The boundary line between two current systems of water of different densities can sometimes
be identified from the way in which driftwood, seaweed and so on stretch out in a more or less
straight line. Tide rips can also be identified from differences in the colour of the seawater or
of the wave pattern on either side of the dividing line.
Apart from tide rips, at sea regular one also meets parallel bands with a clearly different wave
pattern. This phenomenon may be caused by internal waves. These are waves that occur at
the boundary between two different masses of water lying on top of one another. The crests
and troughs of the internal waves affect the wave movement on the surface.