In sky-wave propagation the radio waves which have frequency between 2 MHz to , are reflected back to the ground by the ionosphere. But radio waves having frequency greater than 30 MHz cannot be reflected by the ionosphere because at this frequency they penetrate the ionosphere. It makes the sky-wave propagation less reliable for propagation of signal having frequency greater than 30 MHz . Critcal frequency is defined as the highest frequency that is returened to the earth by the ionosphere. Thus, above this frequency a wave whether it is electronagnetic will penetrate the ionosphere and is not reflected by it.