A Cape Cod Notebook 7/17/07

listenThunderstorm

By Robert Finch

I grew up in the Midwest, the land of violent thunderstorms, and I miss the air-shattering, bone-jarring cloudbursts that frequently punctuated our muggy summer days.

Summer thunderstorms are relatively rare on the Cape and Islands compared with the mainland. The reason is the moderating influence of the ocean. The formation of thunderheads - those dark, towering, anvil-shaped columns of cloud known as cumulo-nimbus - are formed by the rapid upward convection of great masses of warm moist air from the ground. The small amount of land mass on the Cape and Islands and the cool ocean air limit such build-ups, and sea breezes usually disperse the thunderheads when they do form.

When a good storm does come our way, I try to savor it to the full, like lamb chops on my birthday, gnawing the bones clean and sucking out all the marrow, for who knows when it may come again? So, when the first of the season's thunderstorms arrived in early June, I decided to give it my full and undivided intention.

Around six p.m. I became aware of flashes, intermittent and faint, outside the open window. Gradually the bursts of light grew nearer and brighter until it looked as if incendiary bombs were being dropped out in the bay. Now low grumbles of sound began to trail reluctantly after the flashes. In the yard the trees began to stir in a light wind.

I shut off the light and sat in the dark, watching the approach of the storm. It was a formal approach, drawing closer in measured, steady, stately stages, like a regal procession. The flashes of light pealed like bugles, growing ever nearer and more brilliant. Reedy gusts of cool air began rushing through the branches, through the open window, and against my face in palpable waves. The muffled flashes continued, and at length one of them broke out into a textured crackle. The storm instantly went from rumbles of undefined fluctuations to a fully realized event. Lightning flashes broke out in blinding, sharp strobes, illuminating the yard in blanched shudders of light before collapsing back into blackness. Hard on the lightning's heels came the full-blown thunder: long, geared slides of sound, staggered descents and rough ragged blossoms of noise, all overlapping in extended volleys that seemed endless, like the surf.

Never before had I appreciated how fully thunder gives shape to lightning, how its tearing, unfolding architecture brings into relief the charged stairs of the lightning's descents. When the negatively-charged ions in a thundercloud break down into the electrical spark we call a lightning bolt, the air column surrounding it is heated and expands rapidly. It is this expansion of air that creates the shock waves we hear as thunder. Thunder, in other words, is a form of sonic boom. It gives length and dimension to lightning, making the flashes more real and lasting in our memory. Yet thunder and lightning are so fundamentally different in character that only in the past couple of centuries did people suspect there was a causal relationship between the two.

At length the storm passed to the east, as stately in its going as in its approach. The wind died first, the flashes grew fainter and fewer, and the thunder at last sank again into shapeless, flattened rumbles. Residual growls still came from the departing storm, like afterthoughts of violence thrown over its shoulder. Finally, all that was left was the steady patter of the binding, reassuring rain.

Broadcast on July 17, 2007

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