Harper's Round Table, March 3, 1896

by Various

BY W. J. HENDERSON.

What is the Weather Bureau? It is a branch of the national government service whose duty it is to make forecasts of the weather, to estimate and publish the probabilities twice in every twenty-four hours. Its headquarters are at Washington, and it is attached to the Agricultural Department. It was originally a part of the army, for on June 1, 1860, Congress passed an act establishing the Signal Service, and detailing a major and several signal officers to conduct it. In 1863 the Signal Corps was organized. It served through the war, and was then permitted to disband. It was reorganized in 1866, and the weather predictions were a part of its duties until recently. Now the weather service, or, to be more accurate, the Meteorological Bureau, is a separate service.

Its business is to predict the weather as nearly as it can. Most persons are of the opinion that it can do this accurately. At any rate, they blame the observers very severely when, owing to local causes, their predictions, intended to cover a large territory, are not fulfilled to the letter. If they predict showers followed by clearing weather in eastern New York, and it does not clear up in New York city till nine o'clock in the evening, inhabitants of the metropolis are very likely to say unkind things about the observers. They forget that the chief objects of this service are to furnish valuable information to mariners, to the great rice and cotton growers of the South, to the farmers, and to all other persons upon whose prosperity the weather has a potent influence. The fact that John Smith is caught in an unexpected rain and gets his new hat spoiled is not so important as the sailing of a ship, laden with valuable freight, into the teeth of a howling hurricane, of which she might have been warned. The government spends a good deal of money on this service. It costs $5000 to fit out a station, and the yearly allowance for incidentals alone is $500. This is exclusive of the pay of observers and the cost of telegraphing. And there are 182 of these stations at work now.

Twice a day, at 8 a.m. and at 8 p.m., the observations of the weather conditions are taken; and they are immediately telegraphed, in a cipher devised for the purpose, to Washington, at the headquarters. There the facts contained in the reports from the different parts of the country are collated, and the probabilities deduced from them. The bulletins which are printed in the newspapers are sent out, and also weather maps. On these maps are printed lines showing the areas over which certain variations of the barometer exist, and other lines showing the changes in temperature. If you understand the manner in which American weather operates, you can take these maps every day and make pretty good predictions yourself.

As I have said, it is from the local observations that the general predictions are made. In the city of New York the weather is studied away up on top of the tall building of the Manhattan Life-insurance Company. The Local Forecast Observer—that's his official title—is E.. B. Dunn, who, when this was an army service, was Sergeant Dunn. Now the irreverent newspapers call him "Farmer" Dunn. What he does in his office is what all the other observers throughout the country do in theirs. I am going to describe his methods as he described them to me, and then you'll know all about it.

The instruments used in observing the weather are the aneroid and cistern barometers, wet and dry bulb thermometers, wind vane and compass, anemometer and anemograph, and the rainfall. Of all these the barometer is probably the most important. The standard form of the instrument is a tube thirty-four inches long, closed at the top, exhausted of air, and immersed at the bottom in a cup of mercury. The purpose of the barometer is to measure the pressure of the atmosphere. In general, the mercury will stand high in the tube when the weather is fair, and low when it is foul. By noting the minute changes, measured on a graduated scale beside the tube, the observer reads the indications of the barometer. The words "fair," "change," etc., engraved on the front of the instrument are disregarded. They have no significance whatever. The rising or falling of the mercury in the tube is caused by the beginning of those atmospheric changes which precede a storm but are not discernible by our senses. The barometer discerns them for us, and gives warning of weather changes. Of course there are many different conditions which affect the instrument, and the weather observers are instructed in these matters. The aneroid barometer is round, like one of the cheap nickel-plated clocks that are so numerous, and the changes are indicated by a hand moving across a scale on the dial. The weight of the atmosphere is measured not by a column of mercury in a tube, but by the expansion and compression of a small metal box from which the air has been exhausted.

The thermometer, as the reader knows, measures the temperature of the air; and in all readings of the barometer the changes in temperature have to be taken into account. The weather observers use two kinds of thermometers, the dry and the wet bulb. The dry bulb is the ordinary form, which every one knows, and is used to measure heat and cold. The wet has the bulb wrapped in some absorbent material, which is kept soaked with water. Now you know, without my telling you, that the water will cool the bulb, and hence the wet-bulb thermometer will stand lower than the dry. That cold is caused by evaporation, and the evaporating power of the atmosphere depends upon the amount of moisture there is in the air. So you at once see that the difference between the readings of the wet and dry bulb thermometers indicates the amount of moisture in the air. This amount the observers express in percentages of 100; and thus we read of "humidity, 60 per cent." Under ordinary circumstances, when the humidity gets close to 100, the point at which the air is soaked with moisture, it is going to rain. The temperature, however, and also the wind, have a good deal to do with this. The form in which the weather observers use these two thermometers is called the whirling psychrometer. The two instruments are put on the end of an arm, which is fixed on an axle turned by a crank. The observer whirls this around a few times before reading the instrument, for the purpose of making the air act freely on the two bulbs.

The direction of the wind, as every one knows, is shown by a weather vane. Those which are used by the observing stations, however, have an attachment which automatically records on a sheet of paper every variation of the vane, so that the office has an account of the smallest changes of the wind during the twenty-four hours. The speed of the wind is measured by the anemometer. This consists of four half-spheres at the end of four horizontal arms, which centre on an upright axle. The force of the wind causes the arms to revolve, and it has been found that 500 revolutions equal one mile. If the arms revolve 3000 times in an hour, the wind is blowing six miles an hour. The revolving of the upright axle operates a contrivance by which the speed of the wind for every minute in the day is recorded.

The amount of rain which falls is measured in a way which shows what the depth of water would be on a level surface if it did not, in the natural order of things, run off. The rain is caught in a funnel 8½ inches in diameter, so placed as to be protected from all gusts of wind. The record is made in five-hundredths of an inch.

In addition to all these instruments the observers watch the well-known weather signs in the sky. Sunset and sunrise and the various changes in the appearance of the clouds are carefully studied. When a man has spent a year or two of his life in watching all these things, he can make a pretty safe prediction as to the weather for the next twenty-four hours. The Weather Bureau does not profess to foretell the conditions, except in special instances, for more than forty-eight hours.