  An overview is given of the helicity of the velocity field (``kinetic''
helicity to distinguish it from the ``magnetic'' helicity used in
magnetohydrodynamics, astrophysics, and solar physics; or simply
\emph{helicity} in this Chapter) and of the role, which this concept plays in
the modern research in atmospheric physics and atmospheric turbulence. General
information on helicity is presented including some historical comments and a
brief description of the fundamental role that helicity plays in fluid
dynamics. Different applications of the helicity concept are discussed to the
analysis of various dynamic atmospheric processes, including applications to
intense atmospheric vortices, such as tropical cyclones, tornadoes and dust
devils, and also to Ekman boundary layer dynamics. Emphasized are helicity
balance conditions and the important role that helicity fluxes play in their
maintenance. Fundamentals of helical turbulence theory are briefly discussed,
and then emphasis is placed on the helical properties of atmospheric turbulence
within the atmospheric boundary layer. In particular, the pioneering effort of
the research team from the Obukhov Institute of Atmospheric Physics (Moscow) to
measure turbulent helicity in natural atmospheric conditions is described.
