Västerås experiences a humid continental climate with cold winters and warm summers.

Summers tend to be quite unpredictable with sunny spells but with a risk of sudden showers. The sunniest weather usually occurs when high-pressure systems are blocking the low-pressure systems that usually move in from the Atlantic Ocean. Daytime temperatures in July mostly hover around 22 °C (72 °F), but may sometimes exceed 25 °C (77 °F) and occasionally even 30 °C (86 °F).

Winters are usually cold with a snow cover that lasts for several months. Some winters can be mild with longer spells without snow on the ground. The weather differs a lot whether the air masses are coming from the Atlantic Ocean or from the Eurasian continent. In the first case, temperatures over 5 °C (41 °F) might be expected. In the second case, the temperature may not rise above −15 °C (5 °F) in the middle of the day. Lake Mälaren is usually frozen from December until the end of March.