The VLF-noise amplitude variations in Yakutsk during Forbush decreases of c
osmic rays are considered by the. epoch superposition method. Nighttime VLF
-noise amplitudes reflect mainly the activity of their thunderstorm source
in the equatorial region (Africa): summer daytime amplitudes, local thunder
storms activity. Two effect phases can generally be distinguished with resp
ect to the Forbush decrease onset. The VLF noise amplitude decreases by day
0 or -1 during the first phase and increases by day +1 or +2 in the second
phase, exceeding the initial undisturbed amplitude on days -5 or -4.