If a lightning strike is a sufficient distance from the observer, sound from the strike will not be heard. These silent bolts are called heat lightning. Lightning bolts produce thunder, but the thunder sound does not travel all the way to the observer if the observer is too far away. The movement of sound in the atmosphere depends on the atmospheric properties of the air such as temperature and density. Because temperature and density change with height, the sound of thunder is refracted through the troposphere. This refraction results in spaces of volume in which the thunder does not propagate through. The sound of thunder often reflects off the earth's surface. The rumbling sound from thunder is partly due to reflections off the earth's surface. This reflection and refraction leaves voids where thunder can not be heard. The earth's curvature also contributes to people far from the strike from not hearing it. Thunder is more likely to be bounced off the earth's surface before it reaches an observer far from the strike. With this said, the right refraction and reflection can result in people on the earth's surface being able to hear thunder at very far distances from the storm. The reflection and refraction in the troposphere determines who hears the strike and who doesn't. The term "heat" in heat lighting has little to do with temperature. Since heat lightning is most likely to be seen in association with air mass thunderstorms in the warm season, the term "heat" may have been used because these flashes are often seen when surface temperatures are warm. |