a stone into a pool
of water. The spreading ripples are gravity waves. The waves occur
between any stable layers of fluids of different density.
When the fluid boundary is disturbed, buoyancy
forces try to restore the equilibrium. The fluid returns to its
original shape, overshoots and oscillations then set in which propagate
as waves. Gravity or buoyancy is the restoring force hence the
term - gravity waves.
These waves (internal gravity or buoyancy waves) abound in the
stable density layering of the upper atmosphere. Their effects
are visibly manifest in the curls of the stratosphere’s nacreous clouds,
in the moving skein-like and billow patterns of the mesosphere’s noctilucent clouds
and in the slowly shifting bands of the thermosphere’s airglow.
What triggers them? The ‘stones into the pond’ are
disturbances far below in the troposphere, for example, wind flow
over mountain ranges and violent thunderstorms.
Jet stream shear and solar radiation are other sources. An initial
small amplitude at the tropopause increases with height until the
waves break in the mesosphere and lower thermosphere. Their wavelengths
can range up to thousands of kilometres. Their periods
range from a few minutes to days.
They do more than give clouds interesting shapes. They are vital in their role
of transferring energy, momentum and chemical species between the
different atmospheric layers and in the subsequent influence on
upper atmosphere winds, turbulence, temperature and chemistry.