Sea fog: Clearing up a grey area

It was around lunchtime on a calm day in early April when Ilfracombe’s all-weather lifeboat crew were called to the rescue of two sailors, whose yacht had suffered engine and electrical faults. The casualties had no idea of their location because thick fog had reduced their visibility to about 10m.

An RNLI lifeboat with its fog lights on

Photo: RNLI/Nigel Millard

On the phone they simply reported that they could hear the sound of the sea on rocks, so must be close to shore.

Coxswain Andrew Bengey recalls: ‘We had to launch our D class, which is equipped with a compass and chart plotter, just to locate the all-weather lifeboat in the harbour – visibility was so poor, we would have had real difficulty finding it using the boarding boat.

‘Once out to sea, we used radar to pick our way from boat to boat along the coast, as the casualties had no VHF or clue about their location.’

Thanks to Andrew and the crew’s skill and their specialist fog training, the casualties were located and towed safely back to shore. One part of their training challenges the navigator to get a boat from harbour to harbour with blacked-out windows, using onboard AIS, chartplotter and radar.

Andrew adds: ‘Fog is my least favourite sea condition – it distorts sound and light, and makes all the familiar features of the coast look completely different. Lobster pot buoys, for example, suddenly seem huge. Very small vessels like dinghies don’t always show up on the radar, so you’ve got to be extra careful.’

What is sea fog?

Fog forms at sea when humid air is blown over a cooler region of water.

In the case of the UK and Republic of Ireland, it’s often a tropical air mass from the Azores in the Atlantic, blowing over the UK’s south-western approaches and the Irish Sea. As the air cools, it can’t hold as much water vapour and advection fog forms. Most sea fog is of the advection kind, but a small amount of inland fog can drift out to sea too. The cold sea fog that occurs along the east coast of northern England and Scotland is sometimes known regionally as haar, or, further south, as fret.

Met Office Meteorologist Emma Sharples says: ‘Anecdotally, the coast around the south west and south east of England seems to get more than its fair share of fog. There are some steep cliffs around there, which force the sea air to rise (and therefore cool) rapidly, condensing quickly. It’s common all around the UK and Republic of Ireland though, mostly in Spring and Summer when the air temperature warms up but the sea is still relatively cold.’

She adds: ‘Sea fog is tricky to predict with certainty. We measure the sea’s surface temperature with buoys, map it and compare it to the air temperature of approaching fronts, which allows us to say when fog is likely to appear. But it’s a fine art, and dramatic banks of fog can roll in without much warning on what started out as a clear day.’

How to cope with fog

Check the weather forecast throughout the day (not just the moment before you set off), especially if you notice a temperature change, and keep an ear on the shipping forecast while afloat. It’s a common misconception that fog always gets ‘burned off’ by the sun as the day progresses; sea fog can still linger well into the afternoon.

If you find yourself out at sea with dwindling visibility, RNLI Community Safety Product Manager and Ocean Sailor Pip Hare recommends:

  • When you see fog approaching, immediately do a position fix on your GPS and on your paper chart.
  • Make yourself visible and help yourself see other vessels – switch on navigation lights, make sure your radar reflector’s up, and use your radar and AIS (Automatic Identification System).
  • Nominate extra crew for lookout duty and check all are wearing lifejackets.
  • Listen on VHF Ch 16 and, in restricted waters, Ch 13. If entering a harbour, monitor ship movements on the port’s working channel.
  • At a safe speed and a steady course, head for shallower water where you won’t be in any busy shipping lanes. There, drop anchor and sit it out.
  • Use the right fog signals and listen out for other vessels’ signals. These sounds can be distorted in fog, but they’re better than nothing.
  • If you’re really worried you’re stuck in a dangerous place, use your VHF radio to call for help.

A final piece of advice if you find yourself at sea, surrounded by clammy grey fog, is to use your ears. Even low-cost radars these days can be fantastic pieces of kit, but they’re even better with the additional back-up of your own senses. Don’t be tempted to stick on the radio while you’re waiting for the fog to clear, or get distracted by conversation – keep listening out for engine noise, horns, voices and other signs of vessels nearby.


According to the World Meteorological Organization, it’s only defined as fog if water droplets in the air reduce visibility to less than 1km (0.62 miles), described as ‘very poor’ visibility in the inshore/shipping forecast.

If it’s mist or haze, visibility will be greater (about 1–2km or 0.62–1.2 miles), described as ‘poor’.

Fog signals

When fog descends, normal practices regarding speed, lookout, sounds and avoiding collision change.

Section 19 of the International Maritime Organization’s International Regulations for Preventing Collisions at Sea sums up the rules.

Find out more

Get some easily digestible RYA tips on seamanship in fog.

This article originally appeared in Offshore magazine, which is sent to RNLI Offshore members. This membership is ideal for sailors, motorboaters and everyone else who uses the sea for leisure and wants to support the RNLI's lifesavers on the water. Find out more about RNLI membership here.