Lightning: how it forms

National lightning safety awareness week

(Eric Zsolczai)

Lightning is fascinating to watch but also extremely dangerous. In the United States, there are about 25 million lightning flashes every year. Each of those 25 million flashes is a potential killer. While lightning fatalities have decreased over the past 30 years, lightning continues to be one of the top weather killers in the United States. In addition, lightning injures many more people than it kills and leaves some victims with life-long health problems.

How Thunderstorms Develop
All thunderstorms go through stages of growth, development, electrification and dissipation. Thunderstorms often begin to develop early in the day when the sun heats the air near the ground and pockets of warmer air start to rise in the atmosphere. When these pockets of air reach a certain level in the atmosphere, cumulus clouds start to form. Continued heating causes these clouds to grow vertically into the atmosphere. These "towering cumulus" clouds may be one of the first signs of a developing thunderstorm. The final stage of development occurs as the top of the cloud becomes anvil-shaped.

As a thunderstorm cloud grows, precipitation forms within the cloud. A well-developed thunderstorm cloud contains mostly small ice crystals in the upper levels of the cloud, a mixture of small ice crystals and small hail in the middle levels of the cloud, and a mixture of rain and melting hail in the lower levels of the cloud. Air movements and collisions between the various types of precipitation in the middle of the cloud cause the precipitation particles to become charged. The lighter ice crystals become positively charged and are carried upward into the upper part of the storm by rising air.

The heavier hail becomes negatively charged and is either suspended by the rising air or falls toward the lower part of the storm. These collisions and air movements cause the top of the thunderstorm cloud to become positively charged and the middle and lower part of the storm to become negatively charged.

In addition, a small positive charge develops near the bottom of the thunderstorm cloud. The negative charge in the middle of thunderstorm cloud causes the ground underneath to become positively charged, and the positively charged anvil causes the ground under the anvil to become negatively charged.

How Lightning Forms
Lightning is a giant spark of electricity in the atmosphere or between the atmosphere and the ground. In the initial stages of development, air acts as an insulator between the positive and negative charges in the cloud and between the cloud and the ground; however, when the differences in charges becomes too great, this insulating capacity of the air breaks down and there is a rapid discharge of electricity that we know as lightning.

Lightning can occur between opposite charges within the thunderstorm cloud (Intra Cloud Lightning) or between opposite charges in the cloud and on the ground (Cloud-To-Ground Lightning). Cloud-to-ground lightning is divided into two different types of flashes depending on the charge in the cloud where the lightning originates.

Thunder is the sound made by a flash of lightning. As lightning passes through the air it heats the air quickly. This causes the air to expand rapidly and creates the sound wave we hear as thunder. Normally, you can hear thunder about 10 miles from a lightning strike. Since lightning can strike outward 10 miles from a thunderstorm, if you hear thunder, you are likely within striking distance from the storm.

Thunderstorm Development

Thunderstorms typically develop in the warmer months of spring, summer, and fall, but they can occur at any time of the year over most of the United States. There are three basic ingredients needed for thunderstorm development: moisture, an unstable atmosphere, and some way to start the atmosphere moving.

Moisture is necessary to produce the thunderstorm clouds and precipitation. In the summertime, most areas of the United States have sufficient moisture to generate thunderstorms if the other ingredients are present. In the wintertime, thunderstorms favor southern areas of the United States where moisture is more plentiful; however, southerly winds associated with well-developed storm systems can bring sufficient moisture northward to generate thunderstorms at any time of the year, even in the dead of winter.

Atmospheric stability, or more importantly, instability, also plays an important role in thunderstorm development. Rising air is needed to produce clouds, and rapidly rising air is needed to produce thunderstorms. For air to rise rapidly, it must become buoyant compared to the surrounding air. When the atmosphere is unstable, air near the ground can become buoyant and rise rapidly through the atmosphere. In general, the warmer the air is near the earth's surface and the colder the air is aloft, the more unstable the atmosphere is. In addition to temperature, moisture near the ground can also contribute to the instability of the atmosphere.

The third ingredient needed for thunderstorm development is something that will trigger motion in the atmosphere. This may be some sort of boundary such as a front, heating caused by the sun, or cooling aloft. Once a thunderstorm has developed, it will continue to generate boundaries that can trigger additional storms.

In the summer, thunderstorms typically develop in the afternoon when the sun heats air near the ground. If the atmosphere is unstable, bubbles of warm air will rise and produce clouds, precipitation, and eventually lightning.

How thunderstorms become electrified

While the exact details of the charging process are still being studied, scientists generally agree on some of the basic concepts of thunderstorm electrification. The main charging area in a thunderstorm occurs in the central part of the storm where air is moving upward rapidly (updraft) and temperatures range from -15 to -25 Celsius.

At that place, the combination of temperature and rapid upward air movement produces a mixture of super-cooled cloud droplets (small water droplets below freezing), small ice crystals, and soft hail (graupel). The updraft carries the super-cooled cloud droplets and very small ice crystals upward. At the same time, the graupel, which is considerably larger and denser, tends to fall or be suspended in the rising air. The differences in the movement of the precipitation cause collisions to occur. When the rising ice crystals collide with graupel, the ice crystals become positively charged and the graupel becomes negatively charged.

The updraft carries the positively charged ice crystals upward toward the top of the storm cloud. The larger and denser graupel is either suspended in the middle of the thunderstorm cloud or falls toward the lower part of the storm. The result is that the upper part of the thunderstorm cloud becomes positively charged while the middle to lower part of the thunderstorm cloud becomes negatively charged.

The upward motions within the storm and winds at higher levels in the atmosphere tend to cause the small ice crystals (and positive charge) in the upper part of the thunderstorm cloud to spread out horizontally some distance from thunderstorm cloud base. This part of the thunderstorm cloud is called the anvil. While this is the main charging process for the thunderstorm cloud, some of these charges can be redistributed by air movements within the storm (updrafts and downdrafts). In addition, there is a small but important positive charge buildup near the bottom of the thunderstorm cloud due to the precipitation and warmer temperatures.

The charges on the ground are influenced by the charge build up in the clouds. Normally, the ground has a slight negative charge however, when a thunderstorm is directly overhead, the large negative charge in the middle of the storm cloud repels negative charges on the ground underneath the storm. This causes the ground and any objects (or people) on the ground directly underneath the storm to become positively charged.

As the negative charge in the cloud increases, the ground responds by becoming more positively charged. Similarly, the positive charge in the anvil can cause a negative charge to build up on the ground under the anvil (which may extend far from the thunderstorm base).

Types of flashes

There are two main types of lighting: intra-cloud lightning and cloud-to-ground lightning. 

Intra-cloud lightning is an electrical discharge between oppositely charged areas within the thunderstorm cloud.

Cloud-to-ground lightning is a discharge between opposite charges in the cloud and on the ground. Cloud-to-ground lightning can either occur between negative charges in the cloud and positive charges on the ground (a negative flash) or between positive charges in the cloud and negative charges on the ground (a positive flash).

Each cloud-to-ground lightning flash consists of one or more leaders followed by one or more return strokes. The leader is the initial step in the lightning flash and establishes the conductive channel that the electrical discharge (lightning) will take. There are different types of leaders. The most common type of leader is the negative stepped leader. Once a charged leader makes a connection with the ground, the return stroke occurs. The return stroke is simply the rapid discharge of electricity that has accumulated on the leader. We see this discharge as the bright flash of lightning.

Understanding lightning: negative flash

Most lightning flashes are a result of negatively-charged leaders, called stepped leaders. These leaders develop downward in quick steps. Each step is typically about 50 meters (150 ft) in length. Stepped leaders tend to branch out as they seek a connection with the positive charge on the ground. When a branch of the stepped leader reaches within about 50 meters of the ground or some object on the ground, it connects with an upward-developing positive charge, often referred to as an upward streamer.

Upward streamers tend to develop from the taller objects beneath one or more branches of the stepped leader. When the downward-developing negative stepped leader makes contact with an upward-developing positive streamer, referred to as the attachment process, a conductive path is established for the rapid discharge of electricity that we see as a bright flash.

In general, stepped leaders travel at about 200,000 miles per hour, although speeds vary considerably. The highly visible return stroke moves upward through the leader channel at about 200 million miles per hour. The combination of the stepped leader and return stroke happens in just a fraction of a second. While both leaders and return strokes produce visible light (leaders produce a faint light that is more visible at night), they happen so quickly that the human eye cannot distinguish the two; however, high-speed cameras are able to capture the movement of leaders as they move toward ground.

Positively charged lightning 

The previous section discussed flashes produced from downward-propagating negatively-charged leaders, commonly referred to as stepped leaders; however, about 10% of lightning flashes are positive flashes and are produced by downward-propagating, positively-charged leaders. While both positive and negative flashes are deadly, there are significant differences between the two in terms of their formation and behavior.

Positive leaders typically originate from the positive charged area in the upper part of the thunderstorm cloud. Normally, the ground is shielded from this upper positive charge by negative charges in the central part of the storm; however, when upper level winds are stronger than lower level winds and the storm becomes tilted, or when the anvil of the thunderstorm cloud spreads out ahead of or behind the updraft of the thunderstorm, the ground is no longer shielded from this upper charge. If charge differences between this upper level charge and the ground become too large, a downward-moving positively-charged leader can develop.

Because positive flashes often originate away from the main thunderstorm updraft, they often occur in areas some distance from where rain is occurring. Also, because the positive charge center is higher in the atmosphere and a much greater charge differential is needed to initiate a lightning flash, positive flashes occur much less frequently and are much larger distances between flashes. Consequently, many people are caught by surprise by positive flashes. Also positive flashes can precede the main part of the thunderstorm and the rain area by 5 to 10 miles or more.

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