Typical of the South Peninsula region of the San Francisco Bay Area, Palo Alto has a Mediterranean climate with mild, moderately wet winters and warm, dry summers. Typically, in the warmer months, as the sun goes down, the fog bank flows over the foothills to the west and covers the night sky, thus creating a blanket that helps trap the summer warmth absorbed during the day. Even so, it is rare for the overnight low temperature to exceed 60 °F (16 °C).
In January, average daily temperatures range from a low of 38.5 °F (3.6 °C) to a high of 58 °F (14 °C). In July, average temperatures range from 54.9 °F (12.7 °C) to 78.4 °F (25.8 °C). The record high temperature was 107 °F (42 °C) on June 15, 1961, and the record low temperature was 21 °F (−6 °C) on December 24, 1990. Temperatures reach 90 °F (32 °C) or higher on an average of 9.9 days. Temperatures drop to 32 °F (0 °C) or lower on an average of 16.1 days.
Due to the Santa Cruz Mountains to the west, there is a “rain shadow” in Palo Alto, resulting in an average annual rainfall of only 15.32 inches (389 mm). Measurable rainfall occurs on an average of 57 days annually. The wettest year on record was 1983 with 32.51 inches (826 mm) and the driest year was 1976 with 7.34 inches (186 mm). The most rainfall in one month was 12.43 inches (316 mm) in February 1998 and the most rainfall in one day was 3.75 inches (95 mm) on February 3, 1998. Measurable snowfall is very rare in the populated areas of Palo Alto, but 1.5 inches (38 mm) fell on January 21, 1962. A dusting of snow occasionally occurs in the highest (unpopulated) section of Palo Alto near Skyline Ridge, where the elevation reaches up to 2,812 feet.
According to the Köppen Climate Classification system, Palo Alto has a warm-summer Mediterranean climate (Csb).