Guyancourt's winters are chilly, with temperatures often below freezing in January and February, but gradually warming up by March. Spring brings mild and pleasant conditions, with temperatures rising steadily through April and May, reaching averages around 12°C. Summer months of June to August are warm to hot, with highs sometimes exceeding 30°C, while September's temperatures begin to cool down again. Autumn is generally mild, with October's temperatures similar to those in May, before the winter chill returns in November and December.