thunder
霹 centers on a sudden, violent thunderclap—the sharp crack of thunder that accompanies lightning.
霹 combines 雨 (rain) with 辟, which likely contributes the sound and the idea of splitting or cracking, together suggesting a thunderclap that splits the sky.
Rain 雨 falls from the top, and below it 辟 looks like a sudden crack or split. Picture a thunderclap splitting the rainy sky.
For ヘキ, imagine a thunderclap so loud it makes you say 'heck!'—heck -> ヘキ, as the thunder cracks overhead.
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