Nighttime thunderstorms are common in the Valley during the summer. Many of you can recall a situation where you see lightning off in the distance during the evening and someone says “Look, there is some heat lightning.” Heat lightning is usually explained as lightning that forms after a warm day and something that is inherently different than regular lightning. However, after reading this article, you will be well-versed in the physics of nighttime lightning and more importantly, you will be able to explain to your friends that heat lightning is not real.
Heat lightning is not real
Well, at least the notion that lightning is created from heat is not real. The lightning you see in the distance is real, but it is absolutely associated with thunderstorms (unless you are near a wildfire or a volcano which you should high tail it out of there if you are).
Thunderstorms generate lightning due to the collision of ice crystals and small hail in the presence of supercooled water. Each one of these collisions builds up a charge in the atmosphere and eventually leads to the generation of a lightning strike. You can view an explanation of the generation of lightning in thunderstorms below:
Heat lightning has a simple explanation: The phenomenon is simply lightning that is observed from a long distance away.
One observation you might make is you cannot always see the lightning strike hit the ground from a large distance. This is due to the curvature of the Earth and the fact that trees and/or hills could be limited your view.
Why do you not hear thunder from distance thunderstorms?
Now, this is a good question and requires a little talk about the physics of sound. We know that lightning and thunder are inherently linked together, but why does that relationship seem to fail with distant thunderstorms?
First, how does the sound of thunder form from a lightning strike? Lightning strikes are extremely hot and can heat the air around them to 50,000 degrees Fahrenheit (that’s five times hotter than the sun for those keeping up at home). This extreme heating of the air causes it to expand rapidly which generates sound waves.
Now, the speed of sound depends on temperature, humidity, etc. but it is typically around 740 miles per hour at the surface of the Earth. You would think that this would reach you pretty quickly, but the sound waves from thunder do not travel in a straight line.
Normally, the temperature cools with height in the lower atmosphere. Sound travels faster in warmer air than in colder air. Therefore, the bottoms of sound waves typically move faster than the top of the wave in Earth’s atmosphere. This causes the sound wave to bend upward away from the surface of Earth. Therefore, the further away someone is from the lightning strike, the less likely they are to hear thunder.
Sometimes, there is a temperature inversion where it is cool at the surface and warmer above the surface. In this special case, the sound of thunder can be louder and travel longer distances. This often happens with thunderstorms during the morning hours.
The next time your friends are talking about heat lightning you can pull up this article and drop some knowledge on them!