Q: Since thunder and lightning are electrical events, why are they so often accompanied by heavy rain?
A: The electrical activity that manifests itself as lightning is generated by the airborne water.
The thunder is sound waves resulting from the extreme heat of the lightning.
As the National Oceanic and Atmospheric Administration explains, the three ingredients necessary for a thunderstorm are moisture; rising, unstable air; and a lifting mechanism, like hills or mountains or just the collision of masses of warm, wet air and cold, dry air.
When water vapour condenses into a cloud and rises into colder upper regions of the sky, some of it turns into ice crystals, usually with a positive charge, and some becomes water droplets, usually with a negative charge.
When the charges are strong enough, the electricity is discharged as a bolt of lightning.
While some lightning often precedes rain, the main event occurs as a downdraft starts and rain or other precipitation falls.
Eventually, the downdraft overcomes the updraft and the storm dissipates, along with the lightning.
Lightning benefits the earth, keeping its electrical charge in balance and generating protective ozone, the National Severe Storms Laboratory explains. (It also points out that there is no such thing as lightning without thunder, often called heat lightning; the thunder is simply too far away to be heard.)