Thunder and Lightning

Most of us are in awe of lightning. We often sit by the window and watch, as lightning dances across the heaven. But it isnt until the thunder and lightning happen simultaneously that we begin to appreciate the power of our world’s electrical forces. Even in this setting, however, the true impact of lightning escapes us. That’s because lightning rarely grabs the headlines in the same way as hurricanes, winter storms and tornadoes. Lightning does its work in small numbers and in fractions of a second. However, when it strikes, it really strikes! Lightning is basically a gigantic spark that is an electrostatic discharge. It is much like the spark that occurs when you scuff your sock-footed feet across a carpet, in very dry air, and you touch a doorknob and receive a small shock. The only difference is that the lightning is much, much more powerful. The Earth is jolted by lightning a lot! Each day, about 44,000 thunderstorms occur across the planet, with about 2,000 in progress at any moment. Each second, there are about 100 lightning discharges. But the distribution of lightning across the planet favors tropical and middle latitudes and the land masses that lie in these bands.

Thanks to NASA’s space probes, lightning has been detected on other planets in our solar system: the Galileo spacecraft photographed lightning on Jupiter; in 2006, lightning was detected on Saturn thanks to the Cassini spacecraft’s radio instruments. The radio equipment “crackled” much like an AM channel has “static” when thunderstorms are nearby.

While the thunderstorm process creates electric charges that then try to group themselves into charge center, the atmosphere acts as a giant insulator, preventing the transfer of electricity and allowing huge amounts of charge to build up. When the strength of the electric field inside and /or near a thunderstorm reaches a critical level, current starts to flow from one charge center toward an opposite charge center. The outcome, lightning, occurs when charge centers connect. In the process, the temporary charge centers in the atmospheric electric filed come into balance and the ionosphoeric- Earth voltage differential is restored. Lightning discharges can occur between a cloud and the ground (or anything on the ground), cloud and air, and inside and between clouds. When lightning occurs inside a cloud or even between clouds, you may not be able to actually see the flash, just the brightening of the cloud. Lightning between a cloud and ground is typically referred to as “Cloud-to-ground” lightning. Today, we refer to lightning discharge as a flash. Years ago lightning was referred to as a “bolt”, probably a throwback to historical Greek, Roman and Norse times when gods were thought to literally throw “bolts” of lightning.

As Lightning occurs, its high temperature causes the air adjacent to the channel to expand rapidly, generating a sonic boom that we call thunder. Since sound and light travel at different speeds in the atmosphere, it is easy to estimate how far away the lightning is. Lightning travel at the speed at light (186,000 miles (300,000 km) per second and the light from lightning reaches your eyes almost instantaneously. Sound travels much more slowly (around 760 mph (1,223 km/h)). Thus, it takes about five seconds for the sound to travel one mile. (The actual speed of sound is dependent to an extent upon atmospheric temperature; the sound wave can also be affected by wind.)

To get an approximation of how far away the lightning was from you in miles, count the number of seconds between the lightning flash and thunder sound and divide by five. Be careful not to assume that this is a horizontal distance-the thunder may have been produced inside a cloud or in an arc across the sky from one cloud to another.

When the lightning is very close, the sound may appear to be a crack or a boom. Thunder further away, and whose path is at a varying distance from you, will allow sound from many parts of the lightning channel to reach you over several seconds, causing a rumbling or rolling sound. Such variations in sound can also be caused by sound echoing off building, mountainsides and other large objects. Sounds can diminish as it moves through the atmosphere due to attenuation (the sound wave itself loses strength the further it gets away from its source), the curvature of the Earth and even interaction with other thunders. As a result, thunder is rarely heard more than 10 miles (16 km) from its source. Distant thunderstorms, especially if they are “over the immediate horizon” can provide a lightning show without visible lightning channels and without thunder. When this occurs, people may refer to it as “heat lightning”. It was once thought that the air got so warm that it expanded and sparked. In some places where there are few obstructions to visibility (e.g. parts of Kansas, Arizona and Florida)’ it is easy t o see night time flashes from thunderstorms more than 100 miles away. This usually happens in the summer when skies overhead are cloud-free and temperature and humidity values are very high.

Lightning Conductors help to channel a lightning strike to the ground where it can dissipate. These evolved from Benjamin Franklin’s famous “kite in a thunderstorm” experiment in 1752 and his later efforts at developing a lightning conductor. Franklin’s “conductor” was a metal rod or wire through which electrical discharges were led harmlessly to earth. Others who studied lightning around the same time were Thomas Francois D’Alibard of France and GW Richmann, a Swedish physicist. Richmann was the most unlucky of the three as he was struck and killed by lightning during his experiments. Although some recent studies suggest that such protection systems may not be effective, personal and professional experience suggest otherwise. A friend who owns a home on North Carolina’s Outer Banks experienced a lightning strike one summer. She was home at the time. Following the strike, she smelled smoke and called the fire department. The fire was minor and quickly extinguished. The fire department noted that had she not had a lightning system on her home, her home would almost certainly burned down.