — the kind of cold that gets into a coat and stays there.

How September sits in the year

The month is on the way down: 4.1 °C cooler than August, with October dropping another 5.9 °C. The first week and the last are not the same trip.

Daylight in September

13.1 hours of daylight, losing 3.1 hours across the month. That is between Trondheim's extremes of 20.5 and 4.6 hours, so days are long enough not to constrain the plan.

Sea temperature

The sea sits at about 12.9 °C in September — too cold to swim without a wetsuit. This is the warmest the water gets all year.

How busy it gets

Busy. Popular without being frantic; book the things you care about, improvise the rest.

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

September in Trondheim versus nearby

If Trondheim is one option among several, the nearby comparison is worth making. Oslo is a better bet in September — within a degree on temperature, 5 days less rain. Bergen is close enough to be a coin toss — within a degree on temperature, 4 days more rain.

So: September works, with July the better month by 16 points if the dates can move. If they cannot, nothing here should stop you booking.

Every month in Trondheim

See all twelve months in Trondheim, or where else to go in September.