A technique to simultaneously extract particle image velocimetry (PIV) and synthetic schlieren (SS) information from a two-dimensional flow field is proposed and exemplified with simultaneous velocity and density measurements of the eruption of a laminar thermal line plume. The proposed experimental method employs colour separation in conjunction with two video cameras in order to unambiguously separate the conflicting requirements of both measurement techniques, PIV and SS. This allows true simultaneous measurements, in contrast to previously published approaches. In the present experiments, a precision better than 10⁻³ K and 0.02 mm s⁻¹ was obtained for temperature perturbations and velocity, respectively. The measured temperature field shows perturbations of less than 0.1 K as the induced plume first erupts, driving a vortical structure with velocities of around 2 mm s⁻¹ that propagates out in front of the developing thermal structure. The present application of the experimental technique has provided an estimation of the critical Rayleigh number for plume eruption, with excellent agreement with previous works.