The oldest oracle-bone forms of 雷 draw rain falling from the sky above a string of small linked circles — the ancient scribes' way of showing a sound that rolls and repeats, again and again. Over the centuries those rolling circles straightened out into the same neat square grid you already know from 田, "field" — though what rolls through this particular field is thunder, not crops.
A piece of culture
The thunder pattern on bronze Shang and Zhou bronze casters loved that same rolling, repeating rhythm: they carved continuous square scrolls into ritual vessels as background filler, a motif still called 雷纹 — "the thunder pattern" — one of the oldest and most common decorations in Chinese art. The rumble even got its own god: 雷公, the Duke of Thunder, pictured with wings, claws and a drum-shaped mouth, striking down wrongdoers from the sky. To this day the saying 雷声大，雨点小 — "loud thunder, small raindrops" — describes someone who is all talk and no action.