Marine Glossary

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floating

Floating is key concept in weather, especially floating air. If one mass of air will float in another, then it will rise up from the surface. Gentle updrafts of rising air can ultimately lead to violent winds on the surface. The nature of the air with regard to whether or not it will float another mass of air that runs into it is crucial to the subsequent development of local weather.

Density is the key factor in floating. Wood floats in water because its density (typically 0.4 to 0.7) is less than water (1.0) — note ironbark and some ebonies, however, sink because their densities are just higher than 1.0. Diesel oil floats on water because the density of diesel is about 0.9. Fresh water (rain or river runoff with density of about 1.0) floats on salt water (density 1.03 to 1.04) providing the two are not mixed up by currents or waves. Freshwater floating on saltwater has a dramatic effect on sea state in the ocean and on surface current speeds on inland tidal waters.

With floating solids, such as boats, remember it is the density of the entire object that matters, not that of the component parts. A steel boat floats — though steel is much denser than water — because the weight of the structure (steel hull plus enclosed air) is less than the weight of the water displaced by the hull. Since we are dealing with the same volumes (the amount of displaced water), the density of the boat as a whole object is less than that of the water. One need only imagine replacing the enclosed air with water to see the principle.

You can figure how high a floating object will ride in the water because the buoyant force upward on the object is equal to the weight of the displaced water. If a log weighs 8 pounds, it will sink until the volume of the water displaced weighs 8 pounds. In other words, you have to have about a "gallon of log" underwater. A big 8-pound log (low density wood) will ride high in the water; a small 8-pound log, (higher density) will ride low in the water. A water-logged log (mostly water with total density just less than water) will ride almost submerged — a dead head.

With regard to floating air, it is the temperature and moisture content that determines the density of an air mass. Warm moist air is lighter than cold dry air and thus rises in it. This phenomena is not only the ultimate cause of storms and squalls at sea, it is often the driving force of the only wind available along a beach (sea breezes), not to mention that it is the principle behind the general mixing and circulation of the earth's atmosphere as a whole without which we would all perish. Float or die! — it is an important concept.