Where thunderstorms come from
Lightning needs a thundercloud, formally called a cumulonimbus. These clouds grow when warm, moist air rises strongly, cooling as it climbs until its water vapor condenses. Powerful updrafts can carry droplets far above the freezing level, where some turn into ice. A thunderstorm cloud can be many kilometers tall, and the strong up-and-down motion inside it is essential for building up electric charge.
How a cloud becomes charged
Everything is made of atoms containing positively and negatively charged particles, and objects become charged when electrons move from one to another. In a thundercloud, the main idea is that collisions between different kinds of ice particles in the presence of supercooled water droplets (liquid droplets below freezing) transfer charge. Small ice crystals and heavier, soft hail-like pellets called graupel bump into one another in the turbulent air.
The usual explanation is that, in these collisions, the graupel tends to become negatively charged and the lighter crystals positively charged. The updraft carries the light crystals toward the top of the cloud, while the heavier graupel stays lower or falls. The result is a rough layering: positive charge in the upper part of the cloud, negative charge in a region lower down, and often a smaller pocket of positive charge near the base. The details are still researched, and not every step in this picture is fully settled, but it is the standard textbook account.
The negative charge at the base of the cloud pushes electrons away from the ground below and leaves the surface positively charged. Voltage between the cloud and ground, or between regions of the cloud, can reach enormous levels.
How the flash happens
Air is normally a good insulator. When the electric field becomes strong enough, the air breaks down and begins to conduct. A faint, branching channel of charge, called a stepped leader, zigzags downward from the cloud in short jumps. As it nears the ground, objects on the surface such as trees, buildings and people send up streamers of opposite charge. When one connects with the leader, a complete conducting path forms.
Then comes the bright part. A powerful electric current surges along the channel in what is called the return stroke, and this is what we see as the main flash. A single flash often contains several strokes along the same path in rapid succession, which is why lightning can appear to flicker.
Most lightning never reaches the ground. Far more occurs inside a cloud or between clouds. Lightning that does strike the ground is called a cloud-to-ground strike, and these lightning strikes are the ones that pose a danger to people and property.
Why there is thunder
The channel is narrow but carries so much current that it heats the surrounding air to temperatures of tens of thousands of degrees Celsius within a tiny fraction of a second, which is several times hotter than the surface of the Sun. The air expands explosively, creating a shock wave that spreads outward and weakens into an ordinary sound. Another way to put it is that the energy of the discharge is partly converted into heat, light and sound, consistent with energy conservation. The air in the channel also becomes a glowing plasma, a state that is sometimes described as an additional one among the states of matter.
A long rumble rather than a single bang occurs because the lightning channel is long and winding. Sound from different parts of the channel travels different distances to reach you, and it arrives at different times. Echoes from hills and buildings and changes in the air along the path add to the effect.
Why you see the flash first
Light travels at about 300,000 kilometers per second, so for any distance on Earth, the flash reaches you almost instantly. This fits with the speed of light as an upper limit. The sound waves that make up thunder travel at roughly 343 meters per second in air at ordinary temperatures, which is nearly a million times slower.
This gives a rough way to estimate distance. Sound covers about 1 kilometer in 3 seconds, or about 1 mile in 5 seconds. If you count 10 seconds between a flash and its thunder, the strike was roughly 3 kilometers (about 2 miles) away. This is only an estimate, because temperature and wind change the speed of sound slightly. Thunder is rarely heard from farther than about 15 kilometers or so, and heat lightning, the flashes on a warm night with no thunder, is usually just distant lightning whose sound dissipates before reaching you.
Staying safe
This section is general information, not an expert safety plan. Lightning can be dangerous and sometimes deadly. A common recommendation from safety agencies is that if you can hear thunder, you are close enough to be struck, so it is best to move into a fully enclosed building or a hard-topped vehicle and to wait there for a while after the last thunder. Open fields, hilltops, water and isolated tall trees are poor places to be. Follow guidance from local authorities and national weather services.
Different kinds of lightning
Besides the familiar forms, scientists have documented upper-atmosphere events such as sprites and jets, flashes high above thunderstorms that last only a fraction of a second. They are a reminder that the electrical life of storms is still being studied.