Katabatic winds


These can best be thought of as supercharged land breezes. The same mechanism occurs, but much more powerfully as a parcel of cool, heavy air comes tumbling off a high mountain or down a steep valley, often into a harbour or sound below. Such winds are regularly experienced in areas where conditions suit their formation. Look to the study of a good pilot book to keep you informed.


Disturbance to leeward of an obstruction A vessel passing through the lee of a vertical obstruction such as a large vessel or a cliff is likely to suffer major disturbance in the wind for up to 7 times the height of the object. Close in, a 100% reversal of the wind direction may be experienced. Interference will also be felt to windward of the barrier as the air stream rises to pass over it, but only out as far as two or three times its height. A high cliff or mountainous island can produce a ‘falling wind’ in its lee that blows vertically downwards. These are notoriously dangerous because a sailing boat cannot ease her stress by spilling wind. Normally, the further a yacht heels over, the more she spills air naturally from the top of her sails, especially if they are over-sheeted. In a falling wind, her crew’s only redress is literally to drag the canvas down before it lays the yacht flat. Watch the foot of any high cliff with the wind blowing over it for disturbed water. It may be your only indication of impending mayhem.


Gusts in the lee of high ground


Less dramatic but still very noticeable effects occur much further to leeward of certain obstructions, particularly where the interference takes the form of an extended cliff or a flank of high ground. Here, an offshore wind can establish a series of waves not unlike the standing waves or the ‘stoppers’ used by canoeists down-stream of rocks in a rapidly flowing river. As the lower extremities of these waves approach the sea surface, the breeze gusts up, only to drop again further out as the wave swirls upwards, and so on. The outer limits of this type of unrest in the weather can be as much as four miles offshore. Inshore, they can be felt in certain expansive anchorages.


An open-sea squall


Squalls


Squalls are isolated disturbances recognisable by a significant cloud. They produce rain, wind or both, and may affect the wind direction as well. They are, however, comparatively short-lived, with half an hour as a useful reckoning for the likely duration.


In different latitudes and zones squalls behave in various ways. Some areas have local specialities. The white squalls of the Caribbean and the Great Lakes can generate very high winds indeed for brief periods, while some of the villainous-looking black clouds that prowl the Inter-tropical Convergence Zone produce nothing more dangerous than heavy rain. Anyone who has made a trade-wind ocean crossing knows that otherwise pleasant weather can be interrupted regularly by squalls which raise the wind speed by up to 100%. These are a serious nuisance, but fortunately they can be seen clearly as they approach from windward. By day, the cloud simply cannot be missed. Even on a moonless night, the general aura of looming darkness is enough to advise the watch on deck to shorten sail.


Eddy Strong Strong Weaker Weaker


Types of squall and action to take • The line squall - This is usually associated with a cold front or the passing of a trough. A line of cloud spreads across the sky to windward, sometimes in the form of a shallow arch. Cumulonimbus clouds may tower above it. These squalls can prove violent, with lashing rain or hail and a strong, gusty, veering wind (backing in southern hemisphere). Be prepared for this in terms of sail combination. If caught ‘napping’ by a squall from forward of the beam, heave to until the squall passes if your boat can do this properly; otherwise, run off in order to lessen the apparent wind. This tactic might increase the time you are under the


152 | MANUAL OF SEAMANSHIP


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