Speed of Light, Speed of Sound, and Cricket Thermometers
Hello. My name is Glenn Philips.
Ronald “Cheesie” Mack, my friend and classmate, asked me to explain how to calculate the distance to the flashing of a lightning bolt. And I am also going to explain how you can tell the temperature by listening to crickets chirp.
First let me say I very much enjoyed being mentioned in Ronald’s book.
In that book, Ronald wrote:
At school, just as Georgie and I entered class, the room lit up with a flash of lightning.
Instantly Glenn Philips started counting out loud. “One thousand one . . . one thousand two . . . one thousand three . . .” He had just passed seven when a huge clap of thunder surprised us.
A couple of kids shrieked. One dropped a book.
“That lightning is about one and a half miles away,” Glenn announced calmly.
Georgie looked out at the dark sky. “How can you tell?” he asked.
“It takes sound just about five seconds to travel a mile,” Glenn explained. “Light travels almost a million times faster. That’s why we see the lightning flash almost instantly. But it takes a while for the sound to reach us. So, if you count the seconds from the flash until you hear the thunder and divide by five, the result is the number of miles away.”
What Cheesie wrote is accurate, but I’ll explain it in a bit more detail. Sound travels very rapidly. In dry air at 68 °F (20 °C), the speed of sound is 1,126 feet/second (343.2 meters/second)). This is equal to 767 miles/hour (1,234 kilometers/hour). If you’ve ever been a passenger on a commercial jet, they travel at 500-600 miles per hour. Since a mile is 5,280 feet, sound travels a mile in just about five seconds (5 x 1126 = 5,630 feet).
Light travels much, much MUCH faster: about 186,000 miles/second or 671 million miles/hour! It is the fastest movement in the universe. Scientific theory states that NOTHING can travel faster than light.
That means when lightning flashes, the image travels to your eye almost instantly. If you immediately count the seconds until you hear the thunder and divide by five, you get the distance in miles…approximately.
And just for fun, here’s a fairly accurate way to tell the temperature. Listen for a cricket. Then count the number of chirps in 14 seconds and add 40 to get temperature in degrees Fahrenheit. To convert cricket chirps to degrees Celsius, count number of chirps in 25 seconds, divide by 3, then add 4 to get the temperature. You can try it here. I recorded this cricket sound in my backyard.
What do you think the temperature was on that evening?
(Georgie Sinkoff drew the pictures on this page. You can tell because he put a G on the tail of the jet plane!)