Fog on Inland and Coastal Waters

Collection: Field Notes — Old Fashioned Seamanship

Subject: Fog on inland and coastal waters — radiation fog and advection fog, how each forms, how each behaves, how to predict and recognise each type, the North Sea haar, steam fog, navigating in zero visibility without electronics

On a clear September night a tidal river estuary looks perfectly navigable. By dawn it has vanished under a wall of white. The fog is not a surprise if you understand what caused it. The clear sky of the previous afternoon, the lack of wind, the wet mud of the exposed foreshore — all of it was there in advance.

Fog is the weather phenomenon that most directly threatens the safety of a small boat, and it is also one of the most predictable once you understand the two mechanisms that produce it. Tristan Gooley makes the point explicitly in The Secret World of Weather: once you recognise which type of fog you are dealing with, you know where it came from, how dense it will be, and most importantly how and when it will clear. A fog whose type you cannot identify is alarming. A fog whose type you can identify is a problem with a known solution.

There are two main types. Radiation fog forms over land. Advection fog forms over water. They look similar on the water surface but they behave entirely differently and require entirely different responses.

Radiation fog: the overnight fog that maps water

Radiation fog forms by a straightforward cooling process. During a clear night, the land surface loses heat rapidly by radiation into the sky — the same mechanism that creates dew and frost, as described in What Hills Do to Wind in the context of cold air drainage. The surface temperature drops below the dew point: the temperature at which the air becomes saturated and water vapour condenses into tiny suspended droplets. The condensed droplets scatter light and visibility drops.

Gooley identifies the preconditions clearly in The Secret World of Weather: clear sky, light or no wind, and moist air. The clear sky allows heat to radiate out unimpeded. Wind would mix the cooling surface air with warmer air above and prevent the surface temperature from reaching the dew point. The moist air — typically following a period of rain or near significant bodies of water — provides the water vapour that condenses. Remove any one of these three conditions and radiation fog cannot form.

The most useful practical observation Gooley makes is this: radiation fog maps water. If the night has been clear and the ground is not unusually wet from rain, but fog is forming in patches across a landscape, those fog patches are sitting over rivers, lakes, estuaries, and flood-plain ground where water is always near the surface. Radiation fog forms thickest and earliest where evaporation has put most moisture into the air. An estuary approach that shows a line of dense fog filling the river channel while the surrounding hills are clear is telling you exactly where the water is and where the shallow tidal ground runs.

For the coastal sailor arriving at an estuary in early morning, radiation fog concentrated in the river channel while the open sea outside remains clear is a reliable indicator of the tidal state and channel position. The fog is thickest where the water is shallowest and the mudflat evaporation most intense. On a falling tide, the fog patches contract as the mud cools. On a rising tide covering warm shallow ground, they expand.

The behaviour of radiation fog: how and when it clears

Radiation fog is a warm-weather, good-weather phenomenon. It forms under clear skies and is destroyed by the sun. Gooley quotes the ancient observation — attributed to Theophrastus — that wherever there is fog, there is little or no rain. This sounds paradoxical but is physically straightforward: radiation fog forms under the anticyclonic conditions that produce settled weather. It is the signature of a clear night that will be followed by a clear warm day.

The practical consequence for passage planning is that morning radiation fog on an otherwise settled day is a temporary obstacle, not a weather warning. The sun will destroy it. The fog burns from the outside inward, thinning at the edges first, and simultaneously from the top down as the solar heating warms the air above the fog layer faster than the fog itself. The first sign of imminent clearing, as Gooley describes it, is blue sky visible directly overhead while the fog remains at the horizon. Once the sun has broken through the top, the remaining layer burns quickly.

In summer, radiation fog typically clears by 0900 to 1100. In autumn, when the sun is lower and weaker, it may persist until early afternoon. Winter radiation fog can survive all day. Simon Rowell's maxim from Weather at Sea — if there is no wind and fog is present at breakfast, expect it to clear by about 1100 — is a reasonable summer rule of thumb.

The interaction with terrain is important. Cold air drains into valleys at night, which is why valley fog is simply radiation fog concentrated in low ground. A hilltop anchorage or a high harbour sill may be above the fog layer entirely, while a valley anchorage a few hundred feet lower is buried in it. On a passage approaching a hilly coast in morning radiation fog, climbing a little to windward may reveal a sharp fog top above which the sky is entirely clear — the fog is a pool in the valleys, not a blanket over the sea.

Advection fog: the dangerous one

Advection fog forms when warm, moist air is blown over a cold surface — typically cold sea water. The cold surface chills the base of the moist air mass below the dew point, and the water vapour condenses into fog at sea level. Unlike radiation fog, advection fog forms in wind, develops during the day, and cannot be burned off by the sun.

Rowell explains the mechanism precisely in Weather at Sea: the sun warms the fog layer above, but continuously more warm, moist air is being blown in over the cold water surface from elsewhere. The sun cannot overcome the continuous supply of new vapour. The only way advection fog disperses is if the air mass itself changes — if the wind direction shifts to bring in drier, colder air that evaporates the fog rather than replenishing it. This is a front, or a shift from southwesterly Tropical Maritime air to northwesterly Polar Maritime air. It is not a morning mist that burns off by 1000. It persists for as long as the air mass persists.

The diagnostic for advection fog is: fog that develops or persists during the day, associated with an onshore breeze, on a coast exposed to warmer waters from the south or southwest when the sea surface temperature is cold — typically spring and early summer when the sea is still cold after winter. If there is wind with the fog, it is almost certainly advection fog.

The North Sea haar

The haar is the advection fog of the east coast of Scotland and northeast England — one of the most practically significant fog phenomena for British coastal sailors. It forms when Tropical Maritime air, having crossed the warmer waters of the Atlantic, moves eastward over the North Sea. The North Sea surface temperature, cold after winter, chills the base of this air mass and produces a dense fog bank that rolls onshore on any easterly or southeasterly flow.

The haar is characteristically a spring and early summer phenomenon, most common between April and June when the temperature contrast between the incoming warm Atlantic air and the cold North Sea surface is at its greatest. Inland the same air mass is perfectly clear — warm, settled, fine weather. At the coast, zero visibility. The Scottish sea fog that rolls in on an east wind while the hills are clear and dry is not a sign of deteriorating weather in the synoptic sense; it is purely a surface temperature effect at the land-sea interface.

From a passage-planning perspective, the haar has two practical characteristics that matter. First, it can arrive suddenly: a clear morning can produce a dense sea fog within a couple of hours once the east wind brings the moist Atlantic air over the cold water. The barometer gives no warning — pressure is high and steady; the synoptic situation is settled. The first sign is often a lowering of the horizon on the eastern quadrant. Second, once established, it will not clear until the wind direction changes. Waiting it out on a mooring in the hope of mid-morning clearance is futile. The haar will outlast the tide window.

Rowell notes in Weather at Sea that sea fog at Force 7 may be reduced in intensity — the stronger mixing disperses the surface layer slightly — but will not clear. Wind alone does not remove advection fog; only an air mass change does.

Steam fog: the smoking sea

Steam fog — sometimes called Arctic sea smoke — forms by the opposite mechanism to advection fog. When very cold, dry air flows over a relatively warm sea surface, water evaporates rapidly from the sea and immediately condenses in the cold air just above, producing thin wispy tendrils of fog rising from the water like smoke. It is a wintertime phenomenon associated with very cold Continental or Arctic air moving over a sea that has retained some warmth from the preceding autumn.

Gooley describes it in The Secret World of Weather as eerie but usually short-lived: because the air is dry rather than saturated, the tendrils dissolve quickly. Steam fog rarely reduces visibility to dangerous levels in open water but can be thick enough in sheltered anchorages and harbours to be disorienting. Its presence confirms that the air is very cold relative to the water — a relevant fact for hypothermia risk if anything goes wrong, even if the fog itself clears quickly.

Upslope fog: cloud from the sailor's perspective

When moist air is forced uphill by the topography, it cools as it rises and may reach the dew point before the summit, producing fog that fills the hillsides from below. From the summit, this is a cloud. From the hillside, it is fog. Gooley makes the point that the process is identical to cloud formation on a mountain windward face; it is purely a matter of perspective.

The practical relevance for coastal sailing is that a sea fog entering a sea loch or fjord may be partly upslope fog as the fog layer is lifted by the terrain and thickens against the hillsides. A coastline that appears fog-free on the approach from seaward may have dense fog on its inshore side where the rising ground has produced upslope condensation.

The frontal visibility sequence

Gooley summarises the standard visibility sequence through a frontal passage in The Secret World of Weather in terms that are directly practical for passage timing. As a warm front approaches, visibility starts good, then steadily deteriorates as the cloud base lowers and rain falls into the air below. At the warm front, visibility is poor. In the warm sector following the front, visibility may be very poor to foggy — the warm sector air is typically saturated Tropical Maritime air with low cloud base and fog or drizzle. At the cold front, visibility deteriorates again as intense rain falls. After the cold front passage, visibility clears dramatically — the cold sector Polar Maritime air that follows is clean, dry, and often offers exceptional visibility.

The implication for passage planning: timing an arrival or departure for after the cold front passage, rather than in the warm sector, gives a dramatic visibility improvement. A passage delayed by four to six hours to let the cold front through may find conditions that were previously impossible now straightforwardly safe.

Navigating in fog without electronics

The approach to navigating in fog without GPS or radar draws on several of the skills from the Traditional Navigation Techniques series. The lead line, covered in detail in The Lead Line — Depth Sounding, provides continuous depth and bottom type as a position track — the method that North Sea fishermen used as their primary navigation in exactly these conditions. A sequence of soundings at timed intervals, plotted against the chart, gives a position estimate that is independent of visibility entirely.

Sound is more useful in fog than in any other condition. The different character of surf breaking on sand versus gravel versus rock, the echo of a cliff face, the sound of harbour entrance leading marks — all carry bearing information that is normally dismissed when visibility is good. In thick radiation fog over an estuary, the sound of a tidal current over a bar is audible before the bar is visible.

Speed management in fog has one non-negotiable rule: a vessel must be able to stop within the distance of visibility. In zero visibility that is idle ahead or stopped. The Beaufort Scale is not relevant here; it is the visibility, not the wind, that governs safe speed. The tidal stream calculation becomes critical: in fog on a foul tide, the vessel is being set toward danger faster than it can perceive it.

The deepest practical skill is pre-entry pilotage planning for fog arrival. The pilot plan drawn for a night entry, described in What the Sea Does at Night, is the same plan needed for a fog entry: sequence of expected depths, expected bottom types, expected compass course legs, timed at the vessel's speed, between the last confirmed position and the harbour entrance. The difference is that in fog there are no light characteristics to confirm marks. Position must be confirmed by depth and bottom, by timed runs on compass courses, and by sound signals when close enough to hear them.

Tristan Gooley's The Secret World of Weather (Sceptre) covers radiation fog formation, dew point, the water-mapping effect of radiation fog patches, clearance behaviour, advection fog, upslope fog, steam fog, haze, and the frontal visibility sequence. Simon Rowell's Weather at Sea (Fernhurst Books) covers radiation fog and advection fog from a practical forecasting perspective, including the haar mechanism, why sea fog cannot be burned off by the sun, and the conditions for advection fog dispersion.

The navigation skills for fog passages are in the Traditional Navigation Techniques series: The Lead Line — Depth Sounding and Dead Reckoning Without Electronics. The full series index is at Weather Forecasting.