Science in Short Chapters

by W. Mattieu Williams

WEATHER CHART, OCTOBER 6, 1875.

Explanation of Weather Chart.

In these charts the state of the sea—whether “rough,” “smooth,” “moderate,” “slight,” etc., is marked in capital letters; and the state of the weather—as “clear,” “dull,” “cloudy,” “showery,” etc., in small letters. The direction of the wind is indicated by the arrows. Unlike the arrows of a vane, these do not point towards the direction from which the wind is coming, but are flying arrows represented as moving with the wind, and consequently pointing to where the wind is going. The force of the wind is represented in five degrees of strength. 1st. A calm, by a horizontal line and zero—0 thus 0; 2nd. A light wind, by an arrow with one barb and no feathers _____\; 3rd. A fresh to strong breeze, by an arrow with two barbs and no feathers ——>; 4th. A gale, by an arrow with two feathers >——>; and 5th. A violent gale, by an arrow with four feathers >>——>. The temperature—in the shade—is marked in figures with a small circle to the right, indicating degrees—as 60°. These figures stand in the places where the observations are made. The other figures—usually with decimals, and placed at the end of the dotted lines—give the height of the barometer—the dotted line showing where this particular height remained the same at the time of observation. These dotted lines are called “isobars,” or equal weights—the weight or over-head pressure of the atmosphere being the same all along the line.

What must follow from this condition of the atmosphere? Clearly a great flow or rush of air from the south towards the comparatively vacuous regions of the north. The gases of our atmosphere, like the waters of the ocean, are always struggling to find their level, and thereby the winds are produced. The air flows from all sides towards the lowest isobar. But what, then, must be the course of the wind? Will it be in straight lines towards this point? If so, a strange conflict must result when all these currents meet from opposite directions. What will follow from this conflict? A skillful physicist can work out this problem mathematically, but we are not all mathematicians, some of us are not able to follow his formulæ, and, therefore, will do better by resorting to simple observation of other analogous and familiar phenomena. A funnel or any vessel with a hole in the bottom will answer our purpose. Let us fill such a vessel with water, then open the hole, and see what will be the course of the water when it is struggling to flow from all sides to the one point of vacuity. It will very soon establish a vortex or whirlpool, i.e., the water instead of flowing directly by straight lines from the sides to the centre of the funnel, will take a roundabout, spiral course, and thus screw its way down the outlet of the funnel.

This is just what occurs when the air is rushing to fill a comparatively vacuous atmospheric space. It moves in a spiral; and in the Northern Hemisphere this spiral always turns in the same way, viz., in the opposite direction to the hands of a clock when flowing inwards, and vice versâ, or with the clock hands, when the air is overflowing from a centre of high pressure.

In the chart for October 5th both these cases are illustrated. North of Dublin there is a curvature of isobars and an inrush of winds towards a northward low pressure, or vacuous region; while south of Dublin the isobar tends sharply round a high-pressure focus, and the overflowing wind is correspondingly reversed in direction, as shown by the arrows.

The next chart, for October 6th, shows that the overflow has spread northwards as far as Dublin, and the high-pressure focus has also moved northwards. It follows from this that if you know the barometric gradient, and stand with your left hand to the region of low barometer and your right hand to that of the high barometer, the wind will blow against your back, i.e., you will face the direction of the wind, or of those flying arrows on the chart. This interesting and important generalization is called “Buys Ballot’s Law.” In spite of the proverbial fickleness of the winds this simple law is rarely infringed, though it may require a slight modification of statement—inasmuch as the wind does not move in circles round the vacuous space, but in spirals, and thus it blows not quite square to the back, but rather obliquely, or a little on the right side. This is shown by the arrows in the charts, and is most strikingly displayed in the chart for October 6th, between the isobars of 30.3 and 30.5. To take, in Ireland, the position required by Buys Ballot’s Law, one must have stood facing the east, and accordingly, the westerly wind would then blow upon one’s back. In Paris, at the same moment, the position would be facing south-east, and the wind was curving round accordingly. Further south—at Bordeaux or the Pyrenees—the position becomes almost reversed, i.e., facing south-west, and the wind is reversed in equal degree.

Here, then, on these days we had the chief conditions of wind and rain, a steep and increasing barometric gradient, and a flow over our islands of humid air from the south and west regions of the great Atlantic. Strong winds and heavy rains did follow accordingly; and the prophetic warnings of the Meteorological Office, which are conveyed by means of signals displayed on prominent parts of the coast, were fulfilled.

Mr. Scott, the Director of the Meteorological Office, tells us that “The degree of success that has attended our warnings in these islands, on the average of the last two years, has been that over 45 per cent have been followed by severe gales; and over 33 per cent in addition have been followed by wind too strong for fishing-boats and yachts, though in themselves not severe gales; this gives a total percentage of success of nearly 80.”