San Bruno Mountain's ridgeline runs in an east-west configuration, with a considerable area of slopes exceeding 50%, and elevations ranging from 250 feet to 1,314 feet at the summit. The majority of the mountain is found to be bedrock, which is composed of a shale and sandstone mixture, known commonly as greywacke.

The parks' principal resources include 14 species of rare or endangered plant life, as well as host and nectar plants of endangered butterflies. The endangered or threatened butterflies, (San Bruno Elfin, Mission Blue, Callippe Silverspot, and Bay Checkerspot) are found in only a few other places in the world. Another threatened species, the San Francisco Tree Lupin Moth was known to inhabit the area, but urban development destroyed this population. A Habitat Conservation Plan, adopted in the mid-1980's, now ensures the protection of the endangered species and their habitats on the Mountain.

Birds are quite common, especially raptors, and rodent populations are diverse and healthy. Coastal scrub is the only plant community seen in any abundant representation, although there are good examples of coastal strand, oak woodland, chaparral, and riparian habitats.

The relative isolation of San Bruno Mountain has resulted in the creation of unique biotic conditions, especially in the botanical area. The mountain contains a diversity of microenvironments with coastal scrub and grassland communities being the most common. Magnificent wildflower displays are found in on the Mountain in the spring. Some rare and/or endangered plants include:

  • Coast Rock Cress (Arabis blepharophylla)
  • Montara Manzanita (Arctostaphylos montaraensis)
  • Pacifica Manzanita (Arctostaphylos pacifica)
  • San Bruno Mountain (ManzanitaArctostaphylos imbricata)
  • Franciscan Wallflower (Erysimum franciscanum)
  • San Francisco Owl's Clover (Orthocarpus floribundus)
  • San Francisco Campion (Silene verecunda).

The overall temperature is affected by the mountain's exposure to the Pacific Ocean, with dominant westerly winds in the summer bringing moisture-laden fog, while southerly winds carry winter storms. Wind speeds along the ridges can be a dominating experience during a visit to the mountain, with speeds common at 30 miles per hour. Fog is a year-around characteristic, and average rainfall is between 20-25 inches per season. Temperatures are moderate, ranging from lows in the upper 40's to highs approaching 80° F. Visitors are encouraged to wear layered clothing for sudden climatic changes.

Prepare for a safe and enjoyable visit to San Bruno Mountain Park and other San Mateo County Parks by being aware of your natural environment.