Lightning is a giant electrical spark in the atmosphere, within a cloud, between clouds, between a cloud and the surrounding air, or between a cloud and the ground. It forms after a thunderstorm separates positive and negative charge into different parts of the cloud.
How a storm builds charge
Charging happens where the cloud holds a mix of graupel (soft hail pellets), small ice crystals and supercooled liquid drops at temperatures below freezing, 32°F (0°C). The National Severe Storms Laboratory (NSSL) describes rebounding collisions between graupel and ice crystals that transfer charge, leaving the two with opposite charges.
The updraft then pulls them apart. The National Weather Service (NWS) describes lighter, positively charged ice crystals carried toward the top of the storm, while heavier, negatively charged graupel and hail stay suspended or fall toward the lower part of the cloud.
NSSL calls this the noninductive process, because it does not need an existing electric field to polarize the particles, and notes the exact physical mechanisms are not completely understood. Real storms are messier than a two-layer picture, and NSSL notes four main charge regions in the main updraft.
Kinds of flashes
Most flashes never reach the ground. In-cloud lightning, also called intra-cloud, stays inside the cloud, and lightning can also travel from cloud to cloud. NSSL notes roughly 5 to 10 times as many cloud flashes as cloud-to-ground flashes.
Cloud-to-ground lightning links charge in the cloud to the ground. The typical flash lowers negative charge, while a less common type lowers positive charge.
Where thunder comes from
Thunder is the sound of lightning. The NWS notes the air in the channel may reach as high as 50,000°F, and that the rapid expansion and then contraction of that air creates the sound wave.
The NWS notes that thunder can be heard only about 10 miles from the strike.
Safety and limits
The NWS safety message is short: When thunder roars, go indoors. It states there is no safe place outside when thunderstorms are in the area, that hearing thunder means you are likely within striking distance, and that you should wait 30 minutes after the last lightning or thunder before going back out.
A high flash count shows that a storm is electrically active. By itself it does not say the storm will produce damaging wind, large hail or a tornado, so lightning trends work best alongside radar.