Electrokinetic flows lead to promising utilization for mixing, concentration, pumping and have applications from basic studies of convective flows to fully integrated lab on chip developments. Despite these wide applications, electrothermal flow models have been scarcely studied. We find that the model widely used by the microfluidic community does not fit correctly the measured ac electrothermal fluid flows at higher voltages (10 Vpp and above). We thus analyse both theoretically and experimentally the importance of electrothermal coupling and the buoyancy effect. Numerical simulations are compared with micro-particle image velocimetry measurements of the vortices. Our enhanced model successfully matches our measurements over a wide range of conductivities and voltages.