Popular Science Monthly/Volume 53/July 1898/Weather Forecasts: the Manner of Making Them and their Practical Value
|WEATHER FORECASTS: THE MANNER OF MAKING THEM AND THEIR PRACTICAL VALUE.|
WE are not always conscious of the great influence which the weather exerts on our affairs. Fair weather gives zest and interest to everything, while dark clouds depress us and take the life and sparkle from that which was before most attractive. In cases of severe illness the weather sometimes makes all the difference between life and death. Our emotions are largely under its control. The farmer's first thought in the morning and his last consciousness at night relate to the weather. The sailor, the pleasure seeker, the shopper, and the builder are all deeply concerned with the weather, to say nothing of the children, whose lives are quickly limited to the four walls of the house on the approach of bad weather.
It is a matter of so much concern that our Government spends annually about nine hundred thousand dollars for the maintenance of its Weather Bureau, in order that we may know a few hours beforehand what to expect of the elements.
The first attempt at scientific forecasting of the weather was the result of a storm which, during the Crimean War, November 14, 1854, almost destroyed the fleets of France and England. As a storm had raged several days earlier in France, Vaillant, the French Minister of War, directed that investigations be made to see if the
two storms were the same, and if the progress of the disturbance could have been foretold. It was demonstrated that the two were in reality one storm, and that its path could have been ascertained and the fleet forewarned in ample time to reach safety.
In order to intelligently predict the weather for even a small section of the country it is necessary to know the conditions that exist over the whole United States, and over as much of the rest of the world as possible. The Weather Bureau receives observations from one hundred and fifty-four stations. There are four hundred and eight-five miles of telegraph lines and submarine cables operated by the Weather Bureau for connecting with such points as Cape Hatteras, Nantucket, and islands in the Great Lakes and on the Pacific coast.
To understand the plan of work of the Weather Bureau it will be necessary to enter to some extent into the laws of the weather; but it will not be found difficult to see how the forecasters of the bureau, with their greater knowledge in the same directions, are able
to foretell the weather correctly, as they do, in over eighty-two per cent of the predictions. Strange as it may seem, the weather does have laws, laws that are inflexible, so that, if the conditions are correctly understood, the changes in the near future can be confidently predicted. All of these laws have not, however, been discovered, and some that are well known have yet to be satisfactorily explained.
Primarily, the winds result from the sun heating the tropics to a much greater extent than it does the polar blows from each side, and a descending current of air near parallel 32 of each hemisphere, from which the air flows north and south.causing the air to rise in the tropics and flow toward the poles at a high altitude; from which regions it returns toward the equator along the surface of the earth. Owing to the rapidly lessening circumferences of the parallels of latitude toward the poles and other causes, there is an ascending belt of air near the parallel 64, toward which the surface wind
Friction between the surface of the earth and the atmosphere tends to carry the air with the earth in its rotation. As the velocity of the earth's surface is nothing at the poles and increases toward the
equator, those winds which blow toward the equator will lag behind and have a westerly direction, and those that blow toward the poles will retain their greater velocity of the lower latitudes and travel faster than the more northern parallels, resulting in an eastward direction over the earth's surface.
In the temperate zone of the northern hemisphere there is a general northeasterly drift of the surface atmosphere at a fairly regular rate of motion, and this causes our storms almost without exception to travel from west to east. This eastward tendency of all atmospheric disturbances is the basis of all predictions.
In the great ocean of air, as in the ocean of water, there are constantly occurring waves and hollows, or areas where the air is piled to an unusual height, showing increased pressure on the barometer, and areas where the height and pressure are less than the normal. These high and low areas, or "highs" and "lows," as they are technically known, travel in a general northeasterly direction. In Fig. 2 is shown an actual case of a low between two highs in the United States, and it is extremely interesting to notice the laws of the winds around them. The finer, oval lines are used to connect all points having equal barometric pressure. They are known as " isobars." The heavy, arrow-tipped lines show the actual direction of the wind as it was observed on this occasion. It will be seen that the winds flow spirally outward from the highs in the direction of motion of the hands of a watch, while they blow from the high spirally inward toward the center of the low in the opposite direction to that of the watch's hands. These same directions will always be found about a well-defined high or a low. A severe storm and a low will always be found together, and the law of the winds about a low enables navigators to judge of the direction of the center of the storm and to steer away from it in time to avoid disaster. The lows are called cyclones, because of the inward direction of their winds; and the highs are known as anticyclones, because the winds resulting from them flow outward.
The highs are caused by descending currents from the upper air due to increased density which results from cooling by radiation of heat from the upper air. The lows are caused by the air absorbing heat from large areas of the earth's surface where the sun has acted strongly. The heated air ascends and flows outward at the upper levels, reducing the barometric pressure. If the air is moist.
when it rises under the action of heat, its pressure lessens until the moisture is condensed; and the liberated latent heat serves further to heat the air and increase the upward flow. The condensed moisture usually appears as clouds or rain.
The highs are found to enter the United States from only two points. In the winter they usually originate in Alberta to the north of Montana, and they follow either of two well-defined paths in their eastern course. One path leads southeasterly to the Georgia coast, from which locality the high follows the warm, moist air over the Gulf Stream to the Gulf of St. Lawrence, there to pass beyond our observation (Fig. 3). The other path extends across the Great
Lakes and New England to the same destination. In summer the larger number of highs enter our country from the Pacific Ocean about the latitude of Oregon, and either pass northward along the Pacific slope, whence a crossing is made over the Rockies to the northern circuit from Alberta, or through the Salt Lake region to the southern circuit from Alberta. Having determined the circuit which a high will follow, and having found its rate of travel from observation, it becomes largely a matter of calculation to foretell its progress across the continent.
While the highs have only two points of entrance to the United States, the lows or storms originate in nine different districts throughout the country. In Fig. 3 the heavy full lines represent the paths of the highs. The lighter full lines indicate the origins and paths of the lows, and the heavy dotted line is the axis or path of the middle of the cold waves. It will be seen that the lows follow the two circuits, northern and southern, of the highs, and that they occasionally cross over from the southern to the northern circuit. Some of them originate in the West Indies and travel up the Atlantic coast.
Fig. 4 shows the nine districts into which the United States has been divided with reference to the origins of storms and also the paths of the storms. All the paths cross New England, and the chart shows what a harsh, changeable climate this section has. There is no other region on the face of the earth, not even excepting Siberia, where there are such sudden and violent changes of weather as in New England. All storms that visit the United States cross New England and pass off toward the St. Lawrence Gulf, if they do not die out on the way. Lows move on an average at the rate of twenty-five miles per hour.
We have now seen that highs and lows have definite points of origin or entrance into the country, and that they follow well-established tracks. It is clear, therefore, how their location a few hours in advance may be estimated.
Highs are not usually accompanied by rain, but the temperature falls in advance of them as they pass over the country. The lows, on the contrary, are the storms, and usually carry rain with them. The greatest rainfall is usually to the northeast of the center of the low, and the low tends to move toward the point where there is the heaviest rain. The temperature of the air rises in the regions to the east of the low, and falls to the west of it.
Having located the highs and lows that exist and determined what paths they are likely to follow, from the laws that have been
explained, it may be seen how the direction of the wind, the occurrence of rain, and the changes in temperature can be predicted (Fig. 5). fig. 5 is a reproduction of a newspaper weather map. The dotted lines connect all points having the same temperature, temperatures ten degrees apart being chosen. The full lines are lines of equal barometric pressure, there being a line for each one tenth of an inch difference in pressure. The arrows fly with the wind. Rain is indicated by the shaded areas. It can be observed how the winds blow with the hands of a watch around the high, which is central over New Mexico, and in the opposite direction about the low, which is central over Sault Ste. Marie. The temperature is higher to the east of the low, and falls behind the low and before the high in their eastward progress. The rain is seen accompanying the low and to the east of it.
For the purposes of forecasting it is necessary to know the barometric pressure, direction of the wind, and state of the skies. All this information is telegraphed to the Weather Bureau twice daily from
each of the stations of observation, and all other telegraphic business must give way to this for the time being. Each message is, by means of a cipher, usually expressed in ten words or less. The observations are taken at 8 a. m. and 8 p. m., Eastern time. The results are exchanged with the Canadian Weather Bureau. The principal stations are provided with recording instruments, so that a continual record is kept of all these features of the weather. While the instruments appear complicated, they are based on principles that are easy of comprehension.
The barograph consists of a series of corrugated sheet-metal cells placed one upon the other and connected at the upper end of the series to a system of magnifying levers that operate a pen. The pen point rests upon a sheet of paper which is held on a cylinder
having clockwork within it to give it a slow rotation. The varying pressure of the atmosphere causes the cells to contract and expand, and this motion, transmitted through the levers, causes the pen to trace a line on the graduated paper whose co-ordinates represent the pressure of the atmosphere at any given time.
The thermograph is somewhat like the barograph in principle. The element that is affected by the temperature is a metal cell that has the form of a curved and flattened tube, one end of which is secured to the framework and the other end connected by a link to a lever carrying a pen point. It has also a clock-driven cylinder with its graduated sheet of paper. The tube is filled with alcohol, and, as the liquid expands, it straightens the tube and moves the pen over the paper, making an irregular line that represents the ever-changing temperature.
The maximum and minimum thermometers record the highest and lowest temperatures respectively in the twenty-four hours. The former is an ordinary mercurial thermometer with the addition of a contraction in the tube just above the bulb. The heat forces the column of mercury past the contraction, but it can not return to the bulb when the temperature lowers. The minimum thermometer is an alcohol thermometer that has a colored glass float in the liquid. When the alcohol contracts, the skin of the liquid carries the float down with it; but, when the temperature rises, the float remains to mark the lowest temperature.
The amount of moisture present, or the humidity of the air, is determined by a comparison of dry and wet bulb thermometers. They are both ordinary thermometers; but the bulb of the latter is covered with muslin that is wet. In the latest form of instrument the thermometers are mounted on arms carried by a shaft that is rotated by a crank which is geared to the shaft. The motion of the shaft rotates the thermometers in vertical planes and causes the water in the muslin to evaporate more or less rapidly according to the amount of moisture in the air. This evaporation lowers the temperature of the thermometer; and, from tables constructed after long experiments, the degree of moisture can be determined by the difference in temperature-between the two thermometers.
The direction and speed of the wind, the hours of sunshine and cloud, and the amount of rainfall are recorded by the "triple register." The instrument has a cylinder on which is carried a graduated sheet of paper for the record. The cylinder is supported by a horizontal shaft that is turned by clockwork. On the shaft is secured a spiral wire that engages grooves in a bearing of the shaft; so that, as the shaft turns, it and the cylinder are moved along. This would cause a stationary pencil to trace a spiral on the cylinder. The shaft carries two arms parallel to itself that pass
through a yoke which is turned by the clock, this arrangement permitting the shaft to recede from the clock under the action of the spiral. The cylinder revolves four times in twenty-four hours, so that each record passes across the paper four times.
The wind vane is an arrow-shaped vane mounted on a vertical shaft. The tail of the arrow is gradually broadened laterally to give it greater steadiness. A sleeve is fastened on the shaft of the wind vane and is provided with four cams, each of which extends over three eighths of the circumference and the central points of which are a quarter of a circle apart. Four contact levers, having rollers in their ends, hear on these cams; and, when raised
by the cams, the levers touch contact springs having a wire from each that runs to one of four magnets beside the "triple register." The levers and magnets are all connected with batteries. The magnets act on armatures that are carried by levers, one for each magnet, which have pens that make a dot on the paper of the cylinder every time the magnet acts. One or two contact levers are always elevated by the cams to make a contact. The cams are so arranged that only one cam acts when the vane is directed to a cardinal point of the compass; but, when it is between two such points, two of the cams are acting. Thus the eight principal directions of the wind are indicated by the four pens. The clock breaks the circuit of the vane except for an instant each minute, so that the pen in action makes a dot each minute.
The velocity of the wind is registered by an instrument consisting of a vertical shaft which carries four horizontal arms having sheet-metal cups on their ends. The wind acts more strongly on the open faces than on the backs of these cups, and causes them to revolve the shaft. Through gearing connected with the shaft, pins on an index close an electric circuit for every mile the wind travels, and the current through a magnet of the triple register causes a sidewise motion of a pen acting on the paper. The number of jogs in an hour thus made in the line traced by the pen gives the velocity of the wind. The ninth and tenth pins are connected, so that one long jog occurs in the record for every ten miles, making it easy to count the total.
The sunshine recorder is constructed on the principle of the differential thermometer. Inside of a vacuum tube is a tube having a bulb formed on each end, and the inner tube extends into the lower bulb nearly to its bottom. Both bulbs contain air; and the lower one, which is coated with lampblack, has a quantity of mercury in its lower part. The mercury also extends up into the tube. Two wires enter the opposite sides of the inner tube between the bulbs, and these wires form part of the electrical circuit of the one of the magnets of the triple register which magnet occupies a side of the triple register by itself. The armature of the magnet, through a pawl-and ratchet mechanism, gives the pen lever of this magnet a step-by step motion, first to one side and then to the other. This action takes place when the sun, shining on the heat-absorbing lampblack, causes the air and mercury in the lower bulb to expand and force the mercury up the inner tube until it completes the electrical circuit between the wires in the inner tube. The clock breaks the circuit except for an instant each minute. When it is cloudy the pen simply traces a straight line.
The recording rain gauge consists of a cylindrical casing supporting an open funnel above and a reservoir below. Beneath the mouth of the funnel is a pivoted tray or "bucket" divided into two compartments and pivoted so that it can be tipped to bring one compartment under the funnel mouth; and, when that is filled with rain water, it will be overbalanced and tip down, thus pouring its water into the reservoir and bringing the empty compartment under the mouth of the funnel. This tipping of the bucket momentarily closes an electrical circuit including the sunshine-recording magnet of the triple register, and causes the pen to follow a motion in general like the sunshine record, but so irregular as to time, and having either more or less steps in a given time according to the rapidity of the rainfall, that it is Fig. 12.—The Recording Rain Gauge. easily distinguished from the sunshine record. The clock does not interfere with the circuit of the rain gauge.
As the atmosphere extends upward some forty miles, it is evident that observations made at the usual height of a few score feet can not give much idea of the condition existing in the general mass of air, and it is surprising that such successful forecasts are made from these meager data. For some time past efforts have been made to up recording instruments of kites or trains of kites. The kite shown in Fig. 14 is a typical Weather Bureau kite. Its weight is but six pounds, and yet it presents to the wind an area of fifty square feet, and has lifted a weight of one hundred pounds in a wind of twenty-five miles an hour. An altitude of seven thousand feet has been reached with the kites. A special instrument is made for attachment to the kites that electrically records the temperature, pressure, and humidity of the air and the direction of the wind. The combined weight of the instrument and battery is but two pounds and a half. When it is considered that, owing to the weight of the air above, the bulk of the air is comparatively near the surface of the earth, it will be seen how very valuable are observations made at the heights reached by these kites.
It may well be asked: Of what practical value are the Weather Bureau reports and prophecies? As a matter of dollars and cents, does the expenditure of the money necessary to support the Weather Bureau pay? Upon investigation it will be found that it is an immensely profitable investment.
Reports are received weekly from eight thousand special correspondents concerning the effects of the weather on the crops, and these reports serve to set A Weather-Bureau Kite. a value on the products which is in just proportion to the supply, and to enable plans and contracts for the future to be made with reasonable certainty. The records, running back as they do for many years, enable invalids, manufacturers, and agriculturists to find with certainty the locality that is best suited to their needs. The investigations of the Weather Bureau have been directed toward determining the parts of the Unites States where the most constantly humid atmosphere may be found in order that cotton manufacturers may know where their spinning can be most successfully done. Forewarnings of frost enable the truck farmers to protect their produce with a mantle of smoke from a smoldering fire.
The terrible cyclones of the West are frequently foreshadowed by the Weather Bureau long enough in advance to enable people to place themselves and their property in the most protected conditions; and, during the floods of the Mississippi and Missouri Valleys, incalculable savings of life and property result from their warnings. Before the days of the bureau the West Indian hurricanes came unannounced, and sometimes two thousand lives were lost in a single storm. Under the warnings of the Weather Bureau, three such storms have passed in succession without the loss of a single life, and the property saved from one storm in the form of vessels detained
in port would support the service for two years. At Buffalo, in the winter of 1895-'96, by forecasting six very severe storms, one hundred and fifty vessels, valued at seventeen million dollars and carrying eighteen hundred persons, were held in port by the warnings. Every one remembers how the American liner St. Paul went ashore near Long Branch, February 2, 1896. A dispatch from the forecaster informed the captain that at such an hour the wind and tide would present the most favorable opportunity for getting the steamer off. At that time everything was in readiness, and a successful attempt was made. A ship and cargo valued at several millions of dollars were thus saved largely through the effort of the Weather Bureau.
By predictions of the heavy snows, many railroads are saved from complete and lasting blockade, and the immense cattle herds of Kansas, Nebraska, Indian Territory, and Texas are enabled to reach places of safety.
The Weather Bureau not only justifies its existence by its services, but it deserves sympathy rather than ridicule when in the face of such difficulties itsare wrong. The investigations of its scientists are constantly improving its methods, and the error in its predictions is being reduced to very satisfactory proportions.