Thunder is the sound produced by lightning, which generates an immense amount of heat, instantly raising the temperature of the air in its path. This rapid heating causes the air to expand explosively, creating a powerful shockwave that we perceive as the characteristic rumbling or cracking sound. The speed at which this sound reaches an observer is directly tied to the fundamental speed of sound in the atmosphere, which allows for a practical estimation of a storm’s proximity.
The Specific Speed of Sound in Air
The speed of sound in air is not a fixed universal constant, but it is defined under standard atmospheric conditions. At sea level, with a temperature of 20 degrees Celsius (68 degrees Fahrenheit), the speed of sound is approximately 343 meters per second. This speed is often referred to as Mach 1. In more common units, this baseline speed translates to roughly 1,125 feet per second, or about 767 miles per hour.
Factors That Influence Sound Speed
The velocity of sound is highly dependent on the properties of the medium it travels through, with temperature being the most influential factor in the atmosphere. Sound travels faster in warmer air because the air molecules possess more kinetic energy, causing them to vibrate and collide more frequently. This allows the sound wave’s energy to be transferred more rapidly. The speed increases by about 0.6 meters per second for every one-degree Celsius increase in temperature. Humidity also plays a small role; moist air is slightly less dense than dry air, allowing the sound wave to move slightly quicker.
Calculating Distance Using the Flash-to-Bang Method
The most practical application of sound’s speed is the “flash-to-bang” method, a simple technique used to estimate the distance to a lightning strike. This method relies on counting the number of seconds that pass between seeing the lightning flash and hearing the resulting thunder. The measured time interval is dedicated almost entirely to the sound’s travel time, since the light reaches the observer instantaneously. A simple rule of thumb for this calculation is that for every five seconds counted, the lightning strike is approximately one mile away. For instance, if you count 15 seconds, the storm is roughly three miles away from your location.
Comparing Sound Speed to Light Speed
The effectiveness of the flash-to-bang method stems from the massive speed difference between sound and light. While sound travels at a few hundred meters per second, light travels at an incredible 186,000 miles per second, or nearly 300,000 kilometers per second. This difference means the light from a lightning strike reaches your eye in a fraction of a millisecond, even if the storm is miles away. Therefore, the time measured between the flash and the bang is the total time it took for the sound wave to traverse the distance. Because the light’s travel time is negligible, the calculation provides a reliable estimate of the storm’s position.