Winters in Thunder Bay are harsh and cold, with temperatures often dropping below -30°C in January and February. Spring is a slow process, with temperatures gradually warming up over the months of March to May. Summers are mild and pleasant, with average highs around 20-25°C from June to August. Autumn brings a return to cooler temperatures, but still relatively warm compared to winter, with September being one of the warmer months. Overall, temperature fluctuations throughout the year result in distinct seasonal patterns.