What is the phenomenom behind heat lightening?

Heat lightning: Flashes too far away for observer to hear the thunder. Like sheet lightning, these flashes are created by lightning bolts, but are in thunderstorms more than 10 miles away. Trees, buildings and urban noise can cut this distance to less than five miles. It's called heat lightning because it is seen more frequently on hot summer nights when the sky overhead is clear. Often, air molecules and dust particles in the atmosphere refract the light coming from distant lightning, making the bolts or flashes appear orange.
SOURCE:
http://www.usatoday.com/weather/tg/wflash/wflash2.htm