Heat Lightning is actually faint flashes of lightning reflecting outward from distant thunderstorms. These flashes usually do not produce thunder as the storm is sometimes too far away to be heard. The term heat lightning got its name because it often occurs on hot summer nights and does not produce audible thunder. One reason heat lightning can be seen so far away is due to the reflection of the light bouncing off water particles in moist, humid air and as light is scattered throughout the upper atmosphere. The clouds also act as a reflective surface. Lightning flashes can sometimes been up to 100 miles away over flat terrain. Visibilities of less than 10 miles are most common, and sometimes up to 25 miles.
Actual lightning is the result of the discharge of negative ions created from friction of ice and water particles colliding with one another at the base of a cloud. Heat lightning is sometimes an early warning sign of an approaching storm. While lightning usually always produces thunder, there exist reasons why it becomes inaudible to the human ear. Thunder rarely travels more than 10 miles. The refraction of sound within different densities of air may inhibit sound travel. Temperature changes overhead may refract the sound upward. Silent thunder also occurs when airborne matter muffles the sound of thunder, such as heavy snow during a winter storm. Dust and sand storms are capable of muffling the sound of thunder as well.