Mount Hamilton (California)

Mount Hamilton is a mountain in the Diablo Range in Santa Clara County, California. The mountain's peak, at 4,265 feet (1,300 m), overlooks the heavily urbanized Santa Clara Valley and is the site of Lick Observatory, the world's first permanently occupied mountain-top observatory. The asteroid 452 Hamiltonia, discovered in 1899, is named after the mountain. Golden eagle nesting sites are found on the slopes of Mount Hamilton. On clear days, Mount Tamalpais, the Santa Cruz Mountains, Monterey Bay, the Monterey Peninsula, and even Yosemite National Park are visible from the summit of the mountain.

On August 26, 1861, while working for Josiah D. Whitney on the first California Geological Survey, William H. Brewer invited local San Jose preacher (and Brewer's personal friend) Laurentine Hamilton to join his company on a trek to a nearby summit. Nearing completion of their journey, Hamilton, in good humor, bounded for the summit ahead of the rest of the men and claimed his stake. In fact, Brewer suggested the mountain be named after Hamilton, only after Whitney declined to have the mountain named after him (a different mountain was later named Mount Whitney).

The Spanish name for Mt. Hamilton was the Sierra de Santa Isabel and the highest point was originally known as Mount Isabel instead of Mount Hamilton. William Henry Brewer and his fellow geologist, Charles F. Hoffmann, did not know it already had a name, and named it Mt. Hamilton, although they did place Isabel Valley on their map to the east. The "Hotel Santa Ysabel" was built on the road up the mountain in 1885 on Smith Creek. When in 1895, the USGS realized that the peak two miles southeast of Mt. Hamilton was as tall (4,193 ft or 1,278 m), they named it Mt. Isabel.

These mountains are high enough to receive snowfall in the winter, perhaps up to a dozen times. Occasionally, when a cold, wet storm comes in from the Gulf of Alaska or Canada, Mt. Hamilton and the surrounding peaks get significant snowfall. In February 2001, 30 inches (76 cm) of snow fell, and in March 2006, the peak was left with over a foot (30 cm) of snow in one night.

The National Weather Service has had a cooperative weather station on the summit of Mount Hamilton almost since the time that the Lick Observatory opened. It has provided a glimpse of the extreme weather conditions that occur on the Diablo Range, especially in the winter months.

February is the coldest month on average on Mount Hamilton with an average high of 48.7 °F (9.3 °C) and an average low of 37.8 °F (3.2 °C). The warmest month on average is July with an average high of 78.7 °F (25.9 °C) and an average low of 65.1 °F (18.4 °C). Due to frequent thermal inversions during the summer, it is often warmer on Mount Hamilton than in San Jose. The record high temperature of 103 °F (39 °C) was on August 5, 1978. The record low temperature of 7 °F (−14 °C) was on December 21, 1990. The average days with highs of 90 °F (32 °C) or higher is 2.8 days. The average days with lows of 32 °F (0 °C) or lower is 44.9 days.

Annual precipitation averages 23.71 inches (602 mm). Measurable precipitation occurs on an average of 78.2 days each year. The most precipitation in a month was 21.55 inches (547 mm) in December 1955; no rainfall has been common during the summer months. The maximum rainfall in 24 hours was 6.87 inches (174 mm) on December 23, 1955.

Annual snowfall averages 18.1 inches (46 cm). The maximum snowfall in a year was 59.0 inches (150 cm) in 1955. The maximum snowfall in a month was 33.6 inches (85 cm) in January 1950. The 24-hour maximum snowfall of 16.0 inches (41 cm) occurred on April 10, 1965. The deepest daily snow depth was 24 inches (61 cm) in December 1970. Measurable snow has been recorded in every month from October through May.