Here in the UK, we have a slightly undeserved reputation for fog. Certainly, we can be susceptible to it and we can also get it at any time of the year, but compared to the foggiest place on earth (just off Newfoundland where warm and cold oceanic currents converge) we get away very lightly. While they endure around 200 foggy days per year, incidents of fog around Britain’s coastal regions tend to vary between just one and seven per cent.
That’s certainly good news for the recreational boater, but it would be wrong to dismiss the phenomenon altogether because for the unprepared, it can still be a very dangerous thing. As a good Skipper, you should therefore make sure you know the basics.
The basics of fog
As far as the boater is concerned, fog (which is the same as mist but with a visibility at sea of less than 1km) comes in two basic forms. The first is called ‘radiation’ fog and the second is called ‘advection’ fog – and while they are formed in different ways and have different implications for your day on the water, the principles underpinning them are all about one simple thing: the Dew Point.
Now plainly, the air can only contain a certain amount of moisture before it becomes saturated and the water condenses to form visible droplets. It is this point of saturation that is known as the Dew Point. However, because warm air can carry more water than cool air, the Dew Point is not a fixed thing. Instead, it is the temperature at which a given body of air with a given quantity of moisture reaches saturation. If you think about this, it is then quite easy to see how the rapid cooling of a moist air mass might quickly lead to the generation of fog.
Radiation fog is formed by relatively moist air in contact with relatively cold ground. It happens on clear, cloudless nights with minimal wind, when the heat in the land escapes into the atmosphere. As it does so, the air at ground level cools to the Dew Point and fog is formed. You are unlikely to see much radiation fog in the summer, as the nights are rarely long enough or cold enough to generate such conditions, but from early Autumn through to late Spring, it is always a possibility in the UK.
Having formed over land, it can often make its way to the coast via rivers and estuaries but, when it reaches the sea, it doesn’t usually last that long, as the relatively warm sea temperatures, the heat of the sun and the onset of winds tend to disperse it quite quickly. The upshot of this is that, while radiation fog may mist up the marina, making the prospects of a day out look very slim, it will often leave the sea itself fairly clear or disperse altogether as the morning warms up.
Advection fog or sea fog
Advection fog (better known by most of us as sea fog) is formed when warm moist air moves over cold water. The air temperature drops to its Dew Point and the moisture in the air condenses to form fog. This happens most frequently between late spring and mid-summer when inshore waters remain relatively cold and yet the land is relatively warm. The amount of advection fog you are likely to get depends on the disparity between the air’s Dew Point and the seawater below it. If the Dew Point is considerably higher than the sea temperature, then it stands to reason that widespread and relatively long-lasting fog is on the cards. However, unlike radiation fog, advection fog can occur at any time of the day and even in quite strong winds of up to perhaps Force 4 or even 5. It is therefore a more dangerous form of fog for boaters.
What if you get caught in fog?
If you get caught in fog, the first thing to do is reduce your speed to a level which allows you sufficient time to interpret the information provided by your electronic equipment. While you do this, you should turn on your nav lights and use a bright search light to allow others to see you as early as possible. You should also keep as many full-time lookouts as you can spare and make regular sound signals with a proper foghorn in accordance with the dictates of ‘Rule of the Road’.
For us skippers of small powerboats, a combination of Rule 19 (‘for conduct of vessels in restricted visibility’) and Rule 35 basically means you need to do three things: (1) reduce your speed and go slowly enough to give yourself time to take evasive action; (2) navigate with extreme caution; and (3) sound one prolonged blast on the horn at least every two minutes (if you’re making way) or sound two prolonged blasts on the horn at least every two minutes (if you’re underway but stopped in the water).
Additionally, when you organise your navigation plan, it is always wise to arrange a sequence of ‘bolt-holes’. If you make this common practice, it will come in handy if a period of prolonged fog in a relatively busy waterway convinces you that it is not prudent to pursue the more distant destination. And as accurate prediction of fog is not always possible, the regular boater should also go to sea well-equipped, ideally carrying a good modern radar, an effective marine VHF radio, a chart plotter with up-to-date software, a foghorn, a powerful searchlight and considerably more fuel than is required for your ‘blue-sky’ nav plan.
Understanding the basic conditions that generate fog will usually enable you to avoid being caught out by it. If your interpretation of the local forecast (and your advice from the local harbourmaster) suggests that a long-lasting fog is likely to settle on your cruising area, the wisest option for a leisure boater is to stay ashore. But if you do get surprised by fog midway through a sea passage, a combination of the right equipment, a well-sorted nav plan and the application of the seamanship methods outlined above should see you safely (if rather slowly) back to port.