Headline: How the Weather is Foretold.
In former times the chief herald of the weather was the almanac, which ambitiously prophesied a whole year of cold and heat, wet and dry, dividing up the weather quite impartially, if not always correctly.
But the almanac, good as it was now and then, and the weather-wise farmers, correct as sometimes they might have been, were not always able to impart exact information to the country; and they have been thrown quite into the shade of late by one who is popularly known under the somewhat disrespectful title of “Old Prob,” or “Old Probabilities.” He has become the Herald of the Weather to the sailor near the rocky dangerous coasts; to the farmer watching his crops, and waiting for good day to store them; to the traveler anxious to pursue his journey under fair skies; and to the girls and boys who want to know before they start to the woods for a pic-nic, what are the “probabilities” as to rain.
Everyone who reads the daily papers is familiar with the “Weather Record,” issued from the “War Department, Office of the Chief Signal Officer,” at Washington. These reports give a general statement of what the weather has been for the past twenty-four hours all over the country, from Maine to California, and from the Lakes to the South Atlantic States; and then the “probabilities” for the next twenty-four hours over the same broad territory. The annual reports of the Chief Signal Officer show that only in comparatively few instances do these daily predictions fail of fulfillment.
The reasons these prophecies are so true is simple and yet wonderful. The weather itself tells the observer what the weather is going to do, some time in advance, and the telegraph sends the news all over the country from the central signal office at Washington.
We shall see presently how the weather interprets itself to “Old Probabilities.” Although it has proved such a fruitful subject of discourse in all ages, yet I am afraid many people who pass remarks upon it do not really think what the weather is made of. Let us examine its different clements.
The atmosphere has weight, just as water or any other fluid, although it seems to be perfectly bodiless. We must comprehend that the transparent, invisible air is pressing inward toward the center of the earth. This pressure varies according to the state of weather, and the changes are indicated by an instrument called a barometer. Generally speaking the falling of the mercury in the tube of the barometer indicates rain, and its rise indicates fair weather. Sometimes the rise is followed by cold winds, frost and ice. What these changes really indicate, however, can be determined only by comparing the barometric changes at certain hours, in a number of places, very far apart. This is done by the Signal Service.
Observations are made at about one hundred and forty stations, in different stations, in different portions of the country, at given hours, and the results are telegraphed at once to Washington, where our faithful “weather clerk” receives them, reasoning that from them the “probabilities” which he publishes three times in every twenty-four hours.
But the atmosphere varies not only in weight but also in temperature. The thermometer tells us of such changes. Besides this, the air contains a great amount of moisture, and it shows as much variation in this characteristic as in the others. For the purpose of making known the changes in the moisture of the atmosphere, an instrument has been invented, called the “wet bulb” thermometer. We are thus enabled to ascertain the weight of pressure, the temperature and the wetness of the air, and now it only remains for us to measure the force and point out the direction of the wind. This is done by the familiar weather vane and anemometer. The vane shows the direction, and the anemometer is an instrument which indicates the velocity of the wind.
It is by a right of superior understanding of all these instruments that the signal service officer is enabled to tell what the weather says of itself; for they are the pens with which the weather writes out the facts from which the officer makes up his reports for the benefit of all concerned. Thus however wildly and blindly the storm may seem to come, it sends messengers telling just where it arose, what course it will take, and how far it will extend. But it tells its secrets to those only who pay strict attention.
Source: Farmer’s Cabinet, Amherst NH, 9 October 1878; Vol 77, Issue 10, page 1.
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–United States Army Signal Corps–
