CHAPTER Land fog Warm air from the land... 13


... condenses over the cooler sea


Sea fog – Sea fog is a nastier proposition than its land- based cousin. Like all fog, it develops when moisture in the air condenses on contact with a cooler substrate – in this case the sea. Because the air mass generating it is often huge, and the water temperature setting it off may be widespread, it can cover large areas and last a long time. This is the case with a forecast for ‘fog banks’, or ‘extensive fog’. ‘Fog patches’ means that the difference between the sea temperature and the ‘dew point’ of the air mass (the temperature at which it will condense its moisture into fog) is near the critical level, so fog is less certain in density and location.


The classic scenario for sea fog formation occurs up the eastern seaboard of North America, from New England to Newfoundland, where a ‘smoky sou’wester’ blows all the way up from the Caribbean to meet the icy Labrador current. Such fogs can last for weeks. Similar, less dramatic combinations occur in European waters. Because sea fog relies on the water to cool the air below its dew point, a sort of ‘fog shadow’ can show up in a moisture-laden airstream blowing across the lee of a point of land or an island. Typical English Channel examples are to find the Solent clear behind the Isle of Wight in a damp southwesterly, or a substantial area of moderate visibility to the east of Guernsey or Jersey when a southwest wind is filling the western Channel with fog. Being aware of this likelihood can give you the confidence to make a landfall when you might otherwise be losing hope.


Single-station forecasting


As with most hazards, it helps to be forewarned. Even if fog is not forecast, you can often spot it coming. Look out for signs of an impending dew point dump: A halo round navigation lights This is a sure sign of bad news at night. Dew in your eyebrows Another indicator you don’t want.


Sails inexplicably damp Sails suddenly get damp on what seems a nice day? Look out for fog around the corner!


Conduct on deck in fog


Life jackets and harnesses The question of when life jackets are to be worn is better left to the common sense of a skipper than laid down as a set of rules in a seamanship manual. However, there is accepted wisdom in the recommendation that all hands should wear their life jackets in thick fog. The reason for this is less about what happens to them if they fall overboard than their dire peril in the event of being run down. Collision risk is obviously much increased and, if the boat is hit, she may go down faster than people can find their ‘lifers’ and put them on. If they are already around their necks, there will be no unseemly rush for the locker.


The ever-present horror of losing a crew overboard is also exacerbated in fog, because the likelihood of finding a casualty is heavily reduced. It makes sense therefore to treat going on deck as one might after dark. If a harness is considered necessary then, perhaps it ought to be worn in fog also.


Be seen and heard Do all you can to increase your boat’s visibility. Run your navigation lights in daylight; if you’re a sailing boat, hoist your main even if motoring and, above all, remember to exhibit a radar reflector if yours isn’t permanently rigged. Sound your signal every two minutes as laid down in the Collision Regulations (COLREGS). The Lookout It may be sufficient to leave a solitary helmsman to look out alone in good conditions in a small yacht, but in fog this will not do. Unless there is no choice but to maintain a one-person watch, an extra lookout is undoubtedly worth the trouble. If conditions permit, he or she should be stationed on the foredeck. In fifty yards visibility, even thirty feet can make the difference between collision and safety.


MANUAL OF SEAMANSHIP | 129


Fog and poor visibility


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