Winters in Diamond Bar are mild, with temperatures rarely dipping below freezing and averaging around 12°C. Spring brings gradual warming, with April and May seeing average highs reaching the mid-20s to low 30s. Summer months are warm to hot, with July being the hottest month, but even then, nighttime lows remain relatively comfortable. Autumn is similar to spring, with temperatures cooling gradually over September and October before dropping back down in November and December, which can be quite cool.