At sea, fog can be summed up as condensed water vapour or just thick cloud on the surface. Sea fog, or advection fog, forms when relatively warm moist air moves over colder water and cools to its dew point temperature, causing the air to saturate.

Unlike land fog, or radiation fog, sea fog can occur at any time of day and still exist with quite strong winds. It only really clears with a change of air mass – usually with the passage of a cold front. Without the passage of a cold front, it can last for days.

While radiation fog usually occurs on cold, still winter days, sea fog or advection fog is more prevalent in early summer. June can be a particularly bad month in the English Channel, when the water is still relatively cold, with warm moist air coming up from the south-west.

These Sat24.com website images reveal that what appears to be cloud in the satellite view…

Looking out for the forecasts of fog is important, but one simple trick is to go online and use satellite imagery. Sat24.com is a great website to see the last three hours of visual imagery, where fog often shows as a dull, grey and featureless cloud. Next, do a quick comparison with the infra-red satellite imagery and the fog seems to miraculously disappear, whereas other clouds tops still show up.

This is due to the fog being approximately the same temperature as the sea, hence giving a neat confirmation of the existence of fog on the visual picture. You can then return to the visual imagery and more accurately plot the extent of the fog and potentially take avoiding action.

Caught in fog

If you unexpectedly end up sailing in fog, first consider whether you need to keep heading further into it? Would it make more sense to do a quick 180° turn and head back out into clearer visibility?

…is in fact fog that disappears in the infrared view as shown here in the Celtic Sea and North Atlantic

Once we realise we’re sailing in fog, we need to work through a checklist of actions. Note your compass heading. Do we need a more experienced helmsman? It’s easy to quickly find you’re 30° or 40° off course and not notice. Steering is more exacting and even exhausting.

Could it be less stressful to use the autopilot, to allow you to focus on lookout? Just be ready to immediately switch to manual if you need to at a moment’s notice.

Slowing down is certainly good seamanship and the requirement to proceed at a safe speed appropriate to the conditions is detailed clearly in the International Regulations for the Prevention of Collision at Sea (IRPCS Rule 6 – Safe Speed).

Article continues below…

Weather expert Chris Tibbs on how to predict and cope with fog at sea

Radar and AIS help to make navigating in fog safer, but it is still an unnerving experience to sail in…

Navigation briefing: Mike Broughton explains why you still need radar

In a recent feature in Yachting World, my friend and highly respected yachting journalist Ed Gorman wrote a revealing article…

With an increased risk of collision, it’s good practise to ensure everyone is wearing a lifejacket. Maintaining a proper lookout is essential. In ‘pea soup’ fog, stationing a person forwards near the bow can help enormously. If motoring, the bow is a good place to escape the noise to allow the lookout crew member to listen for fog signals, or other hazards. On several occasions when I’ve been sailing in fog, the lookout at the bow has provided vital information to avoid a hazard or navigation mark.

In areas of dense shipping, remember to look up as well as straight ahead, I’m sure I am not the only person to have sighted a large vessel from the helm at the angle of the first spreader!

When ‘in or near areas of restricted visibility’ IRPCS states we shall make the prescribed sound signal, which for sailing vessels is one long blast, followed by two short blasts at intervals not more than two minutes. We need to have a working knowledge of sound signals of other vessels: many don’t appreciate that the fog signal for yachts is the same as vessels involved with fishing, towing, and even vessels ‘constrained by draught’ and ‘not under command’.

Make yourself visible

To help other ships see us, our best chance is to ensure we are clearly seen by the equipment on the bridge, which is radar and AIS (automated information system). Radar is a great help, but not all yachts have it, and it does take concerted focus and an understanding of how best to use it. Nearly all yachts have radar reflectors. Disappointingly tests have shown radar reflectors are not as effective at enhancing our radar signature as many people think. Active radar transponders show up much better.

AIS has been a real step change to aid collision avoidance when sailing in fog, though always remember that not all vessels use it and it is only an aid, albeit a very useful one. AIS can give the speed and course of a vessel as well as the closest point of approach (CPA) and time to the CPA: this is really useful data when navigating in fog. AIS also gives the vessel name, call sign, type and size.

Navigation lights are essential in foggy conditions. In a really thick fog the bow navigation lights can reflect back off the fog, leaving you with an eerie red or green glow.

Do you know how to make the prescribed sound signal in restricted visibility?

The amount of radio traffic tends to rise when sailing in fog, though beware using VHF radio to communicate with the watch keeper of another vessel, which can be fraught with problems.

There have been at least three occasions in the Dover Straits where watch keepers have opted to use radio to try to negotiate collision avoidance, which has ended unhappily in a collision!

These have often come about through misidentification and language difficulties, when both watch keepers should have prioritised normal rules of the road and not allowed themselves to be distracted in an important close quarters situation.

One tactic for dealing with fog on small vessels is to head away from busy shipping channels and sail to shallow water and anchor. Once tethered to the seabed remember to sound your fog signal (for vessels over 12m, ringing a bell for five seconds every minute). Many sailors may not be familiar with the additional signal of one short, one long, one short blast that can be made after the bell if you are concerned of a risk of collision while you are an anchor.

Stopping and racing in fog

Racing in fog creates extra challenges. Fog by night can take you by surprise and it is easy to lose hard fought gains, if the helmsperson loses awareness. If you have the choice of tacking into fog or not when racing, I would take the clear option every time: humans concentrate better when they can see.

The disorientation and confusion that sailing fog can create can easily generate high levels of stress. Mat Sweetman, captain of the J Class yacht Rainbow, suggests: “It is easy to get freaked out in fog, [but] look at is as if it was just a dark night and it gets a whole load less stressful.”

One clue as to the existence of fog at night is an absence of ambient lights and low altitude stars. If you are on the helm and getting close to fog, it is a good idea to start a scan of your yacht instruments. Like a pilot flying into cloud, it is imperative to ‘believe in your instruments’.

A regular scan is most effective but is both tiring and exacting over a long period of time. On top of normal sailing skills such as utilising the feel of your helm, tell tales (if you can still see them) and heel angle, we now need to bring in regular glances at true wind angle, boat speed, and heading. Working out a pattern for your scan is a good discipline for sailing in fog.

Sea fog is renowned over the Grand Banks. On the Transatlantic Race in 2005, fog prevailed for over six days with sustained wind speeds of 25-30 knots. Visibility was mostly less than 150 metres. Sailing in fog for nearly a week is tough going!

Using radar for collision avoidance

• Periodically cycle through the ranges and don’t leave it just on long range. Zoom in to only three miles in busy shipping and even closer when tracking a vessel of interest.
• Leave the sensitivity and clutter settings on auto. Modern radars handle them well.
• If you want to look through a rain cloud, alter ‘rain clutter’ control very briefly, then revert it to automatic every time.
• Use a split screen with radar on dual range, or radar and chartplotter lined up alongside, or radar ‘overlaid’ on the chartplotter.
• Experiment with this before you hit fog.
• Consider setting up an alarm zone to help you detect contacts.
• If your radar display is below decks, have a tested system for communicating to the helm. Some yachts use a private VHF channel.
• Stay clear of busy ferry routes

First published in the November 2020 issue of Yachting World.