Yachting Monthly — November 2017

November 2017 http://www.yachtingmonthly.com 23

Dinllaen in North Wales. The gale was
expected from SSW and the southern
shore of the bay has steep, grassy cliffs
so that the wind was likely to shoot over
their rim and plant gusts in the middle of
the bay. We could not anchor close under
the cliffs in case vertical eddies blew
us towards the beach and into shallow
water. However, a little further out there
was enough room for London Apprentice
to anchor and swing on the inshore side
of the local moorings and this position
served us well (Photo 7). At the height
of the unpleasantness, screaming wind
and driving rain lashed at the boats on
moorings while we were in slightly gentler
conditions but far enough out to be lying
to seaward of our anchor and with our
bows pointing into the deluge (Photo 9).

High ground to windward
Under some conditions, wind flowing
across mountains undulates as a series
of standing waves and occasionally one
of these waves develops into a large
vertical eddy, with a cloud at its centre
(Diagram 4). This rotor cloud remains
stationary or slow-moving while air
flows around it and some of that air
blows back towards the high ground.
Photographs in textbooks generally
feature clean-cut rotor clouds against
a background of blue sky but reality is
often much messier, with a very untidy
cloudscape. The phenomenon is quite
common when a warm, moist airstream
blows across mountains and then out
over the sea, producing dark, lumbering
clouds, random showers of rain and –
most important for yachts – a reversal
or substantial shift in the wind direction
at sea level. For a crew on passage, this
kind of wind shift will be perplexing, and
if a vessel is at anchor she will suddenly
be on a lee shore. Fortunately, the wind
reversal is temporary and a skipper who

EXPERT ON BOARD

9

Sea

Turbulence and a rotor cloud

Strong

downslope

wind

Fluky wind

Rotor cloud

7

Diagram 4

Sea

Katabatic wind

Chilled air

collects in

hollows

Land with chilled

ground surface

Chilled air flows

down steep slopes

understands the cause is most likely to

make correct decisions.

The term ‘katabatic’ (from the Greek)

simply refers to ‘downward’, so might

be applied to any downslope wind, but

in modern meteorology it is used to

denote a mass of cool air, forming against

a cold land surface and then flowing

down a slope because it is denser than

the surrounding atmosphere. Vigorous

katabatic winds can occur when the flank

of a mountain loses radiant heat to a

clear night sky and a layer of chilled air

forms against the rock before rushing

down the slope (Diagram 5). We have

even experienced katabatic violence on

the Dorset Coast, when the high ground

around Worbarrow Bay generated

‘bombs’ of cool air that slid down the

near-vertical cliffs like avalanches,

before bursting outwards over the water.

Anchored boats were sheering around,

straining at their cables and heeling in

the strongest gusts. However, a katabatic

flow soon loses momentum as it spreads

over the sea and a mile offshore there

was complete calm. The gusts could have

been confused with conventional down-

draughts but they died out as soon as

the morning sun began to warm the

steep faces of the chalk.

Diagram 5

Porth Dinllaen anchorage, surrounded by grassy cliffs, with the wind starting to

build. London Apprentice is inshore of the moored boats

Whoosh! The

mooring area at the

height of the gale