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JumpStart - Earth Science
Understanding Lightning
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QUICK FACTS
- One way to find the distance to lightning is to use the 'flash-to-bang' method. Count 5 seconds for every mile the lightning is away from you. Safe shelter must be reached completely in all situations before a flash is 2 to 3 miles away, which is 10 to 15 seconds flash-to-bang.
DID YOU KNOW?
 Lightning is the most dangerous and frequently encountered weather hazard that most people experience each year. It is the second most frequent killer in the United States with nearly 100 deaths and 500 injuries each year. (Floods and flash floods are the number one cause of weather related deaths in the US.)
How are thunderstorms detected?
We can see thunderstorms with a variety of tools. Radars let us see where rain and hail are located in the storm. Doppler radars also let us see how the wind is blowing within and near the storm. Some features of thunderstorms, such as the anvil that spreads out at the top of the storm, can be seen from satellites.
The Center for Analysis and Prediction of Storms (CAPS), located at the University of Oklahoma in Norman, is a National Science Foundation Science and Technology Center whose mission is to demonstrate the practicability of small-scale numerical weather prediction with an emphasis on deep convection.
Read More Information on Lightning --> Here
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LIGHTNING -- ONE OF NATURE'S MOST VIOLENT FORCES
Better Protection Begins with Better Knowledge of Lightning
 Although lightning has been known to be a discharge of electrical energy since Ben Franklin’s kite-flying days, the way electrical charges build up and discharge in clouds is still not fully understood. Researchers throughout the world have attempted to answer these questions so that improved means to detect and measure the charges can be developed.
What is known is that a lightning bolt is the transfer of a positive or a negative electrical charge from one region of a cloud to another, between clouds, or between a cloud and the ground. For such a transfer to take place, the two types of charges must be separated so that the cloud is electrified. Exactly how the charges become separated and where in the cloud they are located are still not completely clear.
Is a Thundercloud Like a Generator?
 However the details may turn out, it is well understood that thunderstorms separate electrical charge. Usually, a positive charge is pumped aloft while a negative charge accumulates near the lower-middle part of the storm. A small amount of positive charge may collect near the base of the storm cloud. It takes energy to separate the charge, and this energy comes from the rapidly rising air currents in the storm. Thus, like a generator, a thunderstorm converts mechanical energy to electrical energy.
Convection and the Formation of Thunderstorms
A thunderstorm is a natural heat engine. On a typical summer day the air is loaded with moisture and the land surface is hot. As the air near the surface is heated by the land, it expands, becomes less dense (hence lighter) and begins to rise. As it rises, it expands further, this time due to the lower pressure higher in the atmosphere rather than due to heating. In fact, as the air expands in the lower pressure, it cools because its internal energy is spread out over a larger volume. When moist air cools enough, it can no longer hold all the water it contained when it was warm. If it were on the ground, dew and fog might form. Aloft, the excess water condenses out as a patch of fog in the sky, which we call a cloud.
When water condenses it releases heat to its surroundings, just as when it evaporates, it absorbs heat (which is why a wet towel cools you on a hot day). The heat released when a cloud forms makes the air rise even more vigorously until a cloud thousands of feet high results. The cloud can continue to grow as long as it has a good source of warm, moist air at its base. As it grows it eventually becomes tall enough for the air in the cloud to cool below the freezing point (0° C).
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 Surprisingly, the water in the parts of the cloud cooler than 0° does not actually freeze until it gets considerably colder: -10° C to about -20° C. Liquid water colder than 0° is called "super-cooled" water. At temperatures below -10° to -20°, water vapor condenses directly to ice (technically, it is called "subliming" rather than "condensing" when this happens). As we will see, it is the mixture of ice and super-cooled water that probably accounts for most thunderstorm electrification.
Cloud droplets are too small to fall as rain, but turbulence in the cloud stirs things up and causes droplets and ice crystals to collide. Droplets may coalesce, and when a super-cooled droplet collides with an ice crystal, it will freeze to the crystal, thus enlarging it. Soon these larger ice crystals begin to fall through the super-cooled water and collect it, growing as they go. When they have fallen enough for the temperature to get above 0°, they melt, becoming raindrops.
Sometimes a small ice pellet will get coated with water and then get blown back up higher by a sudden updraft. Later it can fall again and gather even more water. This can happen several times if the updrafts and turbulence are strong enough. Then some really large ice particles can form and they may not melt before hitting the ground. These large ice particles are called hail.
The Precipitation Charging Theory
The most widely accepted explanation of how thunderstorms separate the charge is based on laboratory experiments and atmospheric observations with aircraft and radar which show that when ice crystals and super-cooled water droplets collide, if they don't coalesce, the pieces which are scattered after the collision are charged. Which pieces get which kind of charge, positive or negative, depends on the temperature, but at temperatures typical of the electrically active part of thunderstorms, the smaller pieces usually get the positive charges. These smaller fragments will be carried aloft by the updrafts while the negatively charged larger remnants fall. This results in charge separation and an upward transport of the positive charge.
The Mechanics of a Lightning Strike
It is a law of nature that positive and negative electrical charges attract each other. The strength of this attraction is called the "electric field." When enough charge has been separated, the force of attraction overcomes the electrical resistance of the air and a giant spark (lightning!) can occur.
Most lightning occurs within or between clouds. The destructive cloud-to-ground lightning bolt occurs much less frequently and can carry either a positive or a negative charge. Of the two, negative lightning is the most common type (about 90 percent). The process involved in generating a lightning stroke explains why lightning usually seeks out and strikes the highest point on the surface.
First, a negatively charged stepped leader from the cloud approaches the ground. During the approach, the leader’s tip causes electric fields on the ground to increase in strength. Positive ions gather around pointed objects as small as pine needles and grass blades and then flow in streams towards the leader. When they get close enough, closure of the cloud-ground circuit takes place and the leader is neutralized. Now a much more powerful return stroke flows through the ionized channel from the ground to the cloud. The grounded object serves as the focal point of the positive ion flow. That object, such as from tree to golfer with an upraised club, is considered "struck" by lightning. The whole process, from leader approach to discharge, takes place in less than a second.
 The return stroke is easily visible to the human eye, with the brightness of more than 100 million light bulbs. Actually, this bolt may have traveled back and forth between the cloud and the ground more than a dozen times — all in less than a second. The entire event is called a lightning flash.
Positive lightning carries a positive charge to the ground. It makes up less than 10 percent of a storm’s lightning strikes and typically takes place at the end of a storm. However, the positive lightning strike has the potential to cause more damage. It generates current levels up to twice as high and of longer duration than those produced by a negative bolt. It’s the long-duration, or "continuing current" components, of lightning that causes heating, burning and metal punctures. For that reason, scientists are especially interested in developing ways to detect the areas of a thunderstorm that develop positive bolts.
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Material on this web page courtesy of NASA
Photo Credit: NOAA Photo Library, NOAA Central Library
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