SYMBOL. h Hail. 7 Lightning. m Misty, hazy atmosphere. o Overcast, the whole sky being covered with an impervious cloud. p Passing, temporary showers. q Squally. r Rain, continued rain. s Snow. t Thunder. u Ugly, threatening appearance of the weather. v Visibility, whether the sky be cloudy or not. w Dew. We should remark here that, though in common parlance the word "weather" is used collectively for the sum of every meteorological element, wind, rain, heat, cold, etc., in this work, and in all synoptic charts, "weather" is used in a more restricted sense to denote whether the actual appearance of the sky is blue, cloudy or otherwise, and whether rain, snow, hail, etc., are falling. Then arrows are placed over each observing station, with a number of barbs and feathers which roughly indicate the force of the wind. By an international convention, the arrows always fly with the wind; that is to say, they do not face the wind like the pointer of a windvane. The scale of force usually adopted is that of Beaufort, which is given opposite. It will be observed that this is a practical scale, based on the amount of canvas a ship can carry. At sea this is certainly a better gauge than any instrumental readings, though there is always a certain disagreement in the estimate of different observers. For land-observations, and those unacquainted with ships, an equivalent of miles per hour and metres per second is given. 12. Hurricane. full and ... Royals, etc. and top-gallant sails etc.... ... Close - reefed topsails and courses Or that with which she could Or that which would reduce her to ... ... ... 65 29.0 ... ... 90 40.0 Or that which no canvas could with- When all this is done, we can see at a glance whether or how wind, rain, cloud, and blue sky are connected with the shape of the isobars. In fact, a synoptic chart gives us, as it were, a bird's-eye view of the weather at the particular moment for which the chart is constructed, over the whole district from which reports have been received. Suppose, now, that after an interval of twenty-four hours another chart is constructed from observations taken over the same area, then we generally find that the shape of the isobars and the position of the areas of high and low pressure have considerably changed, and with them the positions of those areas where the weather is good or bad. For instance, suppose that at 8 a.m. on one morning we find pressure low over Ireland and high over Denmark, with rain over Ireland, cloud over England, and blue sky in Denmark; and that by 8 a.m. on the following day we find that the low-pressure area has advanced to Denmark, and that a new high pressure has formed over Ireland, with rain in Denmark, broken sky in England, and blue sky in Ireland; suppose, too, that the record of the weather, say in London, for those twenty-four hours had been as follows:-cloudy sky, followed by rain, after which the sky broke;-how can an inspection of the two charts help us to explain the weather as observed in London during that day? Our bird's-eye view would show that the rainarea which lay over Ireland in the morning had drifted during the day over England, including London, and covered Denmark by next morning. It would also tell us that the position of the rain was identified with, and moved along with the low pressure. This is the fundamental idea of all synoptic meteorology, but one which can only be thoroughly grasped after a considerable experience in tracing actual cases. It is so different looking at the "ups" and "downs" of the barometer when they are marked on a diagram, and then at any two synoptic charts which refer to the same period, that it is very difficult at first to see any connection at all. In fact, deductions from barograms-as such barometric traces are called-and deductions from synoptic charts are so apparently unconnected that they have hitherto been almost treated as different branches of meteorology. One main feature of this book will be our endeavour to collate these deductions together, and to show how changes in the charts for a large district are simultaneously shown by fluctuation in the instrumental readings at any one place. It must be borne in mind, however, that the whole aim and object of meteorology is to explain weather as it occurs at any place; that is, what successive changes each individual observer will experience. Synoptic charts are only a means to this end. RELATIONS OF WIND AND WEATHER TO ISOBARS. Such, then, is a synoptic chart. Many thousands have been constructed for all parts of the world, and by comparing them the following important generalizations have been arrived at : 1. That in general the configuration of the isobars takes one of seven well-defined forms. 2. That, independent of the shape of the isobars, the wind always takes a definite direction relative to the trend of these lines, and the position of the nearest area if low pressure. 3. That the velocity of the wind is always nearly proportional to the closeness of the isobars. 4. That the weather-that is to say, the kind of cloud, rain, fog, etc.—at any moment depends on the shape, and not the closeness, of the isobars, some shapes being associated with good and others with bad weather. 5. That the regions thus mapped out by the isobars were constantly shifting their position, so that changes of weather were caused by the drifting past of these areas of good or bad weather, just as on a small scale rain falls as a squall drives by. The motion of these areas was found to follow certain laws, so that forecasting weather changes in advance became a possibility. 6. That in the temperate zones sometimes, and habitually in the tropics, rain fell without any appreciable change in the isobars, though the wind conformed more regularly to the general law of these lines. This class of rainfall will be called throughout this work "non-isobaric rain." It will be convenient to take first the broad features of the relation of wind to isobars, which are as follows: First as regards direction. The wind in all cases is not exactly parallel to the isobar, but inclined towards the nearest low pressure at an angle of from 30° to 40°. If you stand with your back to the wind, the lowest pressure will always be on your left hand in the northern hemisphere, and on your right in the southern hemisphere. This is what is commonly known as "Buys Ballot's Law." Then as to velocity. All we need say here is that the velocity is roughly proportional to the closeness of the isobars, and that the measure of the closeness is called the barometric gradient, for in our chapter on wind and calm we will give all necessary details on this branch of the subject. The upshot of these two principles is, that if you give a meteorologist a chart of the world with the isobars only |