HOW LIGHTENING OCCURS / PRODUCTION OF LIGHTENING

All thunderstorms by definition produce lightning. Thunderstorms in the Southeast U.S. are common during the warm season. Therefore, everyone living in this area has experienced lightning at some point. It is an incredible natural phenomenon that amazes most. But, lightning is also very dangerous to humans, especially those involved in outdoor activities. Most people disregard the dangers of lightning because of the relatively low chance of being struck. However, lightning kills more people each year in the United States than all other weather events except flooding. On average, lightning kills 73 people annually. Injuries usually number in the hundreds. One question that is often asked is "Why do some storms produce a little lightning while others produce a tremendous amount?" Also, some have wondered whether there is a correlation between the amount of lightning and severe weather. The answer to these questions and others, including how to protect oneself from lightning, can better be answered with some background on how lightning works.

Introduction to Lightning Production 

The basic principle behind lightning is charge separation. This means a "pool" of positively charged particles becomes separated from a pool of negatively charged particles. A charge potential naturally develops when such a separation occurs. Basic electricity explains that if a charge potential between two areas in space becomes high enough, a discharge will occur. Since air is an insulator, the potential required for electricity to move through it is very high. If a discharge does occur between the two pools, it is called lightning. The obvious question is how does this charge separation get there in the first place?

The past fifty years of thunderstorm research have nearly unanimously concluded that the cloud charging process depends on ice collisions. So for lightning, ice must be present in a developing cumulonimbus and a large portion of the cloud needs to be well below freezing. Most cloud ice formation occurs between the temperatures of -10° and -40° C. Since the air normally cools with an increase in height, these temperatures are located many thousands of feet above the ground. A towering cumulus cloud needs to grow quite tall to reach this region of subfreezing temperatures. The cloud electrification process is not well understood, but is most likely caused by the collision of tiny ice particles and larger ice crystals. Tiny ice particles take on a positive charge during the collision, while the larger crystals are left with a negative charge. Since the smallest crystals are lighter, they are lofted into the upper portion of the cloud by the updraft. Hence, a pool of positive charge forms in the anvil region of the cloud, while the midlevels develop a negative charge pool. Of course, to get a significant separation of charge in the cloud, this collision process is occurring millions upon millions of times.

Diagram courtesy of COMET

So, once enough collisions have occurred, a large electrical potential has formed between the mid and upper levels of the cloud. Since the lowest portion of the cloud is charged negatively, positive charge in the Earth is attracted to its counterpart. Positive charge in the ground pools under the cloud, following the movement of the cloud. After enough cloud charging takes place, a discharge occurs between the cloud and the ground, to neutralize some of the charge.

Since these lightning bolts originate from a negatively charged portion of the cloud, they are termed "negative cloud-to-ground strikes." The vast majority of lightning from a mature thunderstorm originates from this portion of the cloud. They usually coincide with the rain and possibly hail falling from a storm. But, remember the positively charged ice particles that get lofted into the anvil? This charge can get isolated from the main updraft, so it is located well above the ground where a storm is sometimes not even apparent. Lightning also originates from the anvil, in the form of "positive strikes." These are almost always deadly due to the much higher amount of current required to reach the ground. Positive strikes are responsible for a large number of the annual death toll.

Variance in Lightning Strike Frequency 

The amount of lightning produced by any given thunderstorm can vary significantly. Since several important aspects of storm electrification are not well understood, the explanation for the variance in lightning frequency is also sketchy. A few hypotheses can be stated however. Most of the time, the amount of cloud-to-ground lightning produced by a storm is very similar to other storms on the same day in a similar air mass. Much research indicates that the efficiency of the ice forming processes in a thunderstorm correlate well with the amount of lightning produced. A high icing efficiency is usually achieved when the majority of the cloud is below -10° C. This type of environment often occurs during the spring and fall when the freezing level is closer to the ground. During the summer, icing efficiency can be achieved by a cool and dry layer aloft that enhances the ability of water to freeze.

Terrain such as mountains can also increase the amount of lightning yielded by a storm. Since storms initiate higher in the atmosphere above a mountain than over a plain, their updraft cores begin closer to the subfreezing layer. Forecasters at our office frequently notice cloud-to-ground strikes from very weak radar echoes over the mountains, while similar echoes over the low country are not producing any.

Large clusters of thunderstorms that commonly originate in the Plains states are notorious lightning producers. The flash frequency indicated by graphics from the National Lightning Detection Network is often tremendous, sometimes indicating greater than 5000 strikes in fifteen minutes! Those are usually spread over tens of counties, but nevertheless illustrate an amazing amount of electrical activity. Storm clusters are a large group of individual cells and updrafts, so there is an additive quality about their lightning production ability. Also, clusters are often tilted backward with height, so positive charge that gets lofted into the anvil can travel a significant distance behind the leading edge of the storms. This added charge separation seems to increase the total lightning strikes, especially the quantity of positive strikes from the anvils.

Lightning Safety

Protecting yourself and those whose safety you are responsible for is not too terribly difficult. It usually comes down to whether or not it is convenient. Usually, calling off a sporting event or changing your afternoon plans cannot depend entirely upon the forecast. Storms are very common in the Southeast on summer days. But, thunderstorms naturally provide a convenient warning about their approach: thunder! Generally, if you can hear thunder, you are at risk of being struck by lightning. A commonly misperceived notion is that it must be raining before you can get struck. A recent study found that most human lightning casualties occurred before and after the main rain core. Also, as mentioned earlier, positive strikes from the top of a thunderstorm usually strike areas quite distant from the parent storm. At least 10 percent of lightning strikes occur with no visible clouds overhead. Also, positive strikes are nearly always fatal to humans. So to avoid the risk of being struck by lightning, move to a substantial shelter or an enclosed vehicle until the storm passes and thunder can no longer be heard. If caught outdoors without adequate shelter, try not to be the tallest object. Crouch down in a grove of small trees, or at least get as low as possible and away from tall, isolated objects. Always avoid bodies of water such as pools, lakes and the ocean.

When indoors, a few safety rules should be followed as well. Since lightning is a potent source of electricity, it can move about poor electrical conductors with ease. If lightning strikes an object outside your house, it can travel inside via electrical wiring, plumbing, even clotheslines. Generally, avoid all contact or proximity to these types of conduits to vastly cut down on the risk. By remembering a few rules, the risk of being struck or affected by lightning can be greatly reduced. To minimize risk of damage to valuable indoor appliances, unplug as many as possible before a storm hits. Modern surge protectors cannot adequately protect electronics from lightning.

Lightning and Severe Thunderstorms

Lightning not only causes many deaths and injuries each year, but also much damage to property, especially electrical appliances. Numerous people, especially those who have lost property due to lightning, have suggested that the National Weather Service issue warnings for anomalous lightning activity. But, no research has yet linked thunderstorms' severity (hail, wind, tornadoes) to their quantitative lightning production. Although some severe thunderstorms produce an incredible amount of lightning, this is often because they have vigorous updrafts too. Since all thunderstorms produce lightning, which in turn can cause death and destruction regardless of quantity, we cannot issue warnings for lightning. On this note, reports to the NWS about lightning activity are of no use and actually disrupt our ability to warn for severe storms. Lightning activity is easily monitored remotely via the National Lightning Detection Network, displayed in a map format. Reports of hail, strong wind, flooding and storm rotation are much more useful because we cannot detect these events directly.

Conclusions

Lightning is a powerful yet incredible natural occurrence that affects almost everyone at some point in his or her life. From a safe indoor location, its beauty and grandeur can be marveled. Unfortunately, lightning deaths and injuries in the Southeast far outnumber other parts of the country. The main explanation for this is the high population density and large number of thunderstorm days. The abundance of outdoor activities in the Southeast, such as at lakes, sporting events, and farms also contribute to the higher casualties for the area. So, a greater public awareness is necessary to cut down on the number of lightning tragedies. It is imperative that coaches and leaders of outdoor activities be informed about proper lightning safety procedures. Individuals involved in such group activities will rarely if ever take the sole initiative to protect their own life by leaving prematurely. One primary mission of the National Weather Service is to inform the public of proper weather safety procedures to reduce or eliminate casualties. We hope that you will implement this information into your daily activities.