Skysail Training


Fog and Mist - what it is, and what to do...

Remember there is no 'stand on' vessel  in fog - everyone keeps clear, if you hear another vessel forward, slow to a safe speed, stop if necessary, sound fog signal, navigate with caution till danger is past. (IRPCS Rule 19, see foot of this page).  Further actions are required if you have radar.

See also this link Radar collision avoidance  and an informative report on an accident in fog  Vespucci and Wahkuna synopsis

Radar notes are here  radar.pdf - radar reflectors, collision avoidance, etc.

Fog is a frightening experience for the small boat sailor. Radar is a huge help, but you are still relying on others to take the correct actions.  GPS helps to plot and navigate relative to fixed hazards, and AIS at least tells you where the bigger ships are.

Big ships are usually very helpful and alert, but not always......

Recognising the type of fog and the actions to take are vital.  The term fog is normally used when:

'Visibility is less than 1000 metres and relative humidity is more than 95%'

In the Shipping Forecast , what was referred to as 'Fog' is now called 'Very Poor' visibility

Fog is formed by the condensation of water vapour on particles in the air. These particles start to pick up moisture when the relative humidity is only 75%. If the air of high humidity cools, it will become saturated and further cooling will cause condensation of the water vapour into visible drops of water.  These drops of water, although tiny cause any light present to be reflected off in different directions. This is the cause of the whitish valleys that we all know. Another effect is when driving in thick fog at night. If you put your car headlights on full beam, the light will be reflected back towards you. This light is being reflected by the water drops.  So  we can see that the primary cause of fog is the cooling of the 'damp' air. There are several types of fog:

Advection or Sea Fog (advection is another word for wind...)
Sea Fog occurs when warm air flows over a relatively cold sea surface (advection flow). The sea temperature must be below the dew point of the air. This type of fog is frequently experienced around the UK and Channel Islands and often continues in strong winds. A change in wind direction or sea temperature is necessary for the sea fog to disperse.  Cold sea may be brought in by the tide upwelling round an island or coast. The weather forecast may refer to fog as widespread, in banks or patches.

Radiation or Land Fog
Land Fog usually occurs at night the when skies are clear, the air is moist and the land can cool through radiation. It is common in Spring and Autumn when atmospheric pressure is high. The air directly in contact with the ground will be cooled. If this cooling is enough to drop below the dew point then fog will occur. If the air is completely still then dew will form, not fog. Land fog often forms in valleys and may drift out to sea.  It usually clears when the sun warms the air on the following morning; a Met Office rule of thumb is that in October (10th month) it will clear by 10.00 am, and so on.

Sea Smoke or Warm Water Fog
This type of Fog will only occur in a few places. It requires a large difference between air and sea temperature with the air colder than the sea. This type of fog lies generally below 30ft in height from the surface.

Frontal Fog
This type of fog, usually occurs with the passage of a Warm Front where the temperature of the air ahead of the front is very low. Frontal Fog normally takes the form of low cloud enveloping high ground, which may descend to sea level.

As stated above, fog is usually termed as when the visibility is less than 1000 metres. Other definitions of visibility in the Shipping Forecast are:

Mist can simply be defined as:
'Visibility of less than 10km but over 1km with a relative humidity greater than 95%'

Actions in Fog

This is the list the class produced, with just a couple of additions:

If heading for harbour, aim off to make sure of which side you are heading for.  Follow a depth contour to the entrance.

IRPCS Rule 19  Conduct of vessels in restricted visibility

(a) This Rule applies to vessels not in sight of one another when navigating in or near an area of restricted visibility.

(b) Every vessel shall proceed at a safe speed adapted to the prevailing circumstances and conditions of restricted visibility. A power-driven vessel shall have her engines ready for immediate manoeuvre.

(c) Every vessel shall have due regard to the prevailing circumstances and conditions of restricted visibility when complying with the Rules of Section I of this Part.

(d) A vessel which detects by radar alone the presence of another vessel shall determine if a closequarters situation is developing and/or risk of collision exists. If so, she shall take avoiding action in ample time, provided that when such action consists of an alteration of course, so far as possible the following shall be avoided:

(i) an alteration of course to port for a vessel forward of the beam, other than for a vessel being overtaken;

(ii) an alteration of course towards a vessel abeam or abaft the beam.

(e) Except where it has been determined that a risk of collision does not exist, every vessel which hears apparently forward of her beam the fog signal of another vessel, or which cannot avoid a close-quarters situation with another vessel forward of her beam, shall reduce her speed to the minimum at which she can be kept on her course. She shall if necessary take all her way off and in any event navigate with extreme caution until danger of collision is over.

Posted 12th April 2011