Skin Cancer Prevention, Tanning and Vitamin D: A Content Analysis of Print Media in Germany and Switzerland.

Background: Print media are a major source of health information.

Objectives: To analyse press coverage related to skin cancer prevention.

Methods: We conducted a content analysis of print media articles pertaining to skin cancer prevention, solaria and vitamin D published in Germany and Switzerland over a 1-year period between 2012 and 2013.

Self-similar wave produced by local perturbation of the Kelvin-Helmholtz shear-layer instability.

We show that the Kelvin-Helmholtz instability excited by a localized perturbation yields a self-similar wave. The instability of the mixing layer was first conceived by Helmholtz as the inevitable growth of any localized irregularity into a spiral, but the search and uncovering of the resulting self-similar evolution was hindered by the technical success of Kelvin's wavelike perturbation theory. The identification of a self-similar solution is useful since its specific structure is witness of a subtle nonlinear equilibrium among the forces involved.