Fog and Visibility Hazards
How fog forms, where it's common, and how to navigate safely when visibility drops
How Maritime Fog Forms
Fog is simply cloud at the surface — air that has cooled to its dew point, causing water vapor to condense into tiny droplets that reduce visibility to less than 1,000 meters (roughly 0.54 nautical miles; the WMO definition). Understanding which type of fog you are in determines how long it will persist and what will clear it.
Advection fog: the most common and dangerous type for sailors. Warm, moist air flows horizontally over a cooler sea surface. As the air cools to its dew point, fog forms. This type of fog can be thick, persistent (lasting days), and widespread. Classic examples: the California coast (warm Pacific air over cold upwelling water), the Grand Banks of Newfoundland (warm Gulf Stream air over cold Labrador Current water), and the U.S. East Coast in spring and early summer (warm, moist southerly flow over cold coastal water). Advection fog is not driven by local conditions — it is driven by the large-scale air mass movement and will persist until the air mass changes.
Radiation fog: forms overnight when the ground (or calm, shallow water) radiates heat to the sky and cools below the dew point. This is the classic 'morning fog' that burns off by mid-morning when solar heating warms the surface above the dew point. Radiation fog requires calm conditions — wind disperses it. Most common in enclosed harbors, bays, estuaries, and rivers. Less common on the open sea but significant in coastal areas.
Sea smoke (arctic sea smoke): forms when very cold air flows over warmer water. The warm water evaporates into the cold air, which quickly becomes saturated. Seen most dramatically in winter over the Gulf Stream or in arctic regions. Sea smoke looks like steam rising from the surface. It reduces visibility but typically not to the same degree as advection fog.
Frontal fog: thin strips of fog can form along frontal boundaries where warm and cold air masses meet. Less common and less persistent than advection fog, but can create brief localized visibility reduction.
Monitoring the temperature-dew point spread: as covered in the temperature and humidity lesson, a spread below 4°F means high fog risk; below 2°F means fog is forming or imminent. Monitoring this spread on passage is the most direct early-warning tool. When SST is lower than air temperature and a moist maritime air mass is present, advection fog is a serious risk.
On the U.S. East Coast, the summer fog pattern is predictable: warm, moist SW flow over cold nearshore water = fog, especially at night and in the morning. Check the SST chart and the forecast dew point. If the predicted dew point matches or is within 2°F of the SST along your route, budget for restricted visibility. The NOAA forecast often specifically warns for fog — read it.
Why is advection fog more persistent and operationally challenging than radiation fog?
Sea smoke (arctic sea smoke) forms when:
COLREGS Requirements in Fog
Restricted visibility triggers specific requirements under COLREGS that are mandatory — not optional. Every sailor must know these rules and apply them whenever visibility drops.
Navigation lights: all vessels must display navigation lights in restricted visibility, regardless of the time of day. This includes sailing vessels, motor vessels, anchored vessels, and vessels at anchor. The fact that it is daytime does not eliminate the requirement.
Sound signals (Rule 35): vessels must sound fog signals at the prescribed intervals (covered in detail in the COLREGS section). The key signals for recreational sailors:
- Power vessel underway and making way: one prolonged blast every 2 minutes
- Sailing vessel underway: one long + two short blasts every 2 minutes
- Vessel at anchor: rapid bell ringing (~5 seconds) every 1 minute
Safe speed (Rule 6): in restricted visibility, every vessel must proceed at a safe speed that allows collision avoidance within the available stopping distance. For most small vessels, this means significantly slower than normal passage speed. 'Safe speed' is not defined in knots — it depends on vessel maneuverability, sea state, traffic density, and the effectiveness of your detection equipment.
Proper lookout (Rule 5): in restricted visibility, a proper lookout must be maintained by 'all available means' — sight, hearing, and instruments. In fog, hearing becomes critical: listen for fog signals, engine noise, and any sound that might indicate a vessel you cannot see. A separate sound watch (not the helmsperson) is the standard on watch.
Keep clear of channels: if fog is forecast or developing and you are near a shipping channel, get out of it or well to the side of it. Commercial vessels have limited ability to maneuver quickly, and they may be traveling at full speed even in fog, trusting their radar. A sailing vessel at the center of a shipping lane in fog is in a very dangerous position.
Stopping if necessary: Rule 8 in restricted visibility allows — and may require — stopping if you hear an approaching vessel's fog signal forward of the beam and cannot determine the risk of collision. Stopping and waiting is a valid safety action. Forward progress is not always the right goal in fog.
Set a fog watch rotation with someone specifically assigned to listening — not the helmsperson. Fog absorbs sound in unusual ways: a vessel can sound as if it is off your bow when it is actually abeam. Listen in all directions, not just ahead. If you hear a ship's foghorn getting louder, stop, take bearings, and determine its direction before deciding what to do.
A sailing vessel underway in restricted visibility must sound its fog signal how often?
What is the COLREGS safe speed requirement in restricted visibility?
Navigating in Fog: Practical Techniques
Once in fog, the goal is to maintain position awareness, detect other vessels, signal your presence, and make decisions that reduce risk. These are achievable with the right tools and procedures.
Radar: the primary detection tool in fog. On a vessel with a working radar, the watch should be monitoring it continuously in restricted visibility. Key settings: use the range scale appropriate to the traffic density (3–6 nm in open water; shorter in channels). Watch for ARPA targets or manually track targets: are they getting closer? Are they on a constant bearing? If bearing is constant and distance is closing, risk of collision exists.
AIS: Automatic Identification System transmits vessel identification, position, course, and speed. Class A AIS (required on commercial vessels) is visible on an AIS receiver or plotter. Class B AIS is used by many recreational vessels. AIS supplements radar but does not replace it — small vessels, fishing boats, and many commercial vessels in some regions do not carry AIS. Never assume all contacts are visible on AIS.
Radar reflectors: small fiberglass and wooden vessels have very poor radar cross-sections — they may not show up at all on another vessel's radar. A properly mounted radar reflector (or active radar target enhancer) significantly improves detectability. Octahedral passive reflectors must be mounted in the 'catch-rain' position to be effective; active enhancers are more reliable. This is one of the most important passive safety devices aboard.
Fog navigation planning: before entering fog, record your GPS position precisely, note your water depth, identify the nearest shoals and dangers, and mark any shipping lanes or traffic separation schemes on the chart. Know exactly where you are before you lose visual references.
Anchoring in fog: if fog is thick and there is no immediate need to proceed, consider anchoring in safe water away from channels. An anchored vessel in fog is at lower risk than a moving vessel. Ring the anchor bell as required by COLREGS. This is a legitimate option, especially in tidal waters where you can anchor on a shoal, wait for the tide, and proceed when the fog lifts.
Using depth and current: in coastal fog, depth soundings can confirm your track. Following a specific depth contour or staying in a defined depth band is a time-honored fog navigation technique ('running the 20-foot curve' into a harbor). This technique requires a reliable depth sounder and chart, but it provides cross-checks on GPS position and maintains situational awareness when visual cues are absent.
The most dangerous decision in fog is continuing at speed in a shipping lane because 'it should be clear soon' or 'we're almost there.' Near-misses and collisions in fog almost always involve a vessel that was going faster than conditions warranted, in a location it shouldn't have been, without the lookout it was required to maintain. Slow down, get out of channels, sound your signals, and be patient.
What does a target on radar with a constant bearing and decreasing range indicate?
Why do AIS contacts not replace radar in fog?
Summary
Maritime fog is classified by formation mechanism: advection fog (warm air over cold water — persistent, multi-day) and radiation fog (overnight cooling — clears with morning sun). Advection fog is the serious offshore hazard. COLREGS require navigation lights, sound signals, safe speed, and a proper lookout in all restricted visibility. Radar and AIS supplement each other but neither alone is sufficient. Practical fog seamanship: slow down, leave shipping lanes, anchor if safe, use depth soundings as position checks, and never trade safety for schedule.
Key Terms
- Advection Fog
- Fog formed when warm, moist air moves horizontally over a cooler sea surface and cools to its dew point — persistent, large-scale, and driven by the air mass.
- Radiation Fog
- Fog formed by overnight radiative cooling of the surface below the dew point — typically burns off by mid-morning with solar heating.
- Sea Smoke
- Fog-like condensation formed when very cold air flows over warmer water — common over the Gulf Stream in winter and in arctic regions.
- Restricted Visibility
- Any condition — fog, rain, snow, mist — that reduces visibility. Triggers specific COLREGS requirements for lights, sound signals, safe speed, and lookout.
- Radar Reflector
- A passive or active device that increases a vessel's radar cross-section, improving its detectability on other vessels' radar in reduced visibility.
- AIS (Automatic Identification System)
- A transponder system that broadcasts a vessel's identity, position, course, and speed — used by equipped vessels to track traffic in restricted visibility.