MARINE METEOROLOGY
That it behooves a sailor to be weather-wise has always been admitted,
but there was a time, almost within the memory of men now living, when
neither seamen nor landsmen had the remotest conception of the benefits
that a systematic study of the meteorology of the sea was capable of
conferring upon the maritime world. The man who first grasped the
importance of such a study and translated his ideas into facts was the
American naval officer, Lieutenant Matthew Fontaine Maury.
During his brief career at sea Maury became impressed with the
meagerness of the information then available concerning the winds and
currents that aid or hinder the voyages of sailing ships. When, in
consequence of an accident that incapacitated him for shipboard duties,
he was assigned to service in Washington, he began to explore the old
logs of naval vessels, filed in the Navy Department, for notes on
meteorological conditions, and eventually developed a plan of securing
regular observations from both the Navy and the merchant marine. The
result of this undertaking was the publication of the famous Wind and
Current Charts, which revolutionized navigation throughout the world.
The practical value of these charts, of which 200,000 copies were
distributed to the masters of merchant vessels of all nationalities,
was promptly recognized. By taking advantage of the favorable winds
and currents shown on the charts, and avoiding those that were
unfavorable, mariners were able to reduce the average time of a sailing
voyage between the Atlantic and Pacific ports of the United States
by forty days. The money value of the charts to vessels sailing from
the United States to South America and the Far East was estimated at
$2,250,000 per annum. British shipping on all seas is said to have
benefited to the extent of $10,000,000 per annum. Neither was the
utility of the charts limited to the saving of money. The following
episode is cited in Maury’s biography:
“When the _San Francisco_, with hundreds of United States troops on
board, foundered in an Atlantic hurricane, and the rumor reached port
that she was in need of help, everyone looked to Maury as the only man
in the country who could tell where to find the drifting wreck. To him
the Secretary of the Navy sent for information. He at once set to work
and showed how the wind and currents acting upon a helpless wreck would
combine to drift her ‘just here’, pointing to a spot on the chart,
and making a cross mark with the blue pencil he had in his hand. Just
there the relief was sent, and just there the survivors of the wreck
were picked up. This was an incidental result of his study of winds and
currents.”
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.
[Illustration: 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.
In the case of one series of such maps--viz., the daily synoptic
charts of the North Atlantic, begun by Niels Hoffmeyer, of Copenhagen,
and now prepared jointly by the Danish Meteorological Institute and
the Deutsche Seewarte, in Hamburg--the charts have actually been
published and sold, though they are so costly that the number of sets
in libraries throughout the world is probably small. These remarkable
charts present daily pictures of the winds and barometric pressure over
the North Atlantic Ocean and the adjacent continents from 1873 to 1876,
and from 1880 down to a recent date.
From what we have already said it will be seen that the marine
observers cooperating with the United States Weather Bureau and kindred
institutions abroad are all contributing toward the great task of
recording the history of the weather over the oceans from day to day
and assembling data that can be digested in the form of marine climatic
statistics and used as the basis for many scientific investigations.
This concerted undertaking does not, however, constitute the whole
scope of marine meteorology. Every intelligent mariner finds it
necessary to acquaint himself with the laws of the winds, indications
of coming storms, means of determining the proximity of icebergs, the
systems of storm signals used in different countries, the method of
constructing weather maps from wireless bulletins, etc. He ought, in
short, to become an accomplished meteorologist.
One of the classic problems of the navigator is that of handling his
ship in a violent cyclonic storm, especially a tropical hurricane. The
reader will recall that a cyclone, besides traveling as a whole at a
rate of several hundred miles a day, consists of a system of winds
rotating around the center. The result of this double motion is that
the winds on one side of the center are not only more violent than
those on the other, but they are also so directed as to drive a vessel
running before the wind, across the storm track ahead of the advancing
center, while those on the other side tend to drive a vessel to the
rear of the storm. The two halves of the storm area are accordingly
known as the “dangerous” and “navigable” semicircles, respectively.
While this simple statement sets forth the fundamental facts involved,
the actual problem is complicated by many features, such as the fact
that the winds do not blow in circles, but more or less spirally, that
the area of the storm cannot be readily determined, that two storms may
occur in close proximity to each other, etc.
The accompanying diagram, published by the United States Hydrographic
Office, represents a cyclonic storm in the northern hemisphere, the
circles being isobars, or lines passing through places at which the
same barometric pressure prevails (indicated in inches), and the arrows
indicating the direction of the winds. The diagram is thus explained:
“For simplicity the area of low barometer is made perfectly circular
and the center is assumed to be ten points to the right of the
direction of the wind at all points within the disturbed area. Let
us assume that the center is advancing about north-northeast, in the
direction of the long arrow, shown in the heavy full line. The ship _a_
has the wind at east-northeast; she is to the left of the storm track,
or technically in the navigable semicircle. The ship _b_ has the wind
at east-southeast and is in the dangerous semicircle.***A vessel hove
to at the position marked _b_, and being passed by the storm center,
will occupy successive positions in regard to the center from _b_ to
_b_4, and will experience shifts of the wind, as shown by the arrows,
from east through south to southwest. On the other hand, if the storm
center be stationary or moving slowly and a vessel be overtaking it
along the line from _b_4 to _b_, the wind will back from southwest to
east, and is likely to convey an entirely wrong impression as to the
location and movement of the center. Hence it is recommended that a
vessel suspecting the approach of proximity of a cyclonic storm should
stop for a while until the path of the center is located by observing
the shifts of the wind and the behavior of the barometer.”
The movement of the winds around the storm center shown in this diagram
is that of cyclones of the northern hemisphere; i. e., contrary to the
direction of the clock hands. In the southern hemisphere they blow in
the opposite direction around the center.
[Illustration: NAVIGATION OF A SHIP IN A CYCLONIC STORM
(_U. S. Hydrographic Office._)]
By observing the rise or fall of the barometer, the shift of the
winds, and the state of the sea and sky, the experienced navigator
is generally able to lay down on a chart the approximate position of
the storm center and steer his vessel so as to avoid danger. Various
devices, known as “storm cards,” “cyclonoscopes,” etc., have been used
to aid in the process of locating a storm from shipboard observations.
In the Far East mariners use for locating typhoons an ingenious
combination of the storm card and the aneroid barometer, called the
“barocyclonometer,” an invention of the Rev. J. Algué, director of the
Philippine Weather Bureau.
The most important development in marine meteorology in recent years
has been the rapidly increasing use of radiotelegraphy, both by marine
observers in transmitting reports of observations to shore and to other
ships, and by meteorological institutions in issuing weather bulletins
and storm warnings to vessels at sea. The first regular undertaking
in this line was carried out under the auspices of the London _Daily
Telegraph_ in the year 1904. This newspaper arranged with some of
the leading transatlantic steamship lines to furnish weather reports
by wireless from their vessels, and these reports were published in
its columns for several months. The following year the United States
Weather Bureau began, in a tentative way, the collection of wireless
weather reports from off-shore vessels, and similar undertakings were
soon afterward launched in other parts of the world, but for some years
such reports were of little practical value, owing to the limited range
of wireless communication.
[Illustration: A SHIPBOARD WEATHER MAP
Vessels off the American coast can make their own weather maps every
night, using data supplied at fixed hours by high-power radio stations
(shown by stars on the map), together with radio reports from other
vessels. Letters near stations are the code letters used in wireless
bulletins to describe the stations. Vessel reports are indicated by
names. Arrows show direction of wind and the force on the Beaufort
scale (shown by the number of feathers). Besides data for constructing
maps, the radio stations issue forecasts and storm reports for each of
the numbered zones shown off the Atlantic coast and over the Gulf.]
At the present time wireless reports from ships on the Atlantic enable
the forecasters on both sides of that ocean to extend the areas of
the weather maps on which their predictions are based, and reports
from ships are also received to a limited extent by forecasters on our
Pacific coast as well as in the Far East, India, and elsewhere. In
this country such reports have been especially valuable in indicating
the movements of West India hurricanes, and thus have helped to solve
the problem of protecting the vast tonnage that has been attracted
to Caribbean waters by the opening of the Panama Canal. The reciprocal
process of transmitting weather intelligence to vessels by wireless
bulletins, broadcasted at certain hours every day by high-powered
radio stations, has made much more progress. Such bulletins include
information concerning the current and prospective weather, winds and
storms over specified ocean areas, as well as reports of observations
made at a number of land stations, from which it is possible for
vessels at sea to construct their own weather maps. They are thus
enabled to take advantage of favorable winds and to avoid unfavorable
winds and storms. Wireless weather reports from other vessels help to
piece out these shipboard maps.
The meteorological services of all civilized countries adjacent to
the sea display signals along their coasts to announce the coming of
storms dangerous to navigation. One of the earliest devices used for
this purpose was the “aeroclinoscope,” a form of semaphore formerly
employed by the meteorological service of Holland. The position of the
arm of the semaphore indicated the region in which the barometer was
low; i. e., the storm center. In the British Isles, in the middle of
the last century, Admiral FitzRoy introduced the use of canvas cones
and “drums” (i. e., cylinders), which, seen from any direction, have
the appearance of solid triangles and squares against the background of
the sky. The British later abandoned the drum and used the cone only,
pointing up or down for northerly or southerly gales, respectively.
The American storm flag--red with a square, black center--was adopted
by the United States Signal Service (the predecessor of the Weather
Bureau) in 1871. This signal was subsequently amplified by the addition
of red and white pennants to show the expected direction of the wind
at the beginning of the approaching storm. Most countries use lanterns
for night storm signals. In the year 1909 a uniform system of signals,
consisting of cones by day and lanterns by night, was recommended for
use in all countries by an international commission which met in London.
In spite of this recommendation some thirty or forty different systems
of daytime storm signals are now in use in different parts of the
world. On the China coast an elaborate system of signals, consisting of
cones, balls, diamonds, and squares displayed on a mast and yardarms,
indicates the existence of a typhoon anywhere in the neighboring seas,
together with its location and movement.