I really like thunderstorms when I am inside and can observe the heavy rain, lightning and thunder from a dry and safe spot. Thunder is produced at the same time as lightning. But have you ever noticed that thunder always comes after lightning? You can actually tell how far away you are from the thunderstorm by measuring the time difference between the lightning and the thunder. Let's have a look at lightning and at the origin of thunder.

Photo of a lightning bolt by David Selby.

Lightning bolt

In a thunderstorm, clouds are being electrically charged. Some parts of the clouds are charged positively, whereas other parts are charged negatively. Lightning occurs when charges jump suddenly between a negatively charged area, a positively charged area, or the neutrally charged ground. A lightning bolt is a spark made by the strong sudden flux of charges (electrons or ions) through air. (See figure 1.)

Origin of thunder

Within the lightning bolt, the air gets heated up immediately through the sudden electrical current. This sudden strong heating makes the air expand and the air pressure rises locally. Thus, a pressure wave starts and it propagates through the air away from the lightning bolt (see figure 2). When it reaches you, you hear it as thunder because pressure waves are the same as sound waves.

Why do we see lightning before we hear thunder?

We saw that lightning is actually the reason for thunder. Sound travels at a speed of 1130 feet (342 meters) per second. The sound is produced by the lightning bolt and thus, it takes a few seconds until the sound reaches the observer's ears. But light travels so fast that you can almost see the lightning bolt at the same time as when it happens. Because of this, there is a time difference between seeing and hearing lightning's thunder. By counting the seconds between seeing a lightning bolt and hearing its thunder, you can estimate your distance to the thunderstorm. Per second, the lightning bolt and probably the whole thunderstorm is roughly 1100 ft (340 m) away. Rounding the numbers makes it easier to estimate in my head. For example, 3 seconds between lightning and thunder indicates that the lightning struck about one kilometer away.

Figure of a lightning bolt.

Figure 1 (left): You can see a thunderstorm cloud, which is positively charged at the top and negatively charged at the bottom. Electrons discharge through the lightning bolt to the ground.

Figure 2 (right): Electrons are flowing within the lightning bolt, which heats up the air. This leads to a sudden expansion of the air, which produces a pressure wave. This pressure wave mainly propagates perpendicular to the lightning bolt.

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