A further outcome of Maury’s enterprise was the holding, at his suggestion, and by invitation of the United States Government, of an International Maritime Conference, which met in Brussels in 1853, and worked out a world-wide plan for meteorological observations at sea. The work thus begun has since been carried on by the leading maritime nations of the world. In the United States the duty of gathering weather reports on a uniform plan from vessel masters has been intrusted, at different times, to the Hydrographic Office, the Signal Service, and the Weather Bureau. It is now performed by the last-named institution, through its Marine Division, but the Pilot Charts and books of sailing directions (“Pilots”) in which the compiled information is published, are issued by the Hydrographic Office of the Navy.
The modern successors of Maury’s Wind and Current Charts are, especially, the Pilot Charts for the different oceans issued monthly in Washington, and the monthly charts of similar character published by the British and German governments. Apart from these periodical publications, valuable collections of meteorological charts for oceans or smaller marine areas have been published by the British, German, Dutch, Indian, Japanese and other authorities.
The value of such publications has not been lessened by the gradual substitution of steam for sails on ocean-going vessels. While wind is no longer all-important, it is a factor in determining the speed, and hence the earning capacity, of all classes of ocean shipping, and the same is true of marine currents. Fog and drifting ice are, in general, more serious obstacles to steamers than to sailing ships. A glance at one of the Hydrographic Office Pilot Charts will suffice to show that these publications are indispensable to the mariner. On these charts we find, first of all, in the center of each five-degree square of latitude and longitude a “wind rose” showing the frequency of the winds that have been observed in that region from each of the cardinal points, and their average force from each direction. On the charts will also be found the routes recommended for full-power and low-power steamers and sailing vessels, lines of magnetic variation, tracks of storms in past years for the month in question, location and force of currents, average limits and prevalence of fog for the month, recent information about drifting ice and derelicts, descriptions of storm signals, and an abundance of other information of vital importance to the seaman.
OCEAN WEATHER MAP PREPARED FROM VESSEL REPORTS
JAN. 11, 1913, GREENWICH MEAN NOON
U. S. Weather Bureau
Solid black lines are isobars. Arrows fly with the wind, the center of the arrowhead marking the position of the vessel, and the number of feathers denoting the force of the wind on the Beaufort scale. Shading of the head shows degree of cloudiness.
Most of the material used in the preparation of the charts above described is obtained from a great corps of volunteer marine observers, who enter their observations at stated hours in forms provided for the purpose and send these records to the establishment in charge of the work at the end of each voyage. The forms furnished by the United States Weather Bureau prescribe only one regular observation a day, to be taken at Greenwich mean noon. Each observation shows the position of the ship, the direction of the wind, the force of the wind on the Beaufort scale, the height of the barometer, the readings of the dry-bulb and wet-bulb thermometers, the temperature of the water at the surface, the state of the weather, and the kind, amount, and movement of clouds. In order to check the accuracy of the barometric readings, the observer is instructed to read his barometer at prescribed hours on three successive days when in port and send the readings to the Weather Bureau. On receipt of these readings the Bureau compares them with those of the nearest meteorological station, and then mails the observer a “barometer tag,” showing the results of the comparison and the error of his instrument. Besides keeping up these routine observations, the observer keeps a record of fog encountered at any hour of the day and makes detailed reports on storms. Many marine observers also report observations at stated hours by wireless telegraphy.
The enormous fund of information thus collected over the ocean is applied to several purposes besides the construction of Pilot Charts. Our Weather Bureau and certain foreign meteorological institutions prepare daily charts, showing approximately the instantaneous conditions over great oceanic areas, especially the North Atlantic. These maps are analogous to the daily weather maps published for land areas, but the drawing of each map is, necessarily, delayed for several months after the date to which it refers, in order to allow time for the receipt of as many reports as possible from ships at sea. As a rule such charts are prepared in manuscript only, but, though they cannot be distributed after the manner of ordinary weather maps, they are valuable for studies in the institution itself on the movements of storms and other atmospheric processes. They also enable the meteorological officials to answer inquiries concerning the winds and weather that have prevailed over a particular part of the ocean on any specified date. Such inquiries come from vessel owners, underwriters, and others, and the replies are frequently used as evidence in admiralty suits.