The chain temperature → lightning → Schumann resonance

The physical logic of the entire cluster can be summarized in a causal chain:

tropical temperature ↑ → deep convection ↑ → lightning activity ↑ (nonlinearly) → SR intensity ↑.

Higher temperature intensifies convection (updrafts in the humid tropical atmosphere), which promotes the formation of storms. More intense convection and richer ice microphysics lead to a greater frequency of lightning, and because lightning is the source of excitation of the resonances in the Earth-ionosphere cavity, the intensity of the SR also rises Williams 1992.

A fundamental property of the chain is the nonlinearity of the step "temperature → lightning". Lightning activity responds sharply to temperature, so even mild temperature changes show up clearly in the SR records. This sensitivity is what makes the resonances useful as a climatic indicator — but at the same time it complicates the direct conversion of measured intensity back to temperature (see the limitations in #56).