Which is a common first indicator of an approaching thunderstorm?

Question: ​Which is a common first indicator of an approaching thunderstorm?

Answer:

The first and most frequent optical pre-storm signal is a sound, namely a distant rumble of thunder.

Lightning is a sudden electric discharge that takes place in the atmosphere between areas of opposite electric charges. Thus, thunder is the sound produced by it. When lightning occurs, we have electric discharge that quickly warms and expands the surrounding air to form a pressure shock wave that spreads out in all directions. This pressure wave reaches our ears as the more familiar rumble or growl, the sound we call thunder. The document further informs the readers that thunder can be heard beyond quite a distance and sometimes as far as ten miles away when there is no object in the middle of the way and the atmosphere is absolutely clear. As such, it becomes a precursor of an upcoming storm which may be detectable even when other aspects of vision have not picked any features associated with a storm.

For instance, you may be outside when it is just a normal day, and you just hear a low rumbling at the backdrop. This could be your first sign that a thunderstorm is approaching but if you are still outside and the sky above is clear, then you are virtually safe. The loudness and pitch of the thunder can also be affected by the type of lightning whether it is cloud to ground or cloud to cloud, distance of the strike and the weather conditions. Although other visible symptoms such as, cloud coverage becoming more menacing and greyer, a change in temperature, higher humidity or a shift in wind direction can also be early warning signals of an incoming thunderstorm, the strongest early warning signal pointed out in the document is the rumble of distant thunder