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LEARN ONE THING EVERY DAY JULY 1 1916 SERIAL NO. 110

THE MENTOR

THE WEATHER

By C. F. TALMAN Of The United States Weather Bureau

DEPARTMENT OF SCIENCE $3.00 PER YEAR

FIFTEEN CENTS A COPY

Old Probabilities

Shall tomorrow's weather be fair or foul? Blow wind--blow moistly from the South, for I go afishing. "Nay, good friend," exclaims the golfer, "the day must be dry and the wind in the west." The farmer moistens his finger and points it toward the sky. "Rain, come, quickly, for my crops," is his prayer. But the maiden's voice is full of pleading: "Let the sun shine tomorrow that my heart may be light on my wedding day."

* * * * *

And so, through the days and seasons, humanity with all its varied needs, turns anxiously, entreatingly to Old Probabilities. And how is it possible for him to satisfy the conflicting demand? He may, on the same day, please the farmer in the West, the fisherman in the South, the golfer in the northern hills, and the bride in the eastern town. But how can he suit them all in one locality on a single day? Old Probabilities is willing and he loves humanity, but his powers and privileges are limited. There are those who say that it is due to the kind endeavors of Old Probabilities to satisfy everybody that our weather has at times become so strangely mixed.

* * * * *

Old Probabilities is a gentle family name and came out of the affection of the people. The name was a matter of pleasantry. It was given to the Chief of the United States Weather Bureau when the department was first established by Congress, and its source lay in the phrase, "It is probable," with which all the weather predictions began. But Old Probabilities, genial prophet and lover of his fellow men, is passing away, for the officer who organized the Weather Bureau became in time displeased with the name and changed the form of the daily prediction so as to read, "The indications are." The phrase is formal and severe. There is naught but cold comfort in it. Our hearts turn back fondly to Old Probabilities and his friendly assurance: "It is _probable_ that tomorrow will be fair."

THE WEATHER

By CHARLES FITZHUGH TALMAN

_Librarian of the U. S. Weather Bureau_

THE MENTOR . DEPARTMENT OF SCIENCE . JULY 1, 1916

_MENTOR GRAVURES_

CENTRAL OFFICE OF THE U. S. WEATHER BUREAU, WASHINGTON, D. C. A SIMPLE WEATHER STATION A MAJESTIC CUMULUS CLOUD THE OBSERVATORY ON MONTE ROSA LAUNCHING A METEOROLOGICAL KITE THE EFFECTS OF SNOW AND ICE--THE CAMPUS, PRINCETON UNIVERSITY

[Entered as second-class matter, March 10, 1913, at the postoffice at New York, N. Y., under the act of March 3, 1879. Copyright, 1916, by The Mentor Association, Inc.]

It is easy to lay too much stress upon the unimportant aspects of weather. It furnishes a bit of conversation over the teacups; it accentuates the twinges of rheumatism; it spoils a holiday. All this, however, is mere byplay.

The real work of the weather--the work that explains the existence of costly weather bureaus, such as the one upon which our Government spends more than a million and a half dollars annually--is momentous beyond calculation. Consider such facts and figures as these:

The head of the British Meteorological Office recently declared that bad weather costs the farmers of the British Isles about one hundred million dollars a year. In our own country it has been estimated that a difference of one inch in the rainfall occurring during July in six States means a difference of two hundred and fifty million dollars in the value of the corn (maize) crop. The world over, the damage wrought by hail-storms is said to average about two hundred million dollars a year. In the city of Galveston a single hurricane once destroyed twenty million dollars' worth of property and six thousand human lives. Thus we might proceed indefinitely.

The fact is that man's welfare is conditioned to an enormous extent and in an endless variety of ways by the vicissitudes of the atmosphere; hence the study of weather--meteorology--is one of the most important of sciences. It is also one of the most strikingly neglected!

At the office of the Weather Bureau in Washington there is a meteorological library of some thirty-five thousand volumes. But meteorological libraries are rare; meteorological books are scarce in other libraries; and meteorologists are so uncommon that whoever declares himself one is likely to be asked, "What _is_ a meteorologist?"

The "meteors" studied by the meteorologist are not shooting stars, but the phenomena of the atmosphere,--rain and snow, cloud and fog, wind and sunshine, and whatever else enters into the composition of weather and climate.

THE ATMOSPHERE

The ocean of air in which human beings live, even as deep-sea fishes live at the bottom of the liquid ocean, is called the _atmosphere_. Unlike the liquid ocean, it diminishes rapidly in density from the bottom upward. At an altitude of three and one-half miles it is only half as dense as at sea-level. This is higher than the highest permanent habitations of man. Mountain-climbers and balloonists have attained greater altitudes; but above a level of about five miles the air is too greatly rarefied to support life. Balloonists who ascend still higher must carry a supply of oxygen with them. A little above the ten-mile level the air is only one-eighth as dense as at sea-level. The atmosphere extends at least 300 miles above the earth, at which height its density is computed to be only one two-millionth as great as at sea-level.

The weather with which human beings are concerned may be said to extend upward seven or eight miles; _i.e._, to the level of the higher clouds. The layer of the atmosphere lying between sea-level and the upper cloud level has certain characteristics that distinguish it from the air above it, and is known as the _troposphere_.

The heating of the atmosphere by the sun is the beginning of all weather, and the temperature of the air is the most important weather element. As soon as we begin to study atmospheric temperature, we encounter a paradox. The heat of the air is all derived from the sun (except a minute quantity from the interior of the earth, and an infinitesimal quantity from other heavenly bodies), and it would therefore seem at first glance that the upper layers of the atmosphere should be warmer than the lower. Experience proves the reverse to be the case. A mountain overgrown with tropical vegetation on its lower slopes is, if high enough, crowned with eternal snows. A thermometer carried upward in the air shows under average conditions a fall of temperature of one degree (Fahrenheit) for every 300 feet of ascent. This fall of temperature with ascent continues to the upper limit of the troposphere, where the average temperature is something like 70 degrees below zero.

Above the troposphere is a region called the _stratosphere_, or _isothermal layer_, in which an ascending thermometer shows irregular and generally small changes of temperature--not infrequently a rise of temperature with ascent. The exploration of the stratosphere is one of the most fascinating fields of meteorological research, but lies somewhat beyond the scope of an essay on weather. It is carried out chiefly with the aid of small free balloons, some of which (sounding balloons) bear self-registering thermometers and other instruments, while others (pilot balloons) bear no instruments, but show by their movements the drift of the air currents. The greatest altitude ever attained by a sounding-balloon was 21.8 miles; by a pilot-balloon, 24.2 miles. The branch of meteorology dealing with the study of the upper air is called _aerology_.

Reverting to the temperature of man's environment, the reason why the atmosphere is warmest at the bottom is this: The sun's rays come to us from outer space in the form of vibrations in the ether, and warm the air to only a slight extent in passing through it. They are absorbed by the ground, and converted into heat waves. The air is then warmed by contact with the warm ground. Lastly, the warming of the lower air gives rise to air-currents, which distribute the heat through the atmosphere.

BAROMETRIC PRESSURE

If our weather were uniform, it would furnish little matter for conversation; in fact, would hardly be weather at all. Changeableness is the salient feature of weather, and to understand weather changes one must know something about barometric pressure.

Like all other forms of matter, the invisible air has weight. At sea-level it exerts a downward pressure averaging 14.7 pounds to the square inch. Atmospheric pressure is measured by means of an instrument called the _barometer_, in which the weight of the air is balanced against a column of mercury. As the height of the mercurial column varies with the pressure of the air, and is taken as the measure of the latter, we follow the practice of expressing pressure (a force) in linear units (inches or millimeters). This practice is retained even in the use of the aneroid barometer, which contains no mercurial column. Hence, when we say that the average barometric pressure at sea-level is 29.92 "inches," we are really expressing in a roundabout way the weight of the air at that level.

Barometric pressure not only varies somewhat regularly with altitude--diminishing as we ascend--but also less regularly from place to place in a horizontal direction, and from time to time at a given place. In studying the weather meteorologists frequently wish to compare the barometric pressures prevailing at a certain time at a number of places lying in the same horizontal plane. Given a system of meteorological stations scattered over a certain territory, the first step is to secure simultaneous readings of the barometers at these stations. Then, if the stations are at various altitudes, as they commonly are, corrections must be applied to the readings to reduce all to a common plane; the plane adopted for this purpose is sea-level. Since most stations are _above_ sea-level, and since atmospheric pressure diminishes with altitude, reduction to sea-level generally involves applying an _additive_ correction.

THE WEATHER MAP

Now please attend carefully to what follows; because I am going to attempt to put into a minimum number of words the essential facts concerning the _weather map_, the best clue to weather mysteries yet devised by man.

At about 200 stations of the Weather Bureau, distributed over the United States, the barometer and other meteorological instruments are read twice a day; viz., at 8 A. M. and 8 P. M., eastern standard time. The readings are promptly telegraphed in cipher to Washington, where they are entered on a map.

The barometer readings at the different stations, reduced to sea-level as just explained, will vary, say, from 29 to 31 inches. Lines, called _isobars_, are now drawn through places having the same pressure; the intervals between the lines corresponding to differences in pressure of one-tenth of an inch. Lines (_isotherms_) are also drawn to connect places having the same temperature, a little arrow at each station shows the direction of the wind at that point, and various other symbols are used to facilitate the interpretation of the map; but the isobars are more important than anything else.

Here is the weather map for the morning of January 9, 1886. The solid curved lines are isobars, representing barometric pressures ranging all the way from 28.7 to 30.8 inches. It will be seen at a glance that these lines tend to assume roughly circular forms, inclosing regions where the pressure is lower or higher than the average. Moreover, the little arrows (which "fly with the wind") show that the winds round a center of low pressure tend to blow in a direction contrary to that followed by the hands of a clock (in the southern hemisphere the reverse is true), but instead of blowing in circles are inclined somewhat inward toward the center. Round a center of high pressure (in the northern hemisphere) the typical circulation of the winds is exactly opposite ("clockwise," and inclined outward), though the accompanying map does not show this particularly well.

An area of low pressure, with its system of winds, is called a _cyclone_, or _low_. An area of high pressure, with its system of winds, is called an _anticyclone_, or _high_. Note that a cyclone is not necessarily a storm, though the one shown on this map, with its center not far from New York City, was a very violent storm, which, when this map was drawn, was sweeping up the Atlantic coast. (Popular usage applies the term "cyclone" to the tornado.) The strength of the winds in a cyclone depends upon the contrast in barometric pressure between its center and its outer border. A cyclone with crowded isobars always has strong winds; when the isobars are widely spaced the winds are gentle.

These areas of low and high pressure, in addition to their movements about their centers, move bodily across the country, in a general west-to-east direction, at an average speed of over 500 miles a day. This double movement may be compared to that of a carriage-wheel, rotating and advancing at the same time. Most of our cyclones enter the country from the Canadian North-west--though many come from other regions--and nearly all of them pass off to sea in the neighborhood of the Gulf of St. Lawrence. Their route across the country varies greatly, depending in part upon the season.

THE WEATHER IN CYCLONES AND ANTICYCLONES

Barometric pressure is not an element of weather, in the ordinary sense of the term, since the fluctuations of pressure that occur in the human environment are entirely inappreciable to the senses. We have seen, however, that pressure is intimately related to wind, which is a weather element of much importance. In noting that systems of high and low pressure are constantly traveling across the country, and that they are accompanied by winds having fairly definite characteristics in relation to each, we have taken an important step toward bringing order out of the (to the uninitiated) chaotic sequence of weather. Obviously, a system of telegraphic weather reports makes it possible to keep close watch of these wind systems, and, from their locations on today's weather map, to form some idea where they will be tomorrow. Thus the weather forecaster is enabled to give notice of the imminence of those violent winds that destroy life and property at sea, and, to a less extent, on land. There is an element of uncertainty in such predictions--since storms, unlike railway trains, are not confined to fixed routes and regular schedules--but the practised forecaster acquires an instinct that helps him to forestall their vagaries.

Now what is true of wind is also true to a certain extent of the other elements of weather,--they bear typical relations to the distribution of atmospheric pressure. Cyclones are usually preceded by rising temperature and accompanied by cloudiness and rain or snow; anticyclones are usually preceded by falling temperature and attended by fair weather.

Referring again to the map of January 9, 1886, and following the course of the isotherms, or temperature lines, we see that abnormally cold weather prevailed over the Middle Western and Southern States. The isotherm of zero dips far south across northern Texas, Arkansas, Mississippi, Alabama, and Tennessee; while in the upper Mississippi and Missouri Valleys the temperatures were from 20 to 40 degrees below zero. These regions were, in fact, in the grip of a severe "cold wave," which had entered the country a day or two before, preceding the anticyclone here seen central north of Dakota. Cold northwesterly winds were sweeping over the Great Plains, and as far south as the Gulf.

The same map shows typical weather accompanying the cyclone central on the Atlantic coast. From the seaboard west to the Mississippi Valley rain or snow had fallen within the previous twenty-four hours (indicated by shading), and snow (indicated by S) was falling at the moment of observation at a majority of stations within this area. Elsewhere in the same region the weather was cloudy.

The foregoing remarks indicate in a general way the significance of the weather map and the principles upon which scientific weather predictions are based. The endless procession of highs and lows brings to any place on the map constant alternations of heat and cold, storm and sunshine. The forecaster watches the procession, and draws his inferences as to what will happen in this or that part of the country within the next day or two (forty-eight hours is about the limit of his outlook). "Long-range" forecasting is still a thing of the remote future. Forecasts for a week in advance, are, indeed made by the Weather Bureau with the aid of reports from a chain of stations extending round the globe, but these are in very general terms.

In January, 1914, the Bureau began publishing a "daily weather map of the Northern Hemisphere." This publication is, at present, suspended on account of the war.

SOME WEATHER MISCELLANIES

It would require a book, rather than a brief essay, to describe all the vicissitudes of weather, and many books that attempt to do this have been written.[A] We have space here only to mention a few important features of the weather met with in our own country.

[A] See "Brief List of Meteorological Textbooks and Reference Books," 3d ed., by C. Fitzhugh Talman. For sale by the Superintendent of Documents, Washington, D. C. Price 5 cents.

The southern and southeastern part of a cyclone, some hundreds of miles from the center, is a favorite breeding-ground for _thunderstorms_ and _tornadoes_. Thunderstorms of the type known as "heat thunderstorms" also occur with no special relation to cyclonic centers in regions where the ground has been intensely heated. In either case the storm is built up by rapidly ascending air, which cools and condenses its water vapor, first into enormous clouds (_cumulo-nimbus_, or "_thunderheads_"), and then into rain, frequently accompanied by hail. It would be necessary to go to some length to explain the familiar electrical manifestations of the thunderstorm--some points, indeed, are not perfectly clear to meteorologists--but it should be stated that these are always the result, not the cause, of the storm. _Lightning_ is an electrical discharge between cloud and earth, or cloud and cloud, and _thunder_ is simply the violent soundwave set up by the sudden expansion of the heated air along the path of the discharge,--the same acoustic phenomenon that accompanies an ordinary explosion.

A _tornado_ (popularly miscalled a "cyclone") is an extremely violent vortex in the air, usually less than 1,000 feet in diameter. Besides its very rapid rotary motion, it has a progressive motion at a speed averaging forty or fifty miles an hour. Its position at any moment is marked by a black funnel-shaped cloud, which grows downward from the sky and does not at all times reach the earth. A waterspout at sea is an identical phenomenon, though usually less violent. Along its narrow path the tornado demolishes everything,--wooden houses are blown to splinters, trees uprooted or stripped of their branches, structures of heavy masonry laid in ruins. Something like a hundred lives are lost each year in these storms, on an average, and one of them (St. Louis, May 27, 1896) destroyed thirteen million dollars' worth of property.

A _blizzard_ is a high, cold wind, accompanied by blinding snow, which in winter sometimes blows out of the front of an advancing anticyclone, especially in our North-Central States. A similar wind, with or without snow, is called in Texas a _norther_.