Local observations in the tropics show that lightning activity increases strongly with the wet bulb potential, θw, of boundary layer air. The surface wet bulb temperature controls the vigor of convection and the accumulation of ice phase condensate in the mixed phase region of tropical convection, the apparent seat of lightning activity. The Schumann resonance is a global electromagnetic phenomenon driven by global lightning activity. The Schumann resonance amplitude is shown to follow the fluctuations in mean temperature for the entire tropical belt with a sensitivity consistent with the local observations. The manifestation of the ENSO (El Nino–Southern Oscillation) signal in the Schumann amplitude at a single location is consistent with a synchronous warming and cooling over the entire tropical belt.