  We study propagation of electromagnetic surface waves on a metal-air
interface in the case when the wave frequency is below the plasma frequency. We
derive a reduced wave equation for a metal cylinder with a given radius of
curvature. Using the Leontovich boundary condition we find solutions to this
equation which we classify as outgoing and incoming surface waves. We derive
the dispersion relations of the surface waves of both types and argue that the
earlier studies overlooked the waves of the second type although they are the
only type which can propagate on a planar metal-air boundary.
