The air-water interface

Water and air are two different fluids that enter into contact at the ocean’s surface at what is called the air-water interface. This interface, also known as boundary surface or contact discontinuity, plays an important role in ship hydrostatics and in describing the flow of water around a vessel.

Air is a gas made up of a mixture of gasses. Like any other gas, it is compressible. However, because the airspeed around the hull, rig, and sails is just a small fraction of that of the speed of sound, it can be considered as being incompressible for all our purposes. The density of the air depends on the temperature, the atmospheric pressure, and the humidity.

Water is, on the other hand, a liquid. It is incompressible, and its density depends on the temperature and on the salinity. The density of the water is much higher than that of air. At sea level and 15°C the density of air is around 1.225 kg/m3 while, at the same conditions, that of the seawater is 1,026 kg/m3, around 838 times that of the air.

Air and water remain both at their side of the interface. They do not mix as it will be the case, for example, of two gases in contact. Besides, the pressure of the two fluids at the interface is the same (it is just the atmospheric pressure which is the result of the weight of the column of air above it). These two points just mentioned are the necessary conditions for having what is known as a free surface.

At the free surface, the water is “free” to deform creating surface waves. Two common causes for the creation of surface waves are the wind and a vessel sailing through the water. The gravity force plays a significant role in the creation and propagation of surface waves.

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