SFERICS/ˈsfɛrɪks/

Atmospheric discharges.

We can’t hear it without special equipment, but the planet almost continually sings with the sound of low-frequency radio signals that derive from lightning strikes. Because the signals are mostly trapped below the ionosphere, a reflective layer 55 miles above the ground, a suitable receiver can pick them up from thousands of miles away. They sound like twigs snapping or bacon frying. This weird-looking term for them, sferics, is just a respelled version of the last part of atmospherics. The abbreviation appeared around 1940, though the strange noises had first been heard by a German physicist, Heinrich Barkhausen, during World War I. There’s a complete vocabulary of words to describe various types: tweeks come from lightning that is so far away that the high radio frequencies arrive before the low, resulting in a musical set of clicks and tweets; whistlers are slowly descending tones caused by a similar mechanism, but which acts on bursts of radio waves that travel from pole to pole along magnetic lines of force.

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