Lightning and thunder often occur together during thunderstorms, but they are distinct phenomena that travel at different speeds. Lightning is the flash of light that you see in the sky, while thunder is the crashing or rumbling sound that you hear. So which one is actually faster – lightning or thunder?
What Causes Lightning?
Lightning is an electrical discharge that occurs between positively and negatively charged areas within thunderstorm clouds. As updrafts and downdrafts occur within a storm cloud, water droplets and ice crystals collide, causing the lighter particles to accumulate a positive charge and the heavier particles to take on a negative charge. An immense difference in electric potential develops between the top and bottom of the cloud, and eventually a channel of negative charge rushes downward in the form of a stepped leader. When this leader nears the ground, it induces an upward streamer of positive charge. When the two meet, lightning strikes.
The return stroke of electricity that you see flashing in the sky as lightning moves upward at a speed of about 220,000 mph (354,000 km/h). This is equivalent to about 60% of the speed of light! Light can circle the entire planet 7.5 times per second, so lightning is incredibly fast but still not quite as fast as light.
What Causes Thunder?
Thunder is the sound wave created by lightning. When a lightning bolt travels from cloud to ground it heats the air in its path, causing the air to rapidly expand. This expansion creates a sonic wave that becomes the roaring sound of thunder.
Since sound travels much slower than light, you usually see a flash of lightning first, followed by the loud bang of thunder. Sound travels through air at around 767 mph (1,100 ft/s), depending on air temperature and humidity. This makes thunder decidedly slower than lightning.
Comparing Lightning and Thunder Speeds
Let’s break down the speeds:
- Lightning travels at 220,000 mph, or 354,000 km/h
- Thunder travels at 767 mph, or 1,100 ft/s
So lightning is around 300 times faster than thunder. That’s why you see the flash before you hear the thunderclap.
Why the Speed Difference?
The reason for the large difference in speed has to do with the different methods of travel:
- Lightning moves as an electrical discharge between charged particles.
- Thunder moves as a mechanical sound wave through the air.
Electricity can travel much faster than sound, which is limited by the properties of air. This physical difference is why lightning flashes arrive first, with the delayed rumble of thunder behind.
Factoring in Distance
The actual time between seeing the lightning and hearing thunder also depends on how far away the storm is. Light reaches your eyes almost instantly, while sound takes time to travel the distance between you and the lightning strike.
Using the speed of sound, we can estimate the distance to the lightning by timing the interval between the flash and the thunder:
- 5 seconds = about 1 mile (1.6 km)
- 10 seconds = about 2 miles (3.2 km)
- 15 seconds = about 3 miles (4.8 km)
- 20 seconds = about 4 miles (6.4 km)
So if you see a flash and count to 15 before you hear the thunder, the lightning struck about 3 miles away.
Interesting Facts About Lightning vs. Thunder
- Lightning can heat the air it passes through to 50,000°F (27,700°C), which is hotter than the surface of the sun!
- Thunder can range from a loud crack to a long, low rumble depending on what the sound waves reflect off of.
- Lightning travels down from a cloud, while thunder travels up to your ears.
- Summer storms generate more lighting since heat and moisture generate charge separation within clouds.
- Lightning causes an average of 49 deaths and 300 injuries each year in the United States.
The Lightning-to-Thunder Time
Meteorologists use the time delay between seeing lightning and hearing thunder as a way to estimate how far away a storm is. This lightning-to-thunder time is useful for tracking storms and staying safe if you are caught outside in a thunderstorm.
Here is a table showing the approximate distance for different lightning-to-thunder time intervals:
| Time Interval | Distance |
|---|---|
| 5 seconds | 1 mile |
| 10 seconds | 2 miles |
| 15 seconds | 3 miles |
| 20 seconds | 4 miles |
| 25 seconds | 5 miles |
| 30 seconds | 6 miles |
So if you count 20 seconds between a flash of lightning and hearing the thunder, that strike occurred about 4 miles from your location.
Staying Safe in a Thunderstorm
This handy lightning-to-thunder time table can help you estimate when a storm is getting dangerously close. If there are less than 30 seconds between lightning and thunder, it’s time to take shelter inside a sturdy building or vehicle.
Avoid being near trees or open fields since lightning can strike even several miles away from the storm. And never take shelter under trees, which are frequent targets of lightning strikes.
Conclusion
In summary, lightning clearly travels much faster than thunder. A lightning bolt zips between clouds or to the ground at up to 220,000 mph. Meanwhile, the resulting thunder travels at the slower speed of sound, about 767 mph. So lightning is around 300 times faster than its companion thunder.
The next time you experience a thunderstorm, count the seconds between seeing lightning and hearing thunder. This handy method allows you to estimate how far away the lightning struck. More importantly, tracking the lightning-to-thunder time can help you know when to seek shelter from the dangerous bolts flashing across the sky.