The frequency of plume formation in fully‐developed thermal convection is determined experimentally. Because the fluid has a temperature‐dependent viscosity, the cold and hot thermal boundary layers have different thicknesses and viscosities. As a result, plumes are released from these layers with different frequencies. There also appears to be a direct temporal interaction between the cold and hot boundary layers. In particular, whereas there is a single frequency for cold plume formation, hot plumes form with multiple frequencies, one of which is similar to the frequency of cold plumes. The measurements thus suggest that cold sinking “plumes” may induce hot rising plumes.