Winters in Boulogne-Billancourt are cool and mild, with temperatures rarely dropping below freezing. The coldest months are January and February, with average highs around 4-6°C. Spring arrives in March, bringing warmer temperatures that can reach up to 25°C by April and May. Summer is warm but not hot, with July being the warmest month at an average high of 19°C. Autumn is cool again, with temperatures dropping back down into single digits by October and November.