  Collective interaction of light with an atomic gas can give rise to
superradiant instabilities. We experimentally study the sudden build-up of a
reverse light field in a laser-driven high-finesse ring cavity filled with
ultracold thermal or condensed atoms. While superradiant Rayleigh scattering
from atomic clouds is normally only observed at very low temperatures (i.e.
well below $1 \mu$K), the presence of the ring cavity enhances cooperativity
and allows for superradiance with thermal clouds as hot as several $10 \mu$K. A
characterization of the superradiance at various temperatures and cooperativity
parameters allows us to link it to the collective atomic recoil laser.
