Matthew's gravity waves are visible from the ground as well. Frankie Lucena of Cabo Rojo, Puerto Rico, captured them in his recent snapshots of sprites above the hurricane:
Left to themselves, gravity waves would be invisible to the human eye. We see them, however, because they impress their patterns upon a luminous phenomenon called "airglow." Learn more about that in the next article, Amazing Airglow over Easter Island.
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