Atmospheric Sounds

Sferics stands for "atmospherics" and is the generic abbreviation for atmospheric radio frequencies that travel at very low transmission frequencies (Very Low Frequency, from 3 kHz to 30 kHz). They have been classified into three types:

Spherics properly speaking: these are impulsive signals generated by lightning. These radio frequencies travel for thousands of kilometers and resemble the sound of frying bacon. Here's an example.
Tweeks: these signals are spherics that travel in the ionosphere, a dispersive medium in which high frequencies move faster than low frequencies. For this reason, they peak around 2000 Hz and resemble little birds.
Whistlers: the most fascinating. These are descending glissandos, also caused by atmospheric dispersion, which causes high frequencies to arrive before low ones. Here you go.

All these signals were recorded by the INSPIRE VLF radio receiver at NASA's Marshall Space Flight Center in Huntsville, Alabama.

Unfortunately, the link to listen to the INSPIRE receiver live no longer works, although the page still exists.

Another interesting thing is that anyone can receive the VLF band on their PC, with this simple procedure reported by Wikipedia (read PC-based VLF reception).

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