Trondheim, where it's located

The climate of Trondheim is moderately continental, with cold winters, during which the average temperature is a few degrees below freezing, and mild summers.
The city is located inside a large and deep fjord that flows into the west coast, and therefore has a slightly colder and less rainy winter than the coast directly exposed to the Atlantic.
The average temperature of the coldest month (January) is of -1.9 °C (29 °F), that of the warmest month (July) is of 15 °C (59 °F). Here are the average temperatures.
Trondheim - Average temperatures
Min (°C)-5-5-2158111073-1-4
Max (°C)12481316191814942
Min (°F)232328344146525045373025
Max (°F)343639465561666457483936

Precipitation amounts to 875 millimeters (34.4 inches) per year: so, it is at an intermediate level. It ranges from 45 mm (1.8 in) in the driest month (April) to 90 mm (3.5 in) in the wettest ones (July, August, September). Here is the average precipitation.
Trondheim - Average precipitation


Best Time

The best time to visit Trondheim 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.