Thunder is the sound of air exploding. On a warm summer day the sky can be blue and clear in the morning — and then fill with dark, towering clouds by the afternoon. Suddenly a flash of lightning lights everything up, and a few seconds later a heavy rumble rolls across the rooftops. But what is actually crashing up there? The lightning never touches anything. And why does it thunder so much more in summer than in winter? The answer starts with something you know well on a hot day: heat.
What happens when it thunders?
Thunder begins high inside a thundercloud. A thundercloud is not like an ordinary fair-weather cloud — it can be several kilometres tall, all the way up where planes fly. Inside it, tiny ice crystals and water droplets are flung around in strong winds. When they rub and bump together, they spark up electric charge, just like dragging a wool sweater over your head and watching your hair rise. The lightest bits turn positive and gather at the top. The heaviest turn negative and sink to the bottom. The cloud becomes a giant battery.
Eventually the difference between the top and the bottom grows so large that the air can no longer keep the charges apart. A huge spark jumps — a lightning bolt. It heats the surrounding air to almost 30,000°C in a single blink, about five times hotter than the surface of the sun. That air explodes outward so fast that the crash we hear is the sound of the air itself. That is thunder.
Here is the part that surprises most people: the lightning and the thunder happen at exactly the same moment. We only see the lightning first because light races along almost infinitely fast, while sound takes longer to reach our ears. If you want to see how ideas about light, sound, and energy connect, read more about what STEAM learning is all about.
Try it at home: count between lightning and thunder
The next time a storm passes, you can work out how far away the lightning struck — safely, from inside your window. When you see a flash, start counting out loud right away: "one thousand, two thousand, three thousand…" Stop the moment you hear the thunder, then divide the number of seconds by three. That tells you roughly how many kilometres away the bolt landed. Write down each number. Are they getting smaller or larger? Smaller means the storm is coming closer; larger means it is moving away. Always stay indoors during a thunderstorm and watch the weather through the window — never outside, and always with an adult nearby.
Every child is made of good atoms. At Good Atoms we help them discover the big ideas hiding in something as everyday as a summer storm. Explore free content at goodatoms.com.