10.2 Fog Formation: Advection Fog, Radiation Fog & Sea Smoke

10.2 Fog Formation: Advection Fog, Radiation Fog & Sea Smoke

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Moisture Fundamentals & Visibility Definitions

Fog consists of microscopic water droplets or ice crystals suspended in the surface boundary layer.

International Visibility Definitions

• Fog: Visibility reduced below 1,000 metres (approx. 0.54 NM) by suspended water droplets.
• Mist: Suspended water droplets reduce visibility, but it remains above 1,000 metres.
• Haze: Reduced visibility caused by extremely small, dry particles rather than suspended water droplets.

Condensation Physics

• Dew Point ($T_d$): Temperature to which air must be cooled at constant pressure and moisture to reach 100% relative humidity.
• Relative Humidity ($RH$): Ratio of actual vapor pressure to saturation vapor pressure at ambient temperature.
• Temperature-Dewpoint Spread ($T - T_d$): When ambient air cools to within 0°C to 2°C of dew point, condensation is imminent.
• Condensation Nuclei: Microscopic marine salt particles allow droplet condensation at relative humidities as low as 95%–98%.

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Advection Fog (Sea Fog)

Advection fog—termed sea fog—is the most extensive and persistent maritime fog.

Formation Mechanism

Advection fog forms when warm, moist air moves horizontally (is advected) across a cold sea surface whose temperature ($SST$) is below the air's dew point:

1. Conductive heat loss to the cold water cools the surface air layer.
2. Turbulent mixing transfers cooling upward through the boundary layer.
3. Once air cools to its dew point, water vapour condenses into dense fog.
4. The classic source in NW Europe is a Tropical Maritime (mT) air mass from the Azores flowing over cold shelf waters.

Wind Velocity Constraints

Wind provides the horizontal transport and affects vertical mixing:

• Light to moderate wind can sustain the cooling and fog layer. Increasing wind up to about 15 knots can deepen it.
• Stronger mixing often lifts fog into low stratus, but advection fog can also persist in stronger winds. Do not use a fixed wind threshold as proof of clearance.

Persistence & Geographic Distribution

Because sea temperature changes very little diurnally, advection fog can persist through daytime sunshine and may last for days:

• Hotspots: The English Channel, Western Approaches, and Irish Sea during spring and early summer (April–June), when warm, moist air can cross still-cold shelf waters; and the Grand Banks of Newfoundland where the Gulf Stream meets the Labrador Current.
• Clearance: Changes in air mass, surface heating or mixing can disperse it; confirm actual visibility rather than assuming a wind change has cleared the route.

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Radiation Fog (Land Fog)

Radiation fog forms exclusively over land and affects mariners in harbours, estuaries, and rivers.

Formation Mechanism

On clear, calm nights with high surface humidity:

1. The ground emits longwave infrared radiation directly to space, cooling rapidly.
2. Contact air is chilled below its dew point.
3. A light breeze of 2 to 5 knots provides gentle mixing across a shallow layer (10–30 m deep). In calm air, dew or frost may form, or fog may remain very shallow.

Coastal Behavior & Clearing

Radiation fog cannot form over the open sea because water's high heat capacity prevents rapid nocturnal surface cooling. However, dense radiation fog drains down valleys into coastal river mouths, bars, and harbours around dawn:

• Clearing: Often disperses after sunrise as the ground warms, or with increased mixing. Weak winter sunshine, persistent cloud or sheltered terrain can allow it to remain much longer.

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Sea Smoke (Steam Fog / Arctic Sea Smoke)

Sea smoke forms under thermal conditions opposite to advection fog:

• Mechanism: Extremely cold, dry continental or arctic air (typically below -10°C) blows over comparatively warm sea water (+4°C to +8°C).
• Evaporation & Condensation: Intense evaporation from the warm sea saturates the thin contact layer. As warm vapor ascends into freezing air, it immediately condenses into rising, swirling steam plumes.
• Characteristics: Very shallow (10–30 feet high) and turbulent. Mastheads may remain in sunshine while the deck is blinded. Common in high-latitude fjords and the Baltic in midwinter.

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Navigation & Seamanship in Restricted Visibility

Navigating in fog demands strict compliance with Collision Regulations:

COLREGs Rule 19 Compliance

• Safe Speed (Rule 19b & Rule 6): Choose a safe speed using visibility, traffic, stopping and turning ability, sea conditions and the other Rule 6 factors; power-driven vessels keep engines ready for immediate manoeuvre.
• Radar Action (Rule 19d): When choosing a course alteration for a radar-only target, avoid, so far as possible, turning to port for a vessel forward of the beam unless overtaking, or towards a vessel abeam or abaft the beam.
• Rule 19(e): Unless no collision risk has been established, hearing a fog signal apparently forward of the beam requires minimum steerage speed, taking all way off if necessary, and extreme caution until danger is over.

Sound Signals (Rule 35)

Whistle signals below recur within 2 minutes; ordinary anchor bell signals recur within 1minute. Fishing at anchor and RAM vessels working at anchor use one prolonged and two short blasts:

• Power making way: 1 prolonged blast (4–6s).
• Power stopped: 2 prolonged blasts separated by 2 seconds.
• Sailing, RAM, NUC, Fishing, Towing: 1 prolonged + 2 short blasts.
• Ordinary anchor signal: Rapid bell ringing for about 5 seconds every 1minute; add a gong aft on vessels≥100 m. Vessels under 20 m may instead make an efficient sound signal at intervals not exceeding 2 minutes.

Bridge Safety Protocols

1. Maintain an effective visual/auditory lookout; use additional safe positions where they improve sight and hearing.
2. Turn on navigation lights (Rule 20) and hoist an efficient radar reflector.
3. Maintain systematic radar plotting (VRM/EBL) and monitor AIS targets.
4. If revising a route or leaving a traffic lane, obey Rule 10 and check charted depths, hazards and other traffic; shallow water is not an automatic refuge.

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Marine Fog Types Comparison Matrix