There is a particular quiet that arrives with sea fog. The gulls sound muffled, voices carry oddly, and the headland you could see ten minutes ago has simply gone. On the British coast this is not a rare curiosity — it is a routine part of spring and summer weather, and it catches people out every year.

Fog is the one weather hazard that works mainly by removing information. You can dress for rain and brace against wind, but fog takes away the landmarks you were using to judge where you are and what is heading towards you. Understand how it forms and it becomes far less mysterious. Build a few habits and it becomes far less dangerous.

What sea fog actually is

Fog is cloud at ground level: countless tiny water droplets suspended in the air, too small to fall as rain but numerous enough to scatter light. Forecasters draw the line by visibility — mist when you can still see roughly a kilometre or more, fog when it drops below that. Thick coastal fog can cut visibility to a few tens of metres, which on a cliff path or in a shipping channel is very little indeed.

Fog needs two ingredients: moisture in the air, and a way of cooling that air until it can no longer hold the moisture as invisible vapour. That threshold is the dew point. Around the coast, the sea is usually the thing doing the cooling.

Advection fog: why the sea makes its own weather

Most British sea fog is advection fog. Advection simply means air moving from one place to another — here, warm damp air sliding over a colder sea surface.

The sequence is easy to picture:

1. Air picks up moisture, often after crossing warmer land or a warmer stretch of sea.
2. Wind carries it out over water that is colder than the air.
3. The sea chills the lowest layer of air down to its dew point.
4. Water vapour condenses into droplets, and gentle turbulence mixes the fog upwards into a layer that can be tens or hundreds of metres deep.

That is why the classic setup is a light to moderate onshore wind, mild humid air, and a sea that has not caught up with the land. British seas are at their coldest relative to the air in late spring and early summer: inland, May and June can be warm and bright while the water a few miles offshore is still in the low teens or colder. Send a warm southerly or easterly flow across that and you get fog banks sitting stubbornly along the coast while the rest of the country basks.

Because it depends on the wind, sea fog can be persistent. Radiation fog over land — the kind that forms on calm, clear nights and burns off after breakfast — behaves quite differently. Sea fog can hang on for days while the wind keeps feeding it.

Haar, fret and other local names

The regional names for sea fog tell you where it is felt most.

• Haar — the east coast of Scotland and north-east England, especially the Firth of Forth and the Angus and Aberdeenshire coast, driven in off the North Sea by easterly winds.
• Sea fret, or just fret — Yorkshire, Teesside, County Durham and Northumberland. The word is used constantly in places such as Whitby and Scarborough.
• Sea fog or sea mist — the general term, common in the south-west, Wales and around the Irish Sea.

The vocabulary changes with the postcode; the mechanism does not. On the east coast the North Sea is the cold surface, and haar often arrives on an easterly or north-easterly wind in spring and summer, sometimes as a distinct wall of white with blue sky still visible behind it. If you have stood on a seafront in warm sunshine and watched it vanish in twenty minutes, you have met haar.

Out and about: walking, beaches and the coast road

The hazards on a foggy coast are usually the ones you would normally manage by looking.

On foot

• Work out which way is inland before you set out — a bearing, a fence line, the nearest road or village — and carry a paper map and compass or an offline map on a charged phone. Landmarks are exactly what fog removes.
• Stay well back from cliff tops. Fog hides both the edge and any eroded overhang below it, and sound is a poor guide: waves echo off cliffs far beneath the path.
• Check tide times before you go and again on the day. Fog conceals rising water and how quickly a causeway or a sandy bay can be cut off.
• Walk with someone else where you can, and agree a turnaround time rather than just a destination.
• Dress for damp cold. A still foggy headland feels much colder than the forecast figure suggests, so take an extra layer and a torch.
• If you lose confidence in your position, stop. Retracing your steps is almost always better than pressing on and hoping the path reappears.

On the beach

Fog does not block ultraviolet light, so it is entirely possible to get sunburnt on a beach you cannot see the end of. Keep an eye on the tide line and know where the nearest steps or slipway are. Children and dogs can move out of sight in seconds, and shouting carries badly in fog — pick a meeting point and agree it before anyone wanders.

On the road

On a coastal route, dipped headlights are the default. Fog lights are for when visibility is seriously reduced — around 100 metres or less — and should be switched off as soon as it improves, because they dazzle other drivers. Slow down, leave a bigger gap, and use the near-side verge or lane markings to guide you rather than staring at the tail lights in front.

Afloat: fog at sea

At sea, fog is a collision risk before it is a navigation problem. Large ships do not stop, and a small boat's radar signature can be faint. The general rules hold whatever you are in:

• Fix your position before the fog closes in. Afterwards you are working from memory.
• Reduce speed to a pace where you can stop within the distance you can see.
• Make the sound signals for your vessel type — a power-driven vessel under way sounds one prolonged blast at intervals — and listen. In fog, sound is often the first warning of another vessel.
• Post a lookout forward if you have crew, and quieten the engine or the music so you can hear.
• In busy shipping lanes, turn away and get clear rather than crossing blind. If in doubt, head for a known safe haven or anchor well clear of the channel.
• Kayakers and paddleboarders: fog removes the shoreline reference that keeps you oriented. Carry a compass, a whistle and a waterproof means of calling for help, and stay close to shore.

If you are new to navigating in poor visibility, a recognised training course is money well spent. Fog is not the moment to be learning the rules.

Reading the signs before it arrives

Sea fog seldom arrives without any warning. The forecast usually flags it, and the air often gives you clues first.

• Air and sea temperature. Warm, humid air over a colder sea, with any onshore wind, is the setup. If the water is cold for the time of year, be suspicious.
• The dew point. When air temperature and dew point are within a degree or two of each other, the air is close to saturation and only a little cooling is needed.
• Wind direction. Onshore or alongshore winds bring fog to you; a shift offshore usually clears it.
• Forecast wording. Inshore waters and shipping forecasts include visibility. Check the coast, not the nearest inland town, which may be in sunshine all day.
• What you can see. Fog banks often show as a soft grey smudge on the horizon or a bank with a clean upper edge. When the horizon starts to lose its sharpness, believe it.

The five-minute fog check

Before you leave, do four things:

1. Check the visibility and wind direction in the forecast for the coast, not the county town.
2. Check the tide times and note the height.
3. Work out which way is inland and where the nearest road, village or harbour is.
4. Tell someone your route and when you expect to be back.

If the fog arrives while you are out, slow down, stop relying on landmarks alone, keep everyone close, and switch on your lights. Fog is rarely what hurts people on the coast. Being caught without a plan is.

Photo: 13156007 / Pixabay