The ionospheric potential (VI), a measure of the earth's overall electric field intensity, correlates with the classic 'Carnegie curve' diurnal variation in electric field caused by the global distribution of thunderstorms. A comparison of VI variation with satellite images of high tropical cloud fields indicates that equatorial thunderstorms over the continents are dominant in maintaining the earth's electric field. VI time series obtained at just one location may provide a high-resolution measure of intense tropical convection.