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What is air gap?

The NOAA Air Gap system is a tool that measures the vertical clearance between a defined reference point under a bridge and the surface of the water below.

With an air draft of 181 feet, the Gunvor Maersk container ship transits under the Chesapeake Bay Bridge in Maryland at low tide on February 21, 2021. For this transit, NOAA's air gap sensor measured the Bay Bridge clearance (the distance from the water to the underside of the bridge) at 186 feet.

Air gap observations are collected at a high frequency and updated for the public every six minutes to account for changes in water level, vehicular loads on the bridge, air temperature, and other factors. Air gap data and surrounding tides, currents, and meteorological conditions help vessel pilots safely enter and leave many seaports. This information is critical for pilots to safely navigate a ship under a bridge, especially as U.S. seaports grow and vessels continue to increase in size.