Fog and how to cope with it.

Post by: Dee White
01 May 2012

Fog is damp, cold, uncomfortable and potentially dangerous! If you are at sea it increases your chances of running into some hazard, or of another vessel crashing into you. Even if you escape these risks you may simply get lost and not be able to find a safe harbour. So it is well worth considering how to forecast it, and how to deal with it when you are out on the water.

What is fog?

Air is made of various gases, about 1% being invisible water vapour. Fog is produced when the air becomes cooler and cannot hold its water as an invisible vapour, so it condenses into droplets, giving us clouds. When the temperature change takes place near the ground, or sea level, we see the droplets as fog. Dirty atmosphere increases the chances of fog, as water vapour condenses on the particles in the atmosphere. Are you old enough to remember the London smogs?

One of the foggiest places in the world is the Grand Banks off Newfoundland where the cold Labrador Current and the warmer Gulf Stream meet. Some of the foggiest land areas are in Argentina, New Foundland, Chile, Namibia and California, and even in southern Europe, thick and localised fog is often found in valleys and lowlands.

Types of fog you may encounter on the water

  • Radiation fog - this forms inland on still nights, within a moist air mass, with light winds and clear skies. Heat radiates away into space, the air becomes unable to hold its moisture as vapour, so it condenses to form inland fog. It can be blown from the land towards the coast by a katabatic wind (cool air draining from the land to the sea) and down river estuaries and off cliffs. Within a short distance of the shore the air warms and the droplets evaporate to vapour.
  • Advection fog - is the one that most effects sailors. It is caused when warm air is blown across cooler water. Around the UK a warm front arriving from the south west, carries warm moist air from the Atlantic, which condenses as it crosses the cooler water to the west of Europe. If the wind is not strong enough to lift the droplets into the air, the result is fog, but sometimes even a gale force wind cannot shift the fog if the air is too wet, and it can often linger until there is a shift in wind direction. Advection fog can occur almost anywhere around the UK coasts. It is most common in spring and early summer.

Forecasting fog

If we are sailing around the UK, our best way of keeping away from the dangers of fog are by listening to the Shipping Forecast. This gives us two grades of decreasing visibility:

  • Poor visibility - you can see for 1-2 miles
  • Very poor visibility or fog – visibility of 1,000m or less Forecasters still refer to “fog patches” and “fog banks”. Patches tend to come and go, whereas banks extend over a wider area and may linger for a longer time.

If a warm front is forecast you could well expect fog to occur because of the warm, wet air it is bringing in.

So keep listening to successive Shipping Forecasts, taking note of changes and deterioration in visibility. If you hear “good” becoming “moderate to good” and going down the scale to “poor”, you can be relatively sure that fog could be an option in the not too far distant future. Weather websites also forecast visibility, and can be a useful guide if you have access to them. Also keep an eye on what the winds are doing and what is coming your way. A whirling hygrometer will help you to forecast fog by measuring the amount of moisture in the air. Remember that if warm air is approaching and blowing over the cooler waters where you are sailing, you could well end up with fog.

What should you do?

If you are in harbour and wondering whether to venture out in foggy conditions or when fog is forecast, there are certain considerations you should take into account:
Do you have to go?

  • If the visibility is “poor” or fog patches are forecast, you may decide to risk it, especially if you have GPS, radar or AIS to help you.
  • If your fog is radiation fog from the land, the chances are that it will clear as you get out to sea.
  • If your planned route involves crossing busy areas of shipping, it might be sensible to stay put, especially if you have no radar or AIS.
  • If “fog banks” are expected, your electronics will generally get you through, but there is always the risk of them failing and then you’ll probably wish you’d stayed in port. If you have no electronics on board, stay at home.
  • If you are planning a night sail and fog is forecast, don’t venture out. The navigation lights won’t help you, and you will be able to see next to nothing.

If you are already at sea – what do you do?

  • Make sure you know where you are. Even if you have a GPS, take fixes and log your position before visibility goes, and regularly make log entries so you know where you are and what course you are making. Use your paper chart as a back up. It may seem old fashioned but it could get you out of trouble.
  • Take down sails that reduce manoeuvrability.
  • Motor boats reduce speed.
  • Consider changing your passage plan.
  • Make sure everyone is wearing their life jacket in case of collision.
  • Wake anyone who is asleep below deck.
  • Switch on navigation lights.
  • Position extra lookouts on the foredeck. They may be able to see a few more yards ahead, and may well be able to pick up sounds of other vessels or fog horns, away from the cockpit noise.
  • Occasionally take the engine out of gear and listen out for unexpected sounds which might help you estimate distance from land or approaching craft. Even switch it off from time to time if you are sure it can be started easily.
  • Make yourself as conspicuous as you can both aurally and visually. A foghorn will alert others to your approach. Sound it once every 2 minutes making way under power, and one long and two short blasts under sail. Even if you are under power keep your mainsail fully hoisted to make you more obvious. If you have anchored, remember to make the appropriate sound signals – a bell or gong rung for 5 seconds every minute.
  • Consider whether to keep going. This may be the only option but if it is, do everything you can to avoid a collision. If you are in an area with a lot of heavy shipping around, you might be able to manoeuvre into shallower water, too shallow for them to navigate in. You could either wait around or anchor up, but you have to be confident that you are somewhere safe where you won’t be run down. Another option would be to navigate to a place of safety. With a GPS you could enter a waypoint for a safe haven, but try to select a safe entry by steering off 20 or 30 degrees so that if your navigation is not accurate, you at least know which direction you are from your destination. You will need to watch you depth gauge and compare it with the contour lines of the chart to compensate for any inaccuracies between GPS and chart. Contour lines can be a useful guide in finding your destination. Locate the line that leads to the entrance of your harbour and follow it along with your eyes glued to the depth gauge.
  • If you are unhappy with your electronics, make a note to upgrade them before you risk getting caught in fog again. Invest in a good radar reflector or a transponder and make sure they are correctly set up. You may also consider AIS to receive details of other shipping in the area. It is quite cheap in a simple receive-only form, although it is worth paying the extra to have one hooked up to a chart plotter and radar.
  • Its no good having the best electronics in the world if there is no one on board who knows how to use them. I have just read an article of a sea rescue in fog, where the boat was well equipped, but practical knowledge of how to use it was negligible.
  • If you have radar, then use it. Remember that the Colregs require you to use any available methods of navigation when sailing or motoring in fog.

So to sum up – yes fog is dangerous and should be treated with respect. While you would be sensible to take every opportunity to avoid it, you may sometimes not have a choice. So – be prepared both by having a well equipped boat and the knowledge to know how to cope with it.

Author – Dee White

Change units of measure

This feature requires cookies to be enabled on your browser.

Show price in:
Show lengths, beam and draft in:
Show displacement or weight in:
Show capacity or volume in:
Show speed in:
Show distance in: