The falling rain has a coupla
different effects that alter the
water surface, and in doing so
dampens the swell. First up is the
droplet splash with its associated
ring waves. When a raindrop hits
the water surface, it typically
creates a crater with a crown that
evolves into a vertical stalk. This is
followed by radiating ring waves.
These ring waves create a
subsurface turbulence which
dampens the short-period waves
the most. One of the major
scattering features is the collapsing
of the stalk. Scientists have studied
raindrops falling in coloured water
and worked out each drop sends
down one or more masses of water
downwards below the surface in
the form of vortex rings.
These rings descend with a
gradually diminishing velocity and
with increasing size to a distance
of several inches, generally as
much as 18 inches, below the
surface. Each drop sends down a
bunch of rings.
It is not that the drop merely
forces itself down under the
surface, but, in descending, carries
down with it a mass of water. The
rain falling onto water results in as
much motion immediately beneaththe surface as above it. So, besides
the splash and surface-effect which
the drops produce, they cause the
water at the surface to rapidly
change places with the water some
distance below. Such a movement
of water from one place to another
tends to destroy wave-motion.
Another interesting way that
rain reduces the wave height is that
the rain changes the temperature
and salinity of the upper layer of
the sea, which in turn reduces
its viscosity. Warm rain relativeto the sea temp will reduce the
attenuation of surface ripples. If
the temp difference is 10 degrees
Celsius it can dampen waves by 25
per cent!
So, next time you get skunked
by a swell forecast, or are sitting
out the back in a rain squall, you’ll
know exactly what’s happening!Costa Rica, a place where the waves
pump. When it’s not raining.
PHOTO: MATTHEWS