December in Lahti averages a daily high of -1.4°C with nights around -6.1°C. Expect roughly 17.3 wet days totalling 51.5mm. A hard month to visit for comfort alone.
Across the year, the best time to visit Lahti is June–August.
Based on HEINOLA ASEMANTAUS, FI (32km), 1995–2024. 30 years of December data.