As humans, our eyesight is much keener than our hearing. Thus on a dry, warm summer night, it is possible to see lightning reflected off the bottom of clouds as far away as 100 miles while our ears can’t pick up the sound of thunder unless it is within 10-12 miles of us. Thus, the terms heat lightning, dry lightning, and silent lightning have all become synonymous. It’s not really silent; we just can’t hear the thunder. Or sometimes, even if we are closer than 12 miles, the sound of the thunder is refracted by a change in air temperature or density and the normal sound range is diminished. Because heat lightning occurs in the summer, it usually is dry.
The traditional cause of lightning is a disparity in electrical charge of water droplets within a cloud. The lightning happens due to the friction created by ice and water droplets bumping into each other at the bottom of a cloud, thus discharging negative ions. Rising warmth from the friction heat causes polar opposites within the cloud. And Zap – lightning from cloud to cloud or cloud to ground. In addition to earth warming caused by summer heat, fires or volcanic activity can also cause convection leading to pyrocumulus clouds that produce lightning.
Heat lightning, especially when seen over the ocean, can be the warning that thunderstorms are on the way. On land where there are items on the horizon like trees, the lightning can usually only be seen for 30-50 miles but over open water, the distances increase to 100 miles. The height of the clouds also is a factor in our being able to see the lightning produced. “Sprites” are electrically-induced phenomenon in the mesosphere (highest level of the atmosphere) but are not technically lightning.
There may actually be precipation within the clouds; it just doesn’t reach the ground because of the warm dry air beneath them. Heat lightning associated with dry thunderstorms are the primary cause of fire in isolated wilderness regions. While nationwide, 80% of wildfires are caused by humans, 60-70% of forest fires in the American Southwest, Rocky Mountains, and Sierras are caused by lightning. The fires themselves produce more heating and thus the potential for more heat lightning. Since there is no accompanying rainfall, the fires grow with each lightning strike. Even normally cool areas such as the Alps have suffered from heat lightning storms such as the drought related fires of 2003. Heat lightning-caused fires are frequently in remote area, and the first reports of their existence often come from commercial airline pilots.
Heat lightning is difficult to see in the daytime due to the hazy sky. It is only with the contrast of the darkness that we see this extraordinary occurrence. Any information about lightning should always include a warning as to its potential danger. Sometimes, people assume that if they can’t hear thunder that the lightning has to be far away. If lightning can be seen, precautions should be taken to be safe. One never knows what the lightning potential directly overhead may soon be. Heat lightning is a sight to behold on a warm summer night and a reminder of the power and complexity of the natural world we live in.