Sferics, short for "atmospherics", are the impulsive signals emitted by lightning. The frequency range is from a few hertz to millions of hertz. The part of this range that we can hear (the audio range) has frequencies up to about 15 thousand hertz (15 kHz). The spectrograms of sferics are characterized by vertical lines on the frequency-time graph indicating the simultaneous arrival of all of the audio frequencies. The sound of sferics consists of sharp crackling noises like twigs snapping or sizzling noises like bacon frying. Sferics are caused by lightning strokes within a couple of thousand kilometers of the receiver. The VLF radio signal is ducted to the receiver between the surface of the earth and layers in the ionosphere which reflect radio waves. This path is called the earth-ionosphere waveguide.
Tweeks result when sferics are ducted in the earth-ionosphere waveguide distances much greater than a cuple of thousand kilometers. The distance can be as great as halfway around the earth (20,000 kilometers). When ducted over large distances, the VLF radio waves undergo a process called dispersion - that is the higher frequencies travel slightly faster than the lower frequencies. This is especially true of the frequencies between 3 and 2 kilohertz. Tweeks sound much different than sferics. Instead of the sharp crackling sound, tweeks have a quick mujsical sound somewhat like the ricochet sound bullets make (at least in the movies). The spectrogram of a tweek shows a vertical line at the higher frequencies with a curved section (called the "hook") appearing at a frequency of about 2 kilohertz.
Under the right conditions, the VLF signal travels out away from earth and returns by traveling along a magnetic field line. During this long path, dispersion is much greater than with tweeks. While tweeks might disperse a few hundred kilohertz over a few thousandths of a second, whistler show a dispersion of a second or more over several thousand kilohertz. The sound of a whistler is a musical descending tone that lasts for a second or more. On the spectrogram, whistlers appear as long sweeping arcs showing the sequential arrival of the frequencies. It is important to remember that all of the frequencies start out at the same time (a sferic), but the path taken by a whistler is so long that the dispersion of the frequencies is quite pronounced.
