Several authors have now suggested that some interstellar clouds above the plane of the Galaxy are interacting with the Reynolds' layer, the warm ionized gas extending well above (H ≅ 910 pc) the Galactic plane (Reynolds 1993). Characterizing the interaction between these clouds and their surroundings should be useful in understanding one source of interstellar turbulence: vertical shear flows. This paper discusses how studies of the morphology and drag coefficient of falling clouds might be used to constrain the Reynolds number for the flow, and hence the effective viscosity of the warm ionized medium. If arguments based on morphology are correct, the effective viscosity of the warm ionized medium is significantly higher than the classical values. Possible resolutions to this problem are suggested.

Turbulence from Vertical Flows

The spectrum of density and velocity fluctuation in the ionized interstellar medium (ISM) measured by scintillation of pulsars suggests that on small scales much of the structure of the diffuse ionized ISM may arise as the result of turbulent processes. Turbulence arises in regions of viscous shear flows. In the Galaxy, such flows have a large range of outer length scales, and include galactic rotational shear in both the radial and vertical (c.f. Walterbos 1998, this volume) directions, spiral density waves, stellar mass outflows (jets, winds, and explosions), and photoionization-driven flows. The structures formed contain energy over a range of length scales which is ultimately dissipated via viscous (hydrodynamical) and resistive (magneto-hydrodynamical) processes.