Fog Horns
Fog and the Golden Gate Bridge
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.
To aid in the safe travel of vessels as they pass under the Golden Gate Bridge, foghorns have been mounted on the Golden Gate Bridge since its opening in 1937. The foghorns are located in two distinct locations: at the middle of the Bridge (mid-span) and at the south (San Francisco) tower pier.
The Golden Gate Bridge foghorns 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 foghorns 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.
United States Coast Guard, Waterways Management Branch sets the tones and timing pattern of the foghorns. Each foghorn has a different pitch and marine navigational charts give ships the frequency, or signature, of each foghorn.
When the fog rolls in under the Bridge roadway limiting visibility for passing ships, the foghorns are manually turned on (and off) by Bridge workers.
Ship operators heading into the San Francisco Bay steer left of the south (San Francisco) tower pier foghorn and right of the mid-span horn. Outbound vessels stay to the right of the mid-span horn. To read about the setting of foghorns, visit www.uscg.mil/d11/dp/dpw/BRIDGE/NavSignals.htm.
Fog Horns at Mid-Span
At mid-span on the Golden Gate Bridge, there are three foghorns mounted below the roadway level. At mid-span on the Golden Gate Bridge, there are three foghorns mounted below the roadway level. One is on the east side of the Bridge, facing 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 that face 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. The sequence of the two blasts is: 9 second pause starts the sequence, followed by a 1 second foghorn blast, then 2 seconds foghorns are off, then 1 second foghorns blast, then 36 seconds foghorns are off, then 1 second foghorns 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 foghorns are turned on. The mid-span horns have a two-toned sound and have a higher sound than the south tower pier foghorns. The higher of the two tones blasts to the east and the lower of the two tones blasts to the west.
In the late 1970's, the two-tone foghorn 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. 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 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.
Foghorns at the South Tower Pier
There are two foghorns mounted on the south tower pier, about 20 feet above the waterline. 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 foghorns 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 foghorns on the south tower pier are one-tone and have a lower sound than the mid-span horns.