Land-based obstructions Waves running in onto a lee shore may be perfectly harmless until they run into a cliff face or perhaps a breakwater or sea wall. They then bounce back out to sea, meeting those following them in a diametrically opposed wave train. The results can be perilously akin to the inside of a washing machine.
Inside the wave
Anyone who has helmed a boat – sail or power – in a following sea has experienced the feeling that she is carried forward as a wave comes up astern and held back in the trough. It is a subliminal attention to these unseen forces which enables a good helmsman to ‘catch the wave’ and surf with the upper part of its leading face. The reason is the motion of the water particles within the wave. As the wave travels, the water circulates as shown in the diagram, giving rise to higher speeds at the crest than in the trough.
Breaking in open water
We have seen above how a wave has no choice but to break as it approaches water too shallow to support it. This ‘plunging break’ is somewhat different from the ‘spilling break’ that occurs when a wave in deep water is piled up too steeply by the action of other waves, current, or extreme wind to support itself. Any wave that becomes steeper than around 1 in 7 is at risk from breaking. It is the breaking wave that represents the greatest threat to craft caught out in extreme weather.
Waves have a rotational action within them which means that the surface water moves in different directions depending upon its position on the wave. Note how, if travelling downwind, the surface flow will propel you over the crest then rush up to meet you in the trough – a powerful rolling action if you are caught beam-on.
Wind phenomena The barometer
The barometer is the bedrock of single-station forecasting. At sea its readings should be logged every hour, even in the tropics where its movements may be small. At best, a forecast comes from someone who is not where you are,
predicting what is expected to happen, but you can tell better than they can what’s happening now by poking your head out of the hatch. You can also see with your own eyes what the barometric pressure is doing. Interpreting this is not necessarily high science. Even if you don’t understand the full implications of the saturated adiabatic lapse rate, you can reckon that a rapidly falling ’glass’ in mid-latitudes foretells strong winds. These may well back (shift anti-clockwise) in the northern hemisphere or veer clockwise in the southern before the barometer levels out with the approaching weather front. Similarly, if you have been suffering a spell of heavy westerly wind with persistent rain, a sharp rise in the barometer accompanied by a breaking of the cloud is a likely sign that a cold front is arriving, bringing relief, although perhaps not before the breeze has piped up even more for a short time. Diurnal variation A sailor crossing the ocean in tropical latitudes can expect an essentially steady glass which will probably shift up and down a little in a predictable manner as the day progresses. This ‘diurnal variation’ is tabulated in pilot books and should be watched carefully. ‘Highs’ are generally around 1000 and 2200 local time, with ‘lows’ at 0600 and 1800. Any deviation from its regular progress, even by as little as 2mb, can foretell a disturbance which may develop into a full-blown tropical revolving storm. Predictable gale-force winds In higher latitudes where the temperate depression rules, certain rates of barometric movement are virtual guarantees of wind strength. If the breeze starts out fairly light, a fall
of 8mb in 3 hours is a promise that winds of force 8 will arrive within the next 3 hours or so. A 5mb fall in the same period indicates that force 6 is likely to be your lot. Similar predictions can confidently be made with a rapidly rising barometer.
Single-station predictions based on barometric movement are usually associated with moving weather systems. However, strong winds are possible in stable conditions where the isobars are squeezed between two systems. In such cases, the wind may blow a whole gale for days with the glass not moving at all. A classic
150 | MANUAL OF SEAMANSHIP
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