At least in HF, this "dead air" is anything but dead (lightning, channels with lots of fading, things you only catch due to ionospheric effects, etc). Noise is

At least in HF, this "dead air" is anything but dead (lightning, channels with lots of fading, things you only catch due to ionospheric effects, etc). Noise is basically any signal you don't care about in lower frequency bands. Also many transmissions look like wideband noise at higher frequency due to direct sequence spreadspectrum type techniques.