Winters in Upplands Vasby are cold and dark, with temperatures often below freezing, averaging around -3°C in January and February. The coldest months tend to be December and February, with lows dipping as low as -19°C. Spring arrives slowly, with March being a transition month, bringing warmer days but still chilly nights. Summer is mild, with July being the warmest month, averaging 20°C, while autumn brings temperatures back down, with September and October averaging around 12°C and 9°C respectively.