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Page:Popular Science Monthly Volume 32.djvu/669

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call the meteorological significance. What was the relation of the damp to the rain? Why did the prognostic sometimes fail? Why are there many rain-prognostics associated with a tolerably dry air? Why is not all rain preceded by the same set of prognostics? To all these questions no answer could be given. Prognostics had almost fallen into disrepute; they were considered no part of science, and had been supposed to be only suitable for rustics and sailors.

So the subject remained till the introduction of synoptic charts. Then it was soon seen that in temperate regions the broad features of weather depend on the shape of the isobaric lines, and later on it was shown—the author believes, mainly by himself—that nearly all prognostics have a definite place in some shape of isobars, and that all the above questions, formerly insoluble, receive a ready explanation. It has also been demonstrated that prognostics can never be superseded for use on board ship, and that even in the highest developments of weather-forecasting by means of electric telegraph, prognostics often afford most valuable information. But before we attempt to explain how this is done, we must introduce the reader into the elements of synoptic meteorology.

Synoptic meteorology is that part of the science which deals with the results obtained by constructing synoptic charts. Formerly, all meteorology was deduced from the changes which took place in the instrumental readings at any one place during any interval of time, say one day. For instance, a great deal had been discovered as to the connection between a falling or rising barometer and the accompanying rain or wind. Synoptic charts, on the contrary, are constructed by taking the readings of any instrument (say the barometer), or any observations on the sky or the weather (say where rain is falling, or cloud or blue sky is seen), at a large number of places at the same moment (say 8 a. m, at Greenwich). A map of the area or district from which the observations have been received is then taken, the barometer-readings are marked down over their respective places, and then lines are drawn through all the stations where the pressure is equal; for instance, through all the places where the pressure is 29-9 inches (760 mm.), and again at convenient intervals, generally of about two tenths of an inch, say 29·7 inches (755 mm.), 29·5 inches (750 mm.), and so on. These lines are called isobaric lines, or more shortly isobars—that is, lines of equal atmospheric weight or pressure. This method of showing the distribution of pressure by isobars is exactly analogous to that of marking out hills and valleys by means of contour lines of equal altitude.

Similarly, the places which report rain, cloud, blue sky, etc., are marked with convenient symbols to denote these phenomena. 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;