Heat lightning is a misnomer for the faint flashes of lightning on the horizon or other clouds from distant thunderstorms that do not have accompanying sounds of thunder. Heat lightning was named because it often occurs on hot summer nights, and to distinguish it from lightning accompanied by audible thunder and cooling rainfall at the point of observation. Ordinary lightning results from the discharge of negative ions created from the friction of ice and water particles bumping into each other at the bottom of a cloud. Heat lightning can be an early warning sign that thunderstorms are approaching. In Florida, heat lightning is often seen out over the water at night, the remnants of storms that formed during the day along a sea breeze front coming in from the opposite coast.
There are two possible reasons for the lack of thunder. In some cases, the thunderstorm may be too distant to hear the associated thunder from the lightning discharge. Thunder rarely travels more than 10 miles . Other cases can be explained by the refraction of sound by bodies of air with different densities. An observer may see nearby lightning, but the sound from the discharge is refracted over his head by a change in the temperature, and therefore the density, of the air around him. As a result, the lightning discharge seems to be silent. The first situation is far more common, however, and in these cases some fraction of the light emanating from distant thunderstorms (whose distant clouds may be so low to the horizon as to be essentially invisible) is scattered by the upper atmosphere and thus visible to remote observers.
Under optimum conditions the most intense thunderstorms can be seen at up to 100 miles (161 km) or more distant over flat terrain or water when the clouds are illuminated by large lightning discharges, though an upper limit of 30-50 (48-80 km) miles is more common due to topography, items on the horizon like trees and so on, the fact that local visibilities above 25 miles (40 km) are uncommon in most areas, and other low and mid level clouds. Variability of anvil height also contributes -- 45 000 feet (13 715 m) is very common in the mid latitudes for warm season thunderstorms but the range can be from 35 000 (10 665 m) to a current record of 78 000 feet (23 770 m). There is evidence that suggests some cases of heat lightning are, in fact, slow, diffuse discharges of electricity. This is not to be confused with electrically-induced luminosity actually generated at mesospheric altitudes above thunderstorm systems (and likewise visible at exceedingly great ranges), a phenomenon known as "sprites."
Silent lightning also occurs where airborne matter muffles the thunder, such as heavy snow in winter storms (thundersnow) and dust and sand storms. In some instances heavy falling snow has silenced thunder from cloud to ground lightning strokes as close as one to two miles (1.6 to 3.2 km) from the observer and severe dust storms are even more effective in many cases.