Module 13 - Meteorology
Sea Fog
What you will learn
Recognise an advection-fog set-up, identify corroborating evidence and prepare restricted-visibility options before the visual horizon closes.
Sea fog commonly forms when warm moist air moves over colder water and cools to saturation. Wind can transport and sustain it rather than clearing it.
Coastal sea fog is commonly advection fog: relatively warm moist air passes over a cooler sea surface and is cooled to its dew point. Around the UK this can affect windward coasts when the airflow crosses colder water.
The location, movement and persistence depend on wind direction and strength, sea and land temperature, moisture and topography. A small temperature–dew-point spread or low stratus supports concern but does not guarantee fog at the yacht.
Use official visibility forecasts, warnings and nearby observations alongside onboard evidence. Before entry, prepare the restricted-visibility plan: safe speed, engines as appropriate, navigation lights, sound signals, radar and AIS used within their limits, extra lookout, position evidence and an abort or holding option.
| Evidence | What it supports | Limit |
|---|---|---|
| Warm moist air over colder sea | Advection-fog set-up | Does not fix exact location or onset |
| Nearby visibility worsening | Fog or low cloud approaching the route | Station may not represent the yacht |
| Low stratus over water | Low-level saturation nearby | Can lift, lower or remain local |
| Wind | Transport and modification | Does not guarantee clearing |
Act before visibility collapses
Warm moist southwesterly air is moving across cold coastal water. Nearby reports deteriorate from moderate to poor, and low stratus appears over the headland before a narrow entrance.
1. Recognise the set-up
The airflow and colder surface support advection-fog risk.
2. Seek corroboration
Check the latest visibility forecast, warnings, station reports and observed trend.
3. Prepare early
Brief lookout, safe speed, lights, signals, radar/AIS limits and position method.
4. Preserve an option
Choose whether to hold, divert or avoid the entrance before visual references disappear.
Sense check: A breeze may feed and move advection fog; it is not safe to assume that wind will clear it.
Common mistake or limitation
- Assuming fog requires calm wind; advection fog can persist while moist air is transported.
- Treating a narrow dew-point spread as proof that fog must form at a specific time and place.
- Waiting until visibility is lost before preparing restricted-visibility procedures or an alternative.
Recap
- Sea fog is commonly warm moist air cooled over colder water.
- Use several current indicators; no single cue guarantees onset.
- Prepare restricted-visibility actions and an alternative before entry.
Optional quick check
Section 11 of 15
Which condition most directly supports advection sea-fog formation?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Meteorology
This public lesson is part of Revision Module 13. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.