Late yesterday afternoon, a cold front draped across central Missouri, from west to east. As a result, a thick cloud layer formed above Columbia, producing a cold drizzle. Though no lightening or dark skies were visible, waves of thunder rumbled overhead, waxing and waning like the surf on a beach. Checking the local radar, I found that the thunder was originating in large storms to our north and south.
Thunder begins as a shock wave, produced as lightening explodes through the atmosphere. As it propagates outward, the high-frequency sound waves begin to dissipate while the low-pitched sound persists. Nearby cloud-to- ground lightening produces a sharp clap of thunder while thunder from distant storms arrives as a low rumble; in cases where the storm is very distant, the sound wave frequency may be too low for the human ear to detect. Since lightening often develops in a branching pattern (especially in cloud-to-cloud strikes), the sound from its various segments, crossing varied distances, will arrive at our location in sequence, producing a prolonged crackling or rumbling noise.
The persistent rumbling yesterday afternoon likely arose from cloud-to-cloud lightening high in the regional thunderstorms. Numerous branching strikes at high altitude and from distant storms were producing thunder that arrived in Columbia at different times and from different directions. This produced a constant, low-pitched rumble which rose and fell as it was reinforced by new waves of thunder.
