In this thesis we discuss three models describing the dynamics of density-dependent fluids in long lifes pans and on a planetary scale. In such setting the relative displacement induced by various external physical forces, such as the Coriolis force and the stratification buoyancy, is far more relevant than the intrinsic motion generated by the collision of particles of the fluid itself. Such disproportion of balance limits hence the motion, inducing persistent structures in the velocity flow.On a mathematical level one of the main difficulties relies in giving a full description of the perturbations induced by the external forces, which propagate at high speed. This analysis can be performed by the aid of several tools, we chose here to adopt techniques characteristic of harmonic analysis, such as the analysis of the dispersive properties of highly oscillating integrals.All along the thesis we consider boundary-free, three-dimensional domains, and inspecific we study only the case in which the domain in either the whole space or the periodic space . The models we consider are the following ones : primitive equations with comparable Froude and Rossby number and zero vertical diffusivity, density-dependent stratified fluids in low Froude number regime, weakly compressible and fast rotating fluid in a regime in which Mach and Rossbynumber are comparable. We prove that these systems propagate globally-in-time data with low-regularity. Nosmallness assumption is ever made, specific constructive hypothesis are assumed on the initial data when required.