LIGHTNING

We were sailing South of Koh Phi Phi watching the top of the cumulonimbus cloud, a giant bulbous, rounded tower, blowing off West as the afternoon winds picked up, forming a classic anvil head. Lightning flashed in a streak from the leading edge of the anvil head to the gray surface of the ocean, near enough to the squid boat to outshine its lights. The streak was at least 10 kilometers long, typical of lightning strokes.

Lightning has fascinated man throughout history. The ancient Greeks believed that the God, Zeus threw lightning bolts at the Earth. Some American Indian tribes thought lightning resulted from the ThunderbirdĚs feathers and that thunder came from its flapping wings. In Thai mythology, the God Rama Soot threw an axe at the angel MakalaĚs crystal to make lightning and thunder.

Scientists currently believe that warm air rushing upward through a thunderhead rubs electrons off of the cooler air, accumulating them in the higher part of the cloud as a strong negative charge. The amount of water and ice in a thunderstorm play a part in the process, but the actual mechanism is still unknown.

We see two types of lightning discharges, cloud-to-ground and cloud-to-cloud. The stroke starts with a stepped leader, a pathway of negatively charged air created by the accumulated electrons, that moves in a series of microsecond, 50 meter dashes. It reaches from the cloud toward the ground below, looking for positive charges. Before this discharge reaches the ground, protons form a positively charged channel that starts from the ground, moving upward. The two discharges meet shorting the circuit; a return stroke forms which flashes upwards along the ionized channel. We see the original downward stroke, rather than the following return stroke.

In 1989 we discovered high altitude lightning, including Red sprites, and Blue jets produced by electromagnetic pulses colliding with molecules in the lower ionosphere. Sprites are weak luminous flashes that appear directly above an active thunderstorm and happen simultaneously with lightning strokes. They appear as red streamers from the cloud tops up to about 95 km., brightest at 75 km. Below the bright red region, blue tendrils often extend downward to near 40 km. These low-light phenomena are only seen about 1% of the time. Blue jets are distinct from sprites, observed above thunderstorms using low light television systems. They typically flash upward from the top of a thundercloud in narrow cones at about 100 km. per second.

Each lightning bolt generates between 100 million and 1 billion watts of electricity. 100 flashes take place every second from the 1800-2000 thunderstorms happening worldwide. Lightning superheats the air through which it passes to temperatures from 10,000 to 40,000 degrees C. The superheated air expands rapidly, like an explosion, then cools and contracts in an implosion. Together they form shock waves, which we hear as thunder. A lightning strike and resulting thunder occur almost simultaneously. But light travels at 302,600 kilometers in a second, almost a million times the speed of sound. Sound travels at a kilometer every 3 seconds. So the flash of lightning is seen before thunder is heard. By counting the seconds between the flash and the thunder and dividing by 3, you can estimate your distance from the strike in kilometers. Lightning always produces thunder, but if it is more than 20 kilometers away it cannot, usually, be heard, and is frequently misnamed "heat lightning". Close lightning produces a loud, short bang. Lightning that is further away produces a longer rumble as the sound waves echo from the sea or landscape.

Recently, an attorney in Minnesota asked a boatman to row him to the center of the small lake where he was vacationing with his girlfriend. A thunderstorm was in progress. The lawyer stood up in the aluminum bow of the rowboat raised his fist to the sky and shouted, "Give me your best shot!" He was immediately struck dead by a bolt of lightning. The boatman returned his body to his girlfriend, waiting on the shore. You would think a lawyer would know better than to say such things to the gods.