# What Is an Air Gap?

> A container ship approaching a bridge has to fit through a space that keeps changing. The vessel's height above the water stays nearly fixed, yet tides, wind and river flow can raise or lower the surface beneath it. Even the bridge can...

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Byline: ARGO.net Editorial Team
Published: 2026-09-02T17:22:25+00:00
Categories: Explainer, Oceans

![A cargo ship passing beneath a suspension bridge at sunset](https://www.argo.net/wp-content/uploads/2026/09/argo-wave20-53536-pexels-24819003.jpg)

A container ship approaching a bridge has to fit through a space that keeps changing. The vessel's height above the water stays nearly fixed, yet tides, wind and river flow can raise or lower the surface beneath it. Even the bridge can move slightly as temperature and traffic loads change.

Mariners call the vessel's height its **air draft**. The [air gap](https://oceanservice.noaa.gov/facts/air-gap.html) is the measured vertical distance from the water surface to a defined point beneath the bridge. Safe passage depends on comparing those two measurements while preserving a required margin.

The calculation sounds simple, but a few feet can determine whether a large ship passes safely or waits. Modern ports use real-time sensors alongside surveyed bridge heights. Forecasts then help pilots judge the available clearance before committing a vessel to a narrow channel.

## Air gap measures the opening

An air-gap sensor normally looks downward from a bridge and measures the distance to the water. The reference point must be precisely surveyed because clearance figures are useful only when everyone knows which part of the structure defines the top of the opening.

NOAA says its observations are collected frequently and updated for the public every six minutes. The readings can account for water-level changes as well as small shifts related to bridge loading and air temperature. A pilot receives a current measurement rather than relying solely on a printed value.

The term has a different meaning from **under-keel clearance**, the space between a ship's lowest point and the bottom. A deep, tall vessel may face both limits during the same transit. Port planning has to protect the hull below and the mast, cranes or superstructure above.

## Air draft describes the ship

Air draft is measured from the waterline to the vessel's highest relevant fixed point. On a container ship, that point may be the top of the navigation equipment, a mast or the highest tier of cargo. Ballast and fuel use alter how deeply the hull sits. Cargo loading can also change the height above water.

Crews maintain vessel particulars and update the figure for the present loading condition. A ship riding lower generally has less air draft, although it then has less water beneath its keel. Operators cannot solve one clearance problem without considering the other.

Movable equipment complicates the number. Antennas or masts may be lowered on some vessels, while other structures remain fixed. The pilot and bridge team need the operational air draft, including anything that will still project upward during the transit.

A safety margin is added because measurements carry uncertainty and a vessel can move. Waves create heave along with roll and pitch. Passing traffic can generate wake, while squat may lower a vessel underway in shallow water. Each port applies procedures suited to its channel and bridge.

## Water levels move throughout the day

**Tides** are the most predictable source of change in coastal bridge clearance. High water reduces the air gap, while low water increases it. Astronomical predictions provide a useful baseline, but observed levels can depart from the prediction when weather acts on the harbor.

Persistent wind can push water toward a coast or draw it away. Low atmospheric pressure allows the surface to rise slightly and heavy river discharge can lift levels near an estuary. A pilot therefore checks observations alongside tide tables instead of treating the prediction as an exact measurement.

NOAA's [Physical Oceanographic Real-Time System](https://tidesandcurrents.noaa.gov/ports.html), known as PORTS, brings together water levels, currents and meteorological data at participating ports. Some installations include bridge-clearance sensors. The combined display helps explain whether the opening is growing or shrinking as a ship approaches.

Forecast timing also matters. A slow vessel may reach the bridge after the tide has changed appreciably. Pilots plan for the transit window and retain room for delays rather than making the decision from a single observation taken far in advance.

River and storm conditions can produce changes that last much longer than one tidal cycle. Local notices and port restrictions remain part of the decision. The pilot's judgment remains essential even when the sensor itself is operating normally.

## Bridges move as well

**Steel and concrete** expand when they warm and contract when they cool. The movement is small compared with the size of a major bridge, yet close-clearance navigation is concerned with small differences. A sensor mounted on the structure directly observes the resulting distance to the water.

Traffic loads can also deflect a long span. Engineers design bridges to accommodate movement and the measured air gap captures the opening at the sensor location. The lowest point along a vessel's path may be elsewhere. Surveys define where the published clearance applies within the marked navigation channel.

The [U.S. Coast Guard bridge program](https://www.dco.uscg.mil/Office-of-Bridge-Programs/) oversees navigation interests involving bridges across navigable waters. Regulations and permits address fixed spans as well as drawbridges, with operating rules adding waterway-specific requirements. Mariners still have to consult the information issued for the particular waterway.

Drawbridges add scheduling and communication to the problem. Federal [drawbridge regulations](https://www.ecfr.gov/current/title-33/chapter-I/subchapter-J/part-117) specify operating requirements and exceptions. A vessel that cannot clear a closed span may need to request an opening, wait for an authorized period or use another route.

## How pilots make a clearance decision

The pilot begins with the ship's verified air draft and the port's **required margin**. Current air-gap observations show the available space. The team then checks the trend, expected tide, weather and the time needed to reach the bridge.

Communication is essential because the conning officer cannot see the highest point from the bridge of a large ship. Pilots coordinate with the master and tug crews. Vessel traffic services may also support the transit. Speed and steering must remain controlled while the ship is aligned beneath the span.

A positive numerical difference alone may be insufficient. Sensor accuracy and wave motion both contribute uncertainty. The location of the lowest structural element adds another constraint. Port rules may require a larger margin for some vessel types or environmental conditions.

If the margin is too small, waiting for lower water can increase clearance. Cargo arrangements or ballast may sometimes be adjusted before the transit, provided the changes remain safe for stability and under-keel clearance. The conservative choice is to delay when the verified conditions do not meet the local requirement.

The [Maritime Administration's port program](https://www.maritime.dot.gov/ports/strongports) describes ports as connected systems rather than isolated docks. Bridge clearance sits alongside channel depth and berth capacity as a constraint. Landside connections also influence which ships can use a terminal.

## Why real-time clearance data matters

Ships have grown taller as container capacity has increased. A bridge built for earlier traffic can become the controlling height on the route to a modern terminal. Accurate observations let ports use the available opening without weakening the safety margin.

The 2021 passage of the Gunvor Maersk beneath Maryland's Chesapeake Bay Bridge illustrates the scale. NOAA reported an air draft of 181 feet and a measured bridge clearance of 186 feet at low tide. The transit depended on a gap of only a few feet beneath an enormous structure.

**Real-time instruments** do not replace seamanship. Sensors can fail as data becomes stale; conditions may also change between the instrument and the vessel. Time stamps and equipment status help pilots recognize when an electronic reading should not be trusted, while backup procedures provide another safeguard.

The greatest value comes from combining measurements. Water level explains much of the changing opening, meteorological observations reveal forces acting on the harbor and current data helps predict the ship's motion. Together they support a decision grounded in present conditions.

For anyone watching from shore, a ship beneath a bridge may appear to have ample room. From the wheelhouse, **vertical clearance** is a carefully managed quantity. Air gap tells the pilot how much space exists at that moment, while air draft and the safety margin determine whether the vessel should use it.

**Related reading:** [how a nautical mile is defined](https://www.argo.net/what-is-a-nautical-mile/) and [the difference between tide tables and tide charts](https://www.argo.net/tide-table-vs-tide-chart-what-is-the-difference/).

 **Related reading:** [how a nautical mile is defined](https://www.argo.net/what-is-a-nautical-mile/) and [the difference between tide tables and tide charts](https://www.argo.net/tide-table-vs-tide-chart-what-is-the-difference/). **Explore this topic:** [Nautical Chart vs. Map: What Is the Difference?](https://www.argo.net/nautical-chart-vs-map-what-is-the-difference/) and [What Is a Canal?](https://www.argo.net/what-is-a-canal/).