Golden Gate Fog Horn Facts
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.
The original Bridge fog horns, one at mid-span and one at the south pier, remained in use for nearly 50 years. Their deep, baritone sounds guided hundreds of thousands of vessels safely through the Gate, and forewarned San Franciscans when fog was rolling in to envelop the City.
Unfortunately, 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 its one-tone beacon 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, 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 is July through October, they sound over five hours a day.
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, or each fog horn. The current fog horn pattern for the Golden Gate Bridge is as follows: simultaneously, 2 foghorns on the south tower of the Bridge sound together to make a single blast every 20 seconds, while the Bridge's mid-channel fog diaphone sounds two blasts every 40 seconds. Inbound vessel operators heading into the San Francisco Bay steer left of the south tower pier horn and right of the mid-span fog diaphone. Outbound vessels stay to the right of the mid-span fog diaphone.
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.