Ken Endean finds that fine weather can turn violent after dark

I take a close interest in coastal winds, because we spend a lot 
of time cruising in coastal waters and often encounter conditions that are nothing like the predictions of the weather forecasts.

The wildest variations occur when the background wind – the general airflow driven by large-scale weather systems – is blowing off the land, so that local topography and thermal effects can distort the airstream, sometimes with surprising results.

As air is normally invisible, trying to deduce exactly what is happening usually involves studying its visible effects, in the same way as photographs of windy conditions have 
to get their message across by depicting 
the consequences, such as bending trees 
or breaking waves and flying spray.

Even so, a flowing current of air does become visible when it contains mist, such 
as a bank of fog rolling down a valley.

Mupe Bay anchorage is known for its katabatic

Fine weather and clear night skies can generate katabatic down-draughts at Mupe Bay

Such flows are often fairly gentle but we recently experienced a dramatic demonstration when we spent a night in Worbarrow Bay, on Dorset’s Jurassic Coast – roughly half way between Weymouth and Swanage, and two miles east of the better-known Lulworth Cove.

I reckon this is England’s most spectacular coastal anchorage. An outer strip of limestone forms partial breakwaters at both ends of the bay, behind which the sea has eroded softer strata of sands and clays to create curving beaches.

The next band of rock is a chalk ridge, which is gradually succumbing to wave action as its toe is undercut, leading to regular massive landslides as the high cliffs creep steadily inland.

Too good to be true?

We always regard a visit to Worbarrow as a special treat because, for a start, it is within the Lulworth gunnery range and anchoring 
is only possible when the range is closed.

Fortunately, that occurs on many weekends and during the summer school holiday period. Then the weather and sea state have to be cooperative; the curved ends of the bay offer a degree of shelter in winds from south west or south east but a southerly wind or heavy swell will make the place untenable.

Mupe Bay, Dorset

On a calm afternoon in Mupe Bay, the anchored yachts lie to the light SW breeze

The best conditions are with a background wind off the land, which often occurs in fine, high-pressure weather. There may be an afternoon sea breeze but it will probably veer to south west, when the Mupe Rocks provide protection. From Mupe Bay, fine weather also invites crews to climb Bindon Hill, nearly 170m above sea level.

The only snag – which we have encountered several times – is that fine weather and clear night skies can generate powerful katabatic down-draughts as the chalk ridge cools, creating a layer of chilled, denser air that slides down the steep slope.

Last year, arriving in perfect summer weather, we anticipated this phenomenon and chose our anchoring spot carefully, 
on the assumption that we would lie to a south-west sea breeze during the afternoon and then swing to an off-shore wind during the night.

Katabatic at Mupe Bay, Dorset

The condensed water vapour reveals the katabatic airflow sliding from the top of Bindon Hill

On our first try, the anchor made a noisy encounter with limestone boulders; 
it held solidly but we could not be sure what would happen when the direction of pull reversed, so we moved closer inshore until we could lower 
it on to some sand, visible underwater as 
a light patch between darker areas of 
weed-covered stones.

The climb to the top of Bindon Hill is precipitous but not difficult, as steps have been cut in some of the steeper sections, but the path is crammed into the only narrow strip of ground that has definitely been cleared of unexploded ordnance, and its seaward side is encroaching as the top of the cliff progressively crumbles and falls away.

In some places, people who suffer from vertigo should avert their eyes from 
the edge. However, our upward hike was definitely worth the effort, rewarding us 
with views of magnificent scenery along 
the coast.

Continues below…

Later, back at sea level, the breeze had veered and the bay had become a sheltered cove, so we had a swim before returning on board and using our solar shower.

The air, the sea and the shower were all pleasantly warm and relaxing. 
The breeze veered further, to about west, and crews on the anchored yachts settled down for the evening.

A rude awakening

Excitement came during the night, when 
I woke at about 0400. London Apprentice was being buffeted by a strong wind, with roaring, hissing gusts making her sheer from side to side, and when I climbed into the cockpit I was met by an amazing sight.

Where we had earlier had a view of chalk avalanches, the cliffs were now overlaid by 
a different kind of avalanche – a great mass 
of mist that was flowing down from the top 
of Bindon Hill and then spreading out across the bay, with a perceptible ‘bounce’ where the cold airstream hit the water.

Katabatic at Mupe Bay, Dorset

As the sun rose, the base of the mist lifted but strong downdraughts continued to hit the water

The sky above was clear, with a couple of stars visible, and yachts’ anchor lights stood out against the lower gloom, but there was just enough early morning light for photographs.

Three hours later, I took another look. The sun was making itself felt and had probably begun to warm the cliff face, so that the mist was evaporating before it reached sea level.

Round Britain

Ken Endean is an inshore pilotage enthusiast who has made a close study of coastal sea conditions around the British Isles

However, heavy gusts were still smacking down on to the surface and pushing the yachts around their anchors.

Strong downdraughts sometimes occur 
in daytime when a background wind simply rolls over a hill and down its lee slope.

However, in this instance, the night wind 
was certainly katabatic in nature because although the background wind was northerly it was very faint.

In the Navtex 0400 coastal station reports the nearby Portland station recorded a breeze of only 3 knots from 010º.

By 0800, there was a clear view to seaward and thinning haze over Bindon Hill.

At 0900, when we sailed out of the bay, 
the mist had disappeared completely and we even had a light breeze from the south.

The cliffs were now basking in warm sunshine, looking completely innocent and with no hint that they might be a cause of meteorological violence.