Air draft (or draught) is the distance from the surface of the water to the highest point on a vessel, similar to the "deep draught" of a vessel which is measured from the surface of the water to the deepest part of the hull below the surface, but "air draught" is expressed as a height, not a depth. 
The vessel's clearance is the distance in excess of the air draft which allows a vessel to pass safely under a bridge 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.
In 2014, the United States Coast Guard reported that 1.2% of the allisions it investigated in the recent past were due to vessels attempting to pass underneath structures with insufficient clearance.
At several bridges, such as the Gerald Desmond Bridge 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 and ship's masters to aid them in making real time determination of clearance.
- "Marine Safety Alert 090-14: AIR DRAFT IS CRITICAL!" (PDF) (Press release). United States Coast Guard Inspections and Compliance Directorate. 2014-09-09. Retrieved 2015-02-15.
- 2104 Connecticut Boater's Guide (PDF). State of Connecticut Department of Energy and Environmental Protection. p. 60. Retrieved 2015-02-15.