We see lightning before we hear thunder because light travels much faster than sound. Lightning and thunder happen simultaneously during a storm, but light travels at about 300,000 kilometers per second, whereas sound travels around 343 meters per second. This huge difference in speed means the light from the lightning reaches us almost instantly, but the sound of thunder arrives later, creating a delay between seeing the flash and hearing the thunder. The farther away the lightning strike, the longer the delay between the two sensations.

Why Light and Sound Travel Differently

• Light, being an electromagnetic wave, travels at approximately 300,000,000 meters per second in air.
• Sound, which is a mechanical wave, travels much slower, about 340 meters per second in air.
• Because lightning heats air and creates a shockwave, the air expands and contracts rapidly, producing thunder sound waves that lag behind the visual lightning flash.

Practical Implications

• The delay between lightning and thunder helps estimate the distance to a storm.
• Count the seconds between the lightning flash and thunder sound; dividing by three approximates the distance in kilometers (or by five for miles).
• If lightning and thunder seem almost simultaneous, the storm is very close and poses danger.

Thus, the apparent time difference between lightning and thunder is simply due to the different speeds at which light and sound travel through the atmosphere.