According to the international rules of the "road", all seafaring vessels are supposed to maintain "moderate speed" in thick weather, but this rule is variously interpreted and often disregarded. The international rules also prescribe various fog signals, which vessels must make by whistle, foghorn or bell, according to circumstances. In coastal waters foggy weather increases the danger of running aground as well as causing collisions. The waters are well charted, and frequent soundings help the navigator find his way, and he is further aided by fog signals on shore, in the shape of sirens, whistles, bells (including submarine bells) and nowadays, above land, by so-called radio beacons. Acoustic fog signals of all kinds (submarine excepted) are famous for the varying distances at which they can be heard. A signal audible under certain circumstances at a distance of ten miles will, on occasions, be entirely inaudible at two; or, again, there will be certain zones or regions within a mile or two of the signal where no sound can be heard, while the signal is distinctly heard at much greater distances. These eccentricities are probably not due so much to the fog itself as to eddies and other movements of the atmosphere.

According to a well-known publication of the British Meteorological Office, "seamen's weather" includes only three elements of great practical importance-wind, fog, and floating ice. The last of these pertains as much to hydrography as to meteorology, but its prevalence depends upon weather conditions and also exercises a great influence upon the weather of the world, through its effects upon barometric pressure and winds. Ice is more dangerous to steamers than to sailing vessels, on account of the greater speed of the former; and the tragic loss of the Titanic, through collision with an iceberg on Aprill 15, 1912, proved that no vessel is big enough to be safe from it. Fortunately the vigilance of the International Ice Patrol, maintained by the United States Coast Guard, has made the North Atlantic steamship routes fairly safe from the grim menace described in Longfellow's well-known lines- "Southward with fleet of ice Sailed the corsair Death"

Two other outstanding factors in marine meteorology require mention here. The navigator reads his barometer regularly, not because the pressure of the atmosphere affects his well being in any direct way, but because its fluctuations afford a clue to coming storms and shifts of wind. The state of the sea-by which is meant the smoothness or roughness of its surface-is generally classed as a weather feature, because it is mainly dependent on wind. The seaman distinguishes between waves of two kinds. Those set up by the wind blowing at the time and place of observation are called "sea," while waves due to a distant storm or to one that has recently occurred in the vicinity of the ship are called "swell."