Chapter 4 · Storm cloud, flash and thunder
In a thunderstorm
Imagine you watch a storm
Imagine you watch a storm from a window. Inside the tall cloud, ice crystals and soft hail are tossed up and down by strong winds, and they collide again and again. Scientists think that each collision moves a tiny electric charge from one piece to another. The lighter crystals carry positive charge to the top of the cloud, and the heavier pieces carry negative charge to the bottom. The cloud turns into a huge battery. The ground under it becomes positive by contrast, and the air between them holds the voltage back, until it cannot.
Leader and return stroke
First a faint channel, called the stepped leader, zigzags down from the cloud in jumps of tens of metres. When it comes close to the ground, a spark rises to meet it, and the circuit is complete. Then the bright flash runs back up along the channel, the return stroke, at roughly a third of the speed of light. The current is tens of thousands of amperes, and the channel, only a few centimetres wide, heats to about 30,000 kelvin. The surface of the Sun is about 5,800 kelvin, so lightning is roughly five times hotter, for a few millionths of a second.
Thunder and the stopwatch
The air in the channel gets so hot so fast that it expands explosively, and the shock spreads as sound. This is thunder. It rumbles because the channel is long and its sound reaches you from different distances at different times. The flash reaches you at once, but sound travels only about 343 metres per second at 20 degrees Celsius. Count the seconds between the flash and the first thunder, then divide by 3: the result is the distance in kilometres. Worked example: 9 seconds gives about 3 kilometres.
Warm air, faster sound
The speed of sound depends on the air temperature: v = 331.3 × √(1 + T / 273.15), with T in degrees Celsius. At 0 degrees it is 331 metres per second, at 20 degrees 343, and at 30 degrees about 349. So in a hot summer storm thunder arrives a little earlier than in cold air, though the rule of three seconds per kilometre works well in both. Use the second slider in the sandbox to change the temperature and watch the speed of sound move.
When thunder roars
Lightning can strike many kilometres away from the rain, even in a place where the sky above you still looks clear. The weather services therefore give one simple rule: if you can hear thunder, you are close enough to be struck. Go indoors, into a solid building or a car with a hard roof and closed windows, and stay away from water. Do not shelter under a lone tree or stand in an open field. Wait about 30 minutes after the last thunder before going out again.
A planet that flashes
NASA satellites watch the whole planet and count lightning from space. They find about 40 to 50 flashes every second, which makes some 1.4 billion flashes in a year. Most of them happen over land in the tropics, where the sun heats the ground and builds the tallest storm clouds. Many flashes stay inside the cloud or jump from one cloud to another, and only some reach the ground. Each one is a small natural power station that lasts a fraction of a second.
In numbers
- ≈ 30,000 K
- the temperature of a lightning channel
- ÷ 3
- seconds between flash and thunder give kilometres
- ≈ 1.4 bn
- lightning flashes on Earth every year