The NOAA Air Gap system is a tool that measures the clearance between the water surface and the bridge.
Air gap measurements are updated every six minutes to account for changes in water level and bridge height, due to bridge traffic, air temperature, and other factors. Data on air gap along with real-time data on water conditions like tides, currents, and winds, help ship captains safely enter and leave U.S. ports. This information is critical for a ship captain to safely navigate a ship under a bridge.
For more information:
Center for Operational Oceanographic Products and Services
Physical Oceanographic Real Time System
Ship Under a Bridge
NOAA's Air Gap Technology Sends USS New York Down the Mississippi River for Sea Trials
Original article: What is air gap?