Thunder is the sound that lightning makes made up of vibrations that travel through the air! People have been killed from being hit by lightning. According to Lightningsafety.noaa.gov, U.S. lightning deaths in 2017 is 6 (known fatalities to date).
During a thunderstorm, you see a bright flash of lightning. Multiple seconds later, you hear the loud rumble of thunder. How does this happen if thunder and lightning come from the same place and occur at the same time? The answer involves the speed at which sound and light travel.
Light is the electromagnetic radiation within a certain portion of the electromagnetic spectrum. Lightning causes light in the form of plasma, which is created by the electrostatic discharge that take place between the electrically charged regions of two or more clouds, or the electrically charged regions of a cloud and the ground.
The term "light" mostly refers to visible light, which is responsible for the sense of sight in the human eye. Wavelengths around 400-700 nano-meters make up visible light. Light travels at 186,282 meters per second (670,616,629 mph), so in theory, nothing moves faster than the speed of light. The speed of sound is commonly stated as the speed of sound waves through dry air, which is about 343.2 meters per second (768 mph) in air at a temperature near 20°C. This is significantly slower than the speed of light.
This indicates that the reason why you would see lightning first in a thunderstorm is because the visible light waves formed by the plasma in the lightning travels to your eyes much faster than the rumble or crack of thunder reaches your ears. Therefore, when you hear rumbling thunder, the lightning bolt was far away because of the effect of dispersion, which is the action or process of distributing things over a wide area. When you hear a very loud crack or boom, this means that the lightning bolt was near your location.
Of interest, check out these Lightning Myths and Facts shared by Lightningsafety.noaa.gov.