HIGH-VELOCITY CLOUDS

High-velocity clouds (HVCs) consist of neutral hydrogen (HI) at veloci ties incompatible with a simple model of differential galactic rotatio n; in practice one uses \v(LSR)\ greater than or equal to 90 km/s to d efine HVCs. This review describes the main features of the sky and vel ocity distributions, as well as the available information on cloud pro perties, small-scale structure, velocity structure, and observations o ther than in 21-cm emission. We show that HVCs contain heavy elements and that the more prominent ones are more than 2 kpc from the Galactic plane. We evaluate the hypotheses proposed for their origin and rejec t those that account for only one or a few HVCs. At least three differ ent hypotheses are needed: one for the Magellanic Stream and possibly related clouds, one for the Outer Arm Extension, and one (or more) for the other HVCs. We discuss the evidence for the accretion and the fou ntain model but cannot rule out either one.