An electrically insulating liquid layer, confined between horizontal conducting electrodes, the upper of which is warmer, becomes unstable with respect to the onset of steady convection when the electric field reaches a critical value. In a rapidly carying ac field this instability is due to the polarisation body force. We extend Roberts' electrohydrodynamic analysis and test the results against data of Turnbull and Melcher for polydimethylsiloxane. Quantitative between theory and experiment is obtained. We also predict critical voltages of between 0.37 and 9.3 kV to induce convection in 1 mm layers of acetonitrile, nitrobenzene, acetone, chlorobenzene, and cyclohexane when the upper electrode is at 35°C and the lower electrode at 15°C.