Popular Science Monthly/Volume 32/March 1888/Weather-Prognostics
By the HON. RALPH ABERCROMBY.
FROM classic times, down to the commencement of this century, it can hardly be said that this branch of meteorology made any advance. Few, if any, new prognostics had been discovered, and neither their physical explanation nor their meteorological significance had been found out. But about eighty years ago some physical explanations were given. It was found that the air always contained a certain quantity of uncondensed vapor, and means were invented for measuring this amount accurately. From this, the nature and conditions of the formation of dew were discovered, and also that before many cases of rain the air became more charged with vapor. This latter fact gave the explanation of several rain-prognostics. For instance, when walls sweat, stones grow black, and clouds form on hilltops, rain may be expected almost all the world over.
But even when these reasons had been discovered, the science flagged. A large number of rain-prognostics could not be shown by any means to depend on an increase of moisture, and, as vapor can not grow in the air, some explanation was needed to account for its variable quantity. And even when, in a general way, the prognostic had been explained, no clew whatever had been found for what we may 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 It has also been demonstrated that prognostics can never be superseded for use on board ship, and that even in the highest developments of weather-forecasting by means of electric telegraph, prognostics often afford most valuable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; that is to say, they do not face the wind like the pointer of a wind-vane.
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 rain-area 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.
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 of 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 "non-isobaric rain."
In Fig. 1 we give in a diagrammatic form the broad features only
Fig. 1—The Seven Fundamental Shapes of Isobars.
of the distribution of pressure over the North Atlantic, Europe, and the eastern portions of the United States on February 27, 1865. Coast-lines are omitted, so as not to confuse the eye, so also are lines of latitude and longitude; but the foot-note at the bottom of the figure represents the equator, and the top of the diagram would be on the Arctic Circle. All pressures of and under 29·9 inches (760 mm.) are shown with dotted lines, so that the eye sees at a glance the broad distribution of high or low pressure. The whole seven fundamental shapes of isobars will be found there.
Looking at the top of the diagram, we see two nearly circular areas of low pressure, round which the isobars are rather closely packed. Such areas, or rather the configurations of isobars which inclose them, are called "cyclones," from a Greek word meaning a circle, because they are nearly circular, and, as we shall see presently, the wind blows nearly in a circle round their center. Just south of one of the cyclones, the isobar of 29·9 inches (760 mm.) forms a small sort of nearly circular loop, inclosing lower pressure; this is called a "secondary cyclone," because it is usually secondary or subsidiary to the primary cyclones above described. Farther to the left the same isobar of 29·9 inches bends itself into the shape of the letter V, also inclosing low pressure; this is called a "V-shaped depression," or, shortly, a "V." Between the two cyclones the isobar of 29*9 inches projects upward, like a wedge or an inverted letter V, but this time incloses high pressure; this shape of lines is called a "wedge." Below all these we see an oblong area of high pressure, round which the isobars are very far apart; this is called an "anticyclone," because it is the opposite to a cyclone in everything—wind, weather, pressure, etc. Between every two anti-cyclones we find a furrow, neck, or "col" of low pressure analogous to the col which forms a pass between two adjacent mountain-peaks. Lastly, as marked in the lower edge of the diagram, isobars sometimes run straight, so that they do not include any kind of area, but represent a barometric slope analogous to the sloping sides of a long hill. The cyclones, secondaries, V's, and wedges are usually moving toward the east at the rate of about twenty miles an hour; but the anticyclones, on the contrary, are usually stationary for days and sometimes for months together. We should also note that, though the general principles of prognostics and the broad features of the weather in each of these shapes of isobars are the same all over the world, the minute details which we intend to give now apply to Great Britain and the temperate zones only.
We will now take the cyclone separately, and detail the kind of wind and weather which is experienced in different parts of it. In Fig. 2 we give a diagram on which we have written in words the
Fig. 2.—Cyclone Prognostics.
kind of weather which would be found in every portion of a typical cyclone; arrows also show the direction of the wind relative to the isobars and to the center. First let us look at the isobars. We find that they are oval, and that they are not quite concentric, but the center of the inner one we will call the center of the cyclone. Now observe the numbers attached to the isobars; the outer one is 30·0 inches (762 mm.), the inner one 29·0 inches (737 mm.). But suppose the outer one was the same, but the inner one was 29·5 (755 mm.). We should then have two cyclones, differing in nothing but depth; that is, in the closeness of the isobars, or the steepness of the barometric slope. Observation has shown that under these circumstances the general character of the weather and the direction of the wind everywhere would be the same; the only difference would be that the wind would blow a hard gale in the first and only a moderate breeze in the second case; and that what was a sharp squall in the one would be a quiet shower in the other. This is one of the fundamental principles of synoptic meteorology—that the character of the weather and direction of the wind depend entirely on the shape of the isobars, while the force of the wind and intensity of the character of the weather depend only on the closeness of the isobars.
The difference in the details of the weather in a cyclone, or any other isobaric shape which are due to difference in the steepness of the isobars, is called a difference in the intensity of the weather. Hence, when we speak of a cyclone as being intense, we mean that it has steep isobars somewhere. When we come to talk about the general sequence of weather from day to day, we shall find that there is no difference between the cyclones which cause storms and those which cause ordinary weather except intensity. This is another of the fundamental principles of meteorology.
Returning now to our cyclone, the whole of the portion in front of the center facing the direction toward which it moves is called its front, and the whole of this portion may obviously be divided into a right and left front. The other side of the center is, of course, the rear of the cyclone. Then, as the whole cyclone moves along its course, it is evident that the barometer will be falling more or less at every portion of the front, and rising more or less everywhere in the rear, so that there must be a line of places somewhere across the cyclone where the barometer has touched its lowest point and is just going to rise. This line is called the "trough" of the cyclone, because if we look at the barometer-trace at any one place, the "ups" and "downs" suggest the analogy of waves, so that the lowest part of a trace may be called a "trough." Or we may look at the cyclone as a circular eddy, moving in a given direction, and so far presenting some analogy to a wave.
So far for the shape and names of the different portions of the cyclone. Now for the wind. A glance at the arrows will show that, broadly speaking, the wind rotates round the center in a direction opposite to the motion of the hands of a watch. That is to say, that in the extreme front, following the outer isobar, the wind is from the southeast; farther round, it is from the east-northeast; still farther, from the north-northwest; then from about west; and, finally, from the southwest. Then we note that in front the wind is slightly incurved toward the center, and therefore blows somewhat across the isobars, while in rear it has little or no incurvature, and blows nearly parallel to the isobars. The velocity or force of the wind will depend on the closeness of the isobars. In the diagram they are much closer set in rear than in front of the cyclone, and therefore the wind is strongest behind the center.
The weather in a cyclone is somewhat complicated. Some characteristic features depend on the position of the trough, and have nothing to do with the center. For instance, the weather and sky over the whole front of the cyclone—that is, all that lies in front of the trough—is characterized by a muggy, oppressive feel of the air, and a dirty, gloomy sky of a stratiform type, whether it is actually raining or only cloudy. On the other side, the whole of the rear is characterized by a sharp, brisk feel of the air, and a hard, firm sky of cumulus type.
But, on the contrary, other characteristic features are related to the center, and have little to do with the trough. The rotation of the wind, though slightly modified near the trough, is in the main related to the center, and the broad features of the weather in a cyclone are—a patch of rain near the center, a ring of cloud surrounding the rain, and blue sky outside the whole system. The center of the rain-area is rarely concentric with the isobars. It usually extends farther in front than in rear, and more to the south than to the north, but is still primarily related to the center.
This will be readily seen by reference to the diagram; there the drizzle and driving rain extend some distance to the right front, while almost directly behind the center patches of blue sky become visible. Thus a cyclone has, as it were, a double symmetry; that is to say, one set of phenomena, such as warmth, cloud character, etc., which are symmetrically disposed in front and rear of the trough; and another set, such as wind and rain, which are symmetrically arranged round the center. There is reason to believe that what we may call the circular symmetry of a cyclone is due to the rotation of the air, while the properties which are related to the trough are due to the forward motion of the whole system.
We have marked on the diagram the kind of weather and cloud which would be found in different parts of a cyclone. The first thing which will strike us is that the descriptive epithets applied to the sky contain the phraseology of the most familiar prognostics. At the extreme front we see marked "pale moon," "watery sun," which means that in that portion of a cyclone the moon or sun will look pale or watery through a peculiar kind of sky. But all over the world a pale moon and watery sun are known as prognostics of rain. Why are they so? The reason we can now explain. Since a cyclone is usually moving, after the front part where the sky gives a watery look to the sun has passed over the observer, the rainy portion will also have to come over him before he experiences the blue sky on the other side of the cyclone. Suppose the cyclone stood still for a week, then the observer would see a watery sky for a week, without any rain following. Suppose the cyclone came on so far as to bring him under a watery sky, and then died out or moved in another direction, then, after seeing a watery sky, no rain would fall, but the sky would clear. The prognostic would then be said to fail, but the word is only partially applicable. The watery sky was formed and seen by the observer, because he was in the appropriate portion of the cyclone, and so far the prognostic told its story correctly—viz., that the observer was in the front of the rainy area of a cyclone. The prognostic failed in its ordinary indication because the cyclone did not move on as usual, but died out, and therefore never brought its rainy portion over the observer. This is the commonest source of the so-called failure of a rain-prognostic in Great Britain. The reason why all rain is not preceded by a watery sky is because there are other sources of rain besides a cyclone, which are preceded by a different set of weather-signs. Such is the whole theory of prognostics.
The same reasoning which applies to a watery sky holds good for every other cyclone-prognostic. We shall have explained why any prognostic portends rain when we have shown that the kind of sky or other appearance which forms the prognostic belongs to the front of the rainy portion of a cyclone. Conversely we shall have explained why any prognostic indicates finer weather when we have shown that the kind of sky belongs to the rear of a cyclone. It will be convenient, therefore, to describe the weather in different parts of a cyclone, and the appropriate prognostics together.
First, to take those prognostics which depend on qualities common to the whole front of the cyclone, viz., a falling barometer, increased warmth and damp, with a muggy, uncomfortable feel of the air, and a dirty sky.
From the increasing damp in this part of a cyclone, while the sky generally is pretty clear, cloud forms round and "caps" the tops of hills, which has given rise to numerous local sayings. The reason is that a hill always deflects the air upward. Usually the cold caused by ascension and consequent expansion is not sufficient to lower the temperature of the air below the dew-point; but when very damp, the same amount of cooling will bring the air below the dew-point, and so produce condensation.
From the same excessive damp the following may be explained: "When walls are more than usually damp, rain is expected." The Zuñi Indians in New Mexico say that "when the locks of the Navajos grow damp in the scalp-house, surely it will rain." From this we may assume that scalps are slightly hygroscopic, probably from the salt which they contain. Also, owing to excessive moisture, clouds appear soft and lowering, and reflect the glare of iron-works and the lights of large towns. With the gloomy, close, and muggy weather, some people are troubled with rheumatic pains and neuralgia, old wounds and corns are painful, animals and birds are restless, and drains and ditches give out an offensive smell.
A glance at the diagram will show that the barometer falls during the whole of the front of the cyclone. Therefore the explanation of the universally known fact that the barometer generally falls for bad weather is, that both rain and wind are usually associated with the front of a cyclone. When we discuss secondaries, we shall find a kind of rain for which the barometer does not fall; and in our chapter on forecasting for solitary observers we shall explain why it sometimes rains while the barometer is rising, and why there is sometimes fine weather while the mercury is falling.
Now, to take prognostics which belong to different portions of the cyclone-front. By reference to Fig. 2 it will be seen that in the outskirts of the cyclone-front there is a narrow ring of halo-forming sky. Hence the sayings: "Halos predict a storm (rain and wind, or snow and wind) at no great distance, and the open side of the halo tells the quarter from which it may be expected." "Mock suns predict a more remote and less certain change of weather."
Inside the halo sky comes the denser cloud which gives the pale watery sun and moon. Still nearer the center we find rain, first in the form of drizzle, then as driving rain. In the left front we find ill-defined showers and a dirty sky.
We have now come to the trough of the cyclone. The line of the trough is often associated with a squall or heavy shower, commonly known as "a clearing shower." This is much more marked in the portion of the trough which lies to the south of the cyclone's center than on the northern side.
Then we enter the rear of the cyclone. The whole of the rear is characterized by a cool, dry air, with a brisk, exhilarating feel, and a bright sky, with hard cumulus cloud. These features are the exact converse of those we found in the cyclone-front. In the cloud-forms especially we see this difference. All over the front, whether high up or low down, whether as delicate cirrus or heavy gloom, the clouds are of a stratified type. Even under the rain, when we get a peep through a break in the clouds, we find them lying like a more or less thick sheet over the earth. All over the rear, on the contrary, clouds take the rocky form known as cumulus; cirrus is almost unknown in the rear of a cyclone-center in the temperate zone.
In the exhilarating quality of the air we find the meaning of the proverb, "Do business with men when the wind is in the northwest. A northwest wind belongs to the rear of a cyclone, and improves men's tempers, as opposed to the neuralgic and rheumatic sensations in front of a cyclone, which make them cross.
As to the details of the different portions of the rear. Immediately behind the center small patches of blue sky appear. Farther from the center we find showers or cold squalls; beyond them, hard detached cumulus or strato-cumulus; still farther the sky is blue again. In the south of the cyclone, near the outskirts, the long, wispy clouds known as windy cirrus and "mares' tails" are observed. These indicate wind rather than rain, as they are outside of the rainy portion of the cyclone.