Tromsø experiences a subarctic climate as winter temperatures are just cold enough to qualify and the summer season is short. However, precipitation amount and pattern, with maximum precipitation in autumn and early winter, as well as lack of permafrost, are atypical for subarctic areas. Due to the ice-free Norwegian Sea and the westerlies bringing the mild air ashore, winter temperatures in Tromsø are moderated and extremely mild for the latitude.
Tromsø has reputation of accumulating a lot of snow in winter, but on the streets of the city ice often prevails, especially in the first half of the winter. Tromsø’s snowfall pattern is quite erratic and varies substantially between different winters. Thaws with rain in the polar night mid-winter are not uncommon, which melts or wets existing snow, often followed by chilly windy Arctic blasts, creating dangerous ice driving and walking conditions. It is common to see Tromsø inhabitants walking with spikes in their shoes and almost all cars use studded tires. The all-time record for snow depth was set on 29 April 1997, when the meteorological station on top of Tromsøya recorded 240 centimetres (94.5 in) of snow on the ground. In an average winter, Tromsø sees 160 days with at least 25 centimetres (10 in) of snow on the ground (based on 1970–2000 average and recorded at the meteorological office station on top of the island, 100 meters above sea level).
The lowest temperature ever recorded is −18.4 °C (−1.1 °F) in February 1966. That is extremely mild for a location this far north, as it is about the same as the record cold for the entire state of Florida—about 40 degrees latitude further south. At the airport the all-time low is −20.1 °C (−4.2 °F) in February 1985. Despite being a full 10 degrees further north than the Norwegian capital, Oslo, winter temperatures are very similar. The “midnight sun” is above the horizon from 19 May to 27 July (71 days), and the period with continuous night lasts a bit shorter, polar night from 28 November to 14 January (48 days).
Summers are rather cool, sometimes cloudy and rainy, and sometimes sunny, often with large variations from year to year and from month to month, depending on the weather pattern. However, for its latitude its summers are fairly warm, especially for a maritime location. Tromsø has recorded the sunniest month ever in Norway with 430 sunhours in July 1980, and also the sunniest spring month with 381 sunhours in May 2013. The highest temperature ever recorded at the met office 100 m amsl is 30.2 °C (86.4 °F) in July 1972. Outside the city, large areas in the municipality are at some altitude and above the treeline with an alpine tundra climate (ET). The islands to the west at the outer seabord are milder in winter like Måsvik, making this part of the municipality a subpolar oceanic climate (Cfc) zone.
Recent years have seen warming. There have been no overnight air frost in June since 1997, and the recent normal period 1991-2020 shows that the part of the city at low altitude has winters so mild that melting of snow is more likely also in mid-winter; Tromsø-Holt (20 m) coldest month (February) mean is (−2.4 °C (27.7 °F)).