The climate of Tornio is cold continental, with freezing winters and mild summers.
The city is located in central-northern Finland, in the northernmost part of the Gulf of Bothnia (known as Perämeri), which can freeze in winter. Beyond the Swedish border is the twin city of Haparanda.
It is true that we are at the 65th parallel, therefore just south of the Arctic Circle, but due to atmospheric refraction, one can observe the sun at midnight from 19 to 25 June. Moreover, the white nights, when the sun sets but it is not completely dark even at midnight, occur from April 23rd to August 20th.
The average temperature of the coldest month (January) is of -9.9 °C (14 °F), that of the warmest month (July) is of 16.6 °C (62 °F). The lowest record is -41.7 °C (29.4 °F), the highest is 33.6 °C (92 °F). Here are the average temperatures.

Precipitation amounts to 625 millimeters (24.6 inches) per year: so, it is at an intermediate level. It ranges from 35 mm (1.4 in) in the driest months (March, April) to 65 mm (2.6 in) in the wettest ones (September, November). Here is the average precipitation.

The sea temperature ranges between -1 °C (30 °F) and 15 °C (59 °F). So, it is cold all year round. It is useful to recall that sea water freezes at about -2 °C (28.5 °F). Here are the average sea temperatures.

Best Time

The best time to visit Tornio is the summer, from June to August, since it is the mildest season. However, you must take into account some rainy days and a bit of cold at night.