End of storm oscillation

From Glossary of Meteorology

end of storm oscillation

(Abbreviated EOSO.) A characteristic in the electric field signature at the ground near the end of a thunderstorm.

The electric field changes from upward-directed (foul-weather polarity) to downward-directed (fair-weather polarity) and often retains the latter polarity for several tens of minutes before returning momentarily to foul-weather polarity. Positive ground flashes and spider lightning are occasionally observed during the period of reversed field polarity. The origin of the EOSO is only partly understood.

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