Fog Horns
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Fog horns are required at the Golden Gate Bridge for the safe travel of vessels under the span. The foghorn signals are set by United States Coast Guard, Waterways Management Branch. To read about the setting of fog horns, visit www.uscg.mil/d11/dp/dpw/BRIDGE/NavSignals.htm.
Small vessels that do not have radar still use the Bridge fog horns as guides when visibility in the Golden Gate Strait is low. Each horn has a different pitch and marine navigational charts give the frequency, or signature, of each fog horn. Vessel operators heading into the San Francisco Bay steer left of the south tower pier horn and right of the mid-span horn. Outbound vessels stay to the right of the mid-span horn. The original Golden Gate Bridge fog horns, located at mid-span and the south tower pier, with their deep, baritone sounds, have guided hundreds of thousands of vessels safely through the Golden Gate Strait, and forewarned San Franciscans when fog was rolling in to envelop the City.
The Golden Gate Bridge has an influence in directing fog as it pushes up and pours down around the Bridge. "Advection fog" forms when humid air from the Pacific Ocean swoops over the chilly California current flowing parallel to the coast. The fog hugs the ground and then the warm, moist air condenses as it moves across the bay or land. This is common near any coastline. Sometimes, high pressure squashes it close to the ground. By the way, the color of the Bridge is International Orange and was chosen in part because of its visibility in the fog.
FOG HORNS AT MID-SPAN
There are three fog horns at mid-span that are located below the roadway level. One is on the east side of the Bridge and faces east and is 24-1/2 inches long and the horn bell is 11-inches in diameter. Two are on the west side of the Bridge and face to the west and are each 36-inches long with a horn bell that is 18-inches in diameter.
The three horns at mid-span sound as two blasts, in this sequence: 9 second pause to start the sequence, followed by a 1 second horn blast, then 2 seconds horns are off, then 1 second horns blast, then 36 seconds horns are off, then 1 second horns blast, then 2 seconds horns are off, then 1 second horns blast, then 36 seconds horns are off. This pattern continues as long as the horns are turned on. The mid-span horns have a two-toned sound and have a higher sound than the south tower pier fog horns. The higher of the two tones blasts to the east and the lower of the two tones blasts to the west.
FOG HORNS AT SOUTH TOWER PIER
There are two fog horns mounted on the south tower pier, about 20 feet or so up from the water. One is on the east side and faces east, and one is on the west side and faces west. Each horn is 48-inches long with a horn bell that is 23-1/2 inches in diameter.
The two horns sound at the same time as a single blast, in this sequence: 2 seconds horns blast, 18 seconds horns are off, 2 seconds horns blast, 18 seconds horns are off. This pattern continues as long as the horns are turned on. The south tower pier fog horns have a lower sound than the mid-span horns and are one-tone.
ADDITIONAL INFORMATION
In the late 1970's, the two-tone fog horn at mid-span stopped working. One of the horn's two air valves gave way and the two-tone horn became a one-tone horn. But since the mechanism was so old, replacement parts were impossible to find. The hobbled horn continued to sound with just one-tone until 1985, when both of the original horns showed signs of wear, tear and exposure to elements. The original fog horns were replaced by new horns manufactured by the Leslie Air Horn Company. The new single-tone horns (or fog diaphones) differ in frequency or tone from each other, but operate with compressed air, just like the originals.
The fog horns operate, on average over a year, about two and a half hours a day. During March, you'll hear them for less than half an hour a day. However, during the Bay Area's foggy season, which typically occurs during the summer months, they can sound for over five hours a day or for days at a time.