The SR as a "global tropical thermometer" (Williams 1992)

The key idea: the intensity of the Schumann resonance is a sensitive measure of the temperature of the tropical atmosphere. Williams (1992) showed that variations in the SR track changes in tropical temperature, with the connecting link being global lightning activity Williams 1992. The work gave the resonances the nickname "global tropical thermometer" — hence also the name of this entire cluster.

What is essential is that the relationship between temperature and lightning activity (and thus the intensity of the SR) is nonlinear. A small increase in temperature can trigger a disproportionately large rise in lightning activity, because behind the nonlinearity lie the interactions of deep convection and ice microphysics in storm clouds (the formation and separation of charges on ice particles). The sensitivity of the SR to temperature is thus "amplified" precisely by this nonlinear response of lightning activity.

Thanks to this sensitivity, the SR has become a candidate for monitoring long-term temperature trends in the tropics using a single integrating measure, instead of a dense network of ground sensors.