Yachting Monthly - April 2016

PRACTICAL SEAMANSHIP

PHOTOS: COLIN WORK

GRAPHICS: MAXINE HEATH

There are two common types of fog: advection fog, caused

by warm moist air blowing over cold water, and radiation fog,

caused by air that is cooled by the land, reaches its dew point and

condenses as fog before drifting down to the sea.

Advection fog usually occurs earlier in the season when water

temperatures are lower, and southwesterly winds bring in warm,

moist air. Advection fog hugs the sea in winds of up to Force 4,

above which it lifts to become a blanket of low cloud. As the

causes won’t change either until there is a change of air mass

such as a cold front, advection fog can last a while.

The good news is that radiation fog forms over land, so it clears

as you head out to sea. Also, it is associated with high pressure so

the sun normally burns it off, giving a pleasant afternoon.

Warm,

moist air

ADVECTION FOG

RADIATION FOG Heat

Which kind of fog?

The water is warm, moist

air condenses when its

temperature is lowered

to the air’s dew point by

cooler water, creating a

low bank of patchy fog

Land loses heat quickly

at night, via radiation.

The cold ground cools

the air to its dew point

and a shallow layer of fog

forms, which then slides

downhill to the coast

With the plotter below, I used an iPad to check our position while

motoring down the shallower Boat Channel, free of shipping surprises

An eerie shadow

emerges from the fog,

and it turns out to be

the tall ship Kaskelot,

with a lookout posted

on the bowsprit

APRIL 2016 http://www.yachtingmonthly.com 33

iPad on deck – not a wet weather

option but it kept me on deck and

I think was safer than being below.

Radiation fog can be incredibly

thick but it is often patchy

so if you are on deck when it

lifts slightly you can pick up

valuable clues to your position,

such as buoys, and choose the

right moment to cross the main

channel: the kind of clues you

would miss staring at instruments

below. A really important

instrument visible from the helm

position is the depth sounder.

This is critical in Poole, which has

shallow mud banks outside each

of its many channels and we were

leaving on an ebb tide.

A ghost in the mist

We found our way out of Poole

steering courses from buoy

to buoy, using the small boat

channel to avoid the ships and

crossing over the channel when

the visibility improved slightly. We

listened carefully for foghorns.

It was particularly thick when

we reached the entrance of the

harbour, where a large chain ferry

can be diffi cult to negotiate even

in good conditions. I was pretty

anxious that it might set off and

clatter its way towards us so

quickly that we would be unable

to avoid the chain, so I cautiously

kept in the middle of the channel

and listened out for it, unsure

which side it was.

At that moment there was a

foghorn ahead. A large vessel

was coming into Poole. As soon

as we were past the ferry we

steered quickly over to starboard,

the thinking being that, if you

encounter anything moving, a

starboard turn is generally the

least hazardous option. And

it was, because out of the fog

appeared the bowsprit of the

tall ship Kaskelot, bearing a

gesticulating crewman. She was a

stunning and slightly eerie sight.

Once outside the harbour

the small boat channel gave

no problems in the improving

visibility and we enjoyed the rest

of the trip in fi ne sunshine. We

arrived at Portland marina in good

time and it seemed unbelievable

that the fi rst two miles had been

such hard work! W