Historical weather data, all-time temperature records, and monthly climate averages for Diamond Bar, CA. Explore typical weather conditions by week for trip planning, and see how today compares to the historical record.
Diamond Bar sits in the San Gabriel Valley foothills, where summer heat builds fast and stays late — August and early September average highs of 95–97° make it one of the hotter stretches of the year. If you're here today, you're in the thick of the warm season.
Diamond Bar's all-time high of 118° on July 6, 2018 stands as a reminder of how extreme inland Southern California heat can get. On the cold end, the thermometer bottomed out at 29° on January 14, 2013 — a 89-degree spread that tells the full story of this city's range.
Summers run hot, with July and August averaging highs in the low-to-mid 90s and the warmest week of the year — late August through early September — pushing averages to 97°. Winters are mild by most standards, with December averaging 69° and January typically the wettest month of the year. Spring and fall offer the most comfortable window, with April through May highs in the upper 70s to low 80s.