When the atmosphere flashes
Sheet lightning is what it is called when the atmosphere flickers. It vaguely resembles a thunderstorm, except that there is usually no clearly defined lightning and thunder is often not heard.
When the thunderstorm is away
This phenomenon occurs in two ways: there is a lightning strike or a thunderstorm that is so far away that you cannot see the flash itself, for example because it is above the horizon. You only see the layers of air that are lit by the flash. The light from the flash can still be reflected in one or another cloud traveling through the atmosphere, giving the impression of a scattered flicker – as opposed to a sudden thunderstorm in the immediate vicinity.
Lightning discharges in the cloud
The second possibility: in addition to the usual lightning that we know from the thunderstorm – the lightning that strikes from the cloud to the ground – there is also lightning in the clouds. The storm cloud is negatively charged at the bottom and positively charged at the top, so lightning discharges can also occur from the bottom up in the cloud. However, because these flashes are surrounded by the cloud itself, that is, water droplets and ice crystals, the light is scattered in the clouds, making it look like flicker from afar.
Slight humming or no thunderstorms at all
Basically, lightning is always followed by thunder – just because lightning heats the air so that it “knocks out” the sound wave. But when the lightning strikes lightning, the thunderstorm is so far away that you don’t hear the thunder or hear only a faint hum – which can later be so delayed that it no longer associates it with lightning. The further the thunderstorm, the longer the sound required, the more time passes between lightning and thunder.