How waves are formed?
What causes waves to form?
Waves are caused by wind blowing on the surface of the sea i.e. the
air dragging on the water surface. Energy is transferred from the
air to the sea and the waves are the result of this transfer.
It is now generally accepted that waves are formed by two
mechanisms. The first starts by producing small waves from a
completely flat sea whilst the second takes over from the first at a
certain point and allows the waves to grow into bigger ones until
some limit is reached whereby they cannot grow any further.
Capillary Waves
At the beginning if wave generation are the small waves or bumps
called capillary waves. These are produced from a flat calm sea .The
main factor involved in the generation of these small weaves is the
fact that the wind when it blows does not blow uniformly .The wind
will never blow completely horizontally ,it also goes up and down
and changes directions slightly. These disturbances are enough to
start to push air down on the sea surface making tiny up and down
motions on the surface of the water itself.
These tiny vortices tend to drift about like small tornados and
occasionally one randomly follows the wave it just created and it
adds a bit more energy to the wave and so the wave increases in
height and strength. To understand more clearly. Imagine having a
small hairdryer with an on and off switch. As you point it down at
the water, you switch on the dryer for a second and then turn it
off. The air that was pushing down from the hairdryer caused a
depression directly under the hairdryer and a lump from the water
displaced. If the dryer switches off, the depression will swing back
up and the lump will spring back down .this is the creation of a
wave motion.
If you follow one of the waves with the hairdryer switched on, the
wave would grow in size as you put more energy from the hairdryer
in. The growths of waves in this fashion are linear in nature. They
grow bigger and bigger at a steady rate with time. Once these waves
have reached centimetres high the second mechanism of wave growth
takes over.
Exponential Growth
The second mechanism is self perpetuating. As soon as some the
capillary waves start to exist, the surface becomes roughened on the
surface. This modifies the air over the surface producing larger
vortices called turbulent eddies. These vortices are not random but
instead follow the waves. The waves will grow because the vortices
increase the pressure over the troughs of the waves and decrease the
pressure over the crests. As long as the wind blows the turbulent
eddies will grow and this increases the waves grow even more which
makes the eddies even bigger and so on.
In summary as the surface get waves on it, it gets rougher. As it
gets rougher the wind can grip it more and more and so builds the
waves up higher. This continues in an exponential way with time.
Overall the bigger the waves the quicker they grow. These waves are
no longer called capillary waves but gravity waves. The name derives
from the fact that gravity is the restoring force i.e. gravity will
reduce these waves back to a capillary waves , while surface tension
will restore capillary waves to a flat sea state
