Abstract: Recent research in flow visualization is focusing on the analysis of time-dependent, but mass-less particles. However, in many application scenarios, the mass of particles - and their resulting inertia - is essential in understanding fluid mechanics. This includes critical processes, such as dust particles interacting with aircraft (e.g., brown-or white-out effects) and particle separation based on density variation. In this paper, we contribute a generalized description of mass-dependent particle trajectories and apply existing unsteady flow visualization methods to the mass-dependent case. This comprises the extension of common concepts, i.e., path lines, streak lines, and time lines. Furthermore, we introduce a new class of integral curves, called mass lines that effectively visualizes mass separation and captures mass-related features in unsteady flow fields that are inaccessible using traditional methods. We demonstrate the applicability of our method, using a number of real-world and artificial data sets, in which mass is a crucial parameter. In particular, we focus on the analysis of brown-out conditions, introduced by a helicopter in forward flight close to the ground.