The ease with which a navigational passage is made depends considerably on the weather conditions encountered. Heavy seas, adverse winds, or fog, for example, will add to the difficulties of a voyage. A navigator, therefore, requires forewarning of weather likely to endanger his craft or impede his passage. Weather is the condition of the atmosphere at any particular moment; climate is the mean condition of the atmosphere over a period of 30 years or so, and Meteorology is the science of the atmosphere, embracing both weather and climate. A yacht 'or small boat mariner does not require a detailed knowledge of Meteorology, but he must know sufficient about the subject to enable him to make the best use of a Shipping Forecast as broadcast on the radio and to understand an official weather map as issued by a Weather Centre and reproduced in certain newspapers. This entails an understanding of the nature and causes of weather, and the limitations of forecasting it. The offshore mariner, whether sail or power, cruising or racing, cannot have too much weather information and weather knowledge, and should take steps to acquire a familiarity with Meteorology comparable to his knowledge of navigation, pilotage, seamanship and safety.

There are two means of obtaining weather forecasts available to the navigator: weather predictions issued by the Meteorological Office and weather predictions based on the navigator's own observations of the weather. It might be thought that the daily predictions made by experts in the Meteorological Office, where vast numbers of observations made simultaneously at hundreds of stations over a very large area are frequently available for analysis and accurate prediction, would be infinitely more reliable than predictions made by a single amateur observer with limited resources, inadequate (and sometimes inaccurate) instruments, observing weather restricted to his own immediate locality. The truth is, however, that there are certain limitations to both methods of prediction. Weather analyses made by professional forecasters are not always 100% right; Meteorology is not a perfect science, and forecasters are not wizards, able to produce weather reports out of a hat or to fill up blank spaces in a synoptic chart over the oceans. Furthermore, forecast messages usually include an expectation of weather in specified areas for a period of 12 or 24 hours, but however hard a forecaster may try, it is seldom (in temperate latitudes, anyway) that he can satisfactorily describe weather for so long a period over such a necessarily large area; the forecast does, of course, give you a picture, though necessarily somewhat incomplete, of the kind of weather you are likely to experience, but it cannot tell you the time at which a change will occur in your vicinity, nor can it always cater for vessels near the border of an area. It follows, therefore, that unless you play your own part by watching your own weather like a true sailor, you will be let down from time to time if you trust implicitly in official forecasts. A good navigator may use buoys as a guide, but he also keeps an eye on transits or lines of bearings ashore, and on the lead or echo-sounder, because he knows buoys can sometimes go adrift. The same navigator who buries his head in his newspaper synoptic chart or broadcast forecast and predicts fine weather because, according to that chart or broadcast he is entering a ridge of high pressure, whereas in fact the ridge is ahead of its anticipated position and an intense low is rapidly approaching (as would be evident to an observant navigator who knows what to look for in the sky, wind behaviour, and his barometer), is behaving like the proverbial ostrich.

A yachtsman or small-boat navigator, therefore, without going too deeply into the subject of Meteorology, should be able to: (a) understand a broadcast weather forecast for shipping (including the description of the synoptic situation), (b) interpret an analysis synoptic weather chart as issued by the Meteorological Office or republished in certain newspapers, (c) predict the weather on his proposed voyage for the next 12 or 24 hours from such synoptic chart, and (d) observe and understand his own weather to see whether it is conforming to the predicted pattern or whether there are danger signs indicating a significant change in that predicted pattern.

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