As well as preventing oxidation, a cross-flow of inert gas over the powder bed extracts process by-products, making it critical for process repeatability and build quality during the additive manufacturing of metallic parts. In this study, high-magnification schlieren imaging was employed to visualise the interactions between the cross-flow, laser plume and individual powder particles for the first time. Novel 3D multiphysics simulations of the cross-flow, including the vapour jet and laser plume, were used to calculate energy, momentum and species transport over the processed area in order to understand and quantify the observed interactions. During processing, metallic fumes as well as a significant number of airborne powder particles and agglomerates were observed to remain close to the powder bed, over the scanned area. These particles interact in a range of stochastic collisions that affect the build consistency. Refractive index gradients were formed in the atmosphere above the island scans, which extended many tens of mm beyond the processing area. The rate of extraction of these by-products, regulated by the velocity of the cross-flow, was related to the quality of the builds. A cross-flow perpendicular to the laser scan direction was observed to remove airborne particles from adjacent laser scans most effectively, limiting laser-particle and particle-particle interactions. The model demonstrated that the refractive index gradients away from the vapour jet are due to the metal concentration rather than the temperature of the heated gas.