The first daily weather maps were issued in connection with the Great Exhibition of 1851 in London. The data were collected by the Electric Telegraph Company and transmitted to London over its wires. These maps were published and sold daily (excepting Sundays) from Aug. 8 to Oct. 11, 1851. The first official weather map of the United States Weather Service was prepared in manuscript on Nov. 1, 1870, and on Jan. 14, 1871, the work of manifolding the maps for distribution was begun at Washington. Previous to the publication of this government map, Professor Cleveland Abbe had issued in Cincinnati, with the support of the Chamber of Commerce of that city, the first current weather maps published in the United States (Feb. 24 to Dec. 10, 1870). In France, daily weather maps have been published continuously since Sept. 16, 1863.

Two things are essential for the publication of a daily synoptic weather map; first, simultaneous meteorological observations over an extended area; and, second, the immediate collection of these observations by telegraph. The weather map of the United States is based on simultaneous observations made at about 150 stations in different parts of this country, besides several coÖperating stations in Canada, Central America, Mexico, and the West Indies. At each of our stations, whose location may be seen on any weather map, the Weather Bureau employs one or more observers, who, twice a day, at 8 A.M. and 8 P.M., “Eastern Standard Time,” make regular observations of the ordinary weather elements, i.e., temperature, pressure, humidity, wind direction and velocity, precipitation, cloudiness, etc. The instruments at these stations are all standard, but the completeness of the equipment varies according to the importance of the station. The 8 A.M. observations are the only ones now generally used in the preparation of weather maps. When the Weather Service was first established, tri-daily charts were for some time issued from the central office in Washington. On April 1, 1888, the number was reduced to two a day, and on Sept. 30, 1895, a further change was made, and now there is but one map a day.

The 8 A.M. observations, as soon as made, are corrected for certain instrumental errors, and the barometer readings are reduced to sea level. The data are then put into cipher, not for secrecy, but to facilitate transmission and to lessen the chances of error, and are telegraphed from all parts of the country to the central office of the Weather Bureau in Washington. Besides sending their own messages to Washington, all the important stations of the Weather Bureau receive, by a carefully devised system of telegraphic circuits, a sufficient number of the reports from other stations to enable their observers to draw and issue local weather maps.

The observations are received at the central office of the Weather Bureau in Washington by special wires, and are usually all there within an hour after the readings were made. As the messages are received in the forecast room, they are translated from the cipher back again into the original form, and the data are entered upon blank maps. The official charged with making the forecasts then draws upon the maps lines of equal temperature, lines of equal pressure, lines of equal pressure-change and temperature-change during the past 24 hours. These several sets of lines, together with those showing the regions of precipitation during the past 24 hours, furnish the necessary data on which the forecasts can be based. In other words, the forecast official has before him, on the several maps, a bird’s-eye view of the weather conditions over the United States as they were an hour before, and also of the changes that have taken place in these conditions during the preceding 24 hours. Thus, by knowing the general laws which govern the movements of areas of high and low temperature, of fair and stormy weather, across the country, he can make a prediction as to the probable conditions which any state or section of the country will experience in 12, 24, or 36 hours.

In a later chapter some suggestions will be given for studies of forecasting.

The forecasts made in Washington, and printed on the Washington daily weather map, relate to all sections of the United States, and include predictions of cold waves, killing frosts, storm winds, river floods, and the like, besides the ordinary changes in weather conditions. These forecasts, as soon as made, are at once given to the local newspapers and to the press associations. They are also sent by telegraph to all regular stations of the Weather Bureau, and to all stations at which cautionary or storm signals are to be displayed, along the Atlantic or Gulf coasts, and on the Great Lakes.

The Washington weather map is about 24 by 16 inches in size, and is newly lithographed each day. The total number of maps issued from the central office during the fiscal year ending June 30, 1898, was 310,250. In addition to these, there are now 84 stations of the Weather Bureau in different parts of the country, at which daily weather maps are issued and local forecasts made. These latter forecasts are made by a corps of local forecast officials, each of whom has to make the weather prediction for his own district. At first, and until within a few years, one predicting officer in Washington made all the forecasts for the country, but it was found better to have the country divided into geographical sections, over each one of which the meteorological conditions are fairly similar, and to have a local forecast official in charge of each section. These local forecast officials have the double advantage of being able to study the weather conditions over the whole country, as sent them by telegraph each morning, and also of knowing the special peculiarities of their own regions. This enables them to make more accurate predictions than can be made by an official who may be one or two thousand miles distant, in Washington.

The greater portion of the maps issued at the map stations outside of Washington are prepared by what is known as the chalk-plate process, suggested by Mr. J. W. Smith, local forecast official at Boston. This process is as follows: A thin covering of specially prepared chalk, ¹/₈ of an inch in thickness, is spread upon a steel plate of the size of the prospective weather map. On this chalk are engraved, by means of suitable instruments, the various weather symbols, the lines of equal pressure and of equal temperature, and the wind arrows. The plate is then stereotyped in the ordinary way, and printed on a sheet prepared for the purpose, which has a blank outline map of the United States at the top, and space in the lower half for the forecasts, summary, and tables.

The size of the chalk-plate map itself is 10 by 6¹/₂ inches; the size of the whole sheet, which includes also the text and tables, 16 by 11 inches. Weather maps prepared by the chalk-plate process are now issued from 28 of the 84 stations which publish daily maps. At the remaining stations the maps are prepared by a stencil process, the size of the map being 13¹/₂ by 22 inches. The total number of weather maps issued at the various stations during the fiscal year 1897-1898 was 5,239,300.

Besides recording the usual meteorological data, and publishing weather maps and forecasts, the various stations of the Weather Bureau serve as distributing centers for cold wave, frost, flood, and storm warnings. These warnings are promptly sent out by telegraph, telephone, and mail. Besides these usual methods of distributing forecasts, other means have also been adopted. In some places factory whistles are employed to inform those within hearing as to the coming weather; railway trains are provided with flags, whose various colors announce to those who are near the train fair or stormy weather, rising or falling temperature; and at numerous so-called “display stations,” scattered all over the country, the forecasts are widely disseminated by means of flags.