Air draft is the distance from the surface of the water to the highest point on a vessel, similar to the "deep draft" of a vessel which is measured from the surface of the water to the deepest part of the hull below the surface, but "air draft" is expressed as a height, not a depth.

The vessel's clearance is the distance in excess of the air draft that allows a vessel to pass safely under a bridge

Bridge

A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...

 or obstacle such as power lines, etc. A bridge's clearance is most often noted on charts as measured from the surface of the water to the under side of the bridge at Mean Highest High Water (MHHW) which is the most restrictive clearance. The height of the tide at any time below its highest point at MHHW will then increase the clearance under the bridge.

At several bridges, such as the Gerald Desmond Bridge

Gerald Desmond Bridge

The Gerald Desmond Bridge is a through arch bridge that carries four lanes of Ocean Boulevard from Interstate 710 in Long Beach, California, west across the Cerritos Channel to Terminal Island...

 in Long Beach, CA, NOAA has installed an "Air Gap" measuring device that accurately measures the distance from its sensor on the bridge to the water surface and can be accessed by marine pilots

Maritime pilot

A pilot is a mariner who guides ships through dangerous or congested waters, such as harbours or river mouths. With the exception of the Panama Canal, the pilot is only an advisor, as the captain remains in legal, overriding command of the vessel....

and ship's masters to aid them in making real time determination of clearance.