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Lightning is one of the most spectacular meteorological phenomena and the most common severe weather event to affect people directly. Below are answers to frequently asked questions about lightning.

The Earth's electrical structure

## The Earth's electrical structure

### What is the Earth's charge?

The Earth is electrically charged and acts as a spherical capacitor. The Earth has a net negative charge of about a million coulombs, while an equal and positive charge resides in the atmosphere.

The electrical resistivity of the atmosphere decreases with height to an altitude of about 48 kilometres (km), where the resistivity becomes more-or-less constant. This region is known as the electrosphere. There is about a 300,000 volt (V) potential difference between the Earth's surface and the electrosphere, which gives an average electric field strength of about 6 V/metre (m) throughout the atmosphere. Near the surface, the fine-weather electric field strength is about 100 V/m.

### What charges the earth?

The atmosphere is not a perfect insulator, and there is a small current between the electrosphere and the Earth. Negative charge leaks from the Earth and rises to the electrosphere. This is called the fair weather electric and it is about 2,000 amperes (A) at any given moment. At this rate, the Earth's charge would dissipate in less than an hour, but, as it turns out, lightning recharges the Earth's surface by delivering negative charges back to the surface.

### How many lightning flashes occur each day?

There are roughly 2,000 thunderstorms in progress around the world at any one time, producing about 30 to 100 cloud-to-ground flashes each second or about five million flashes a day.

### If there is a fair weather electric field strength of 100 V/m, why can't I set up two plates 10 centimetres (cm) apart, which could act like a battery and power my Walkman?

You can't do this because batteries don't work that way. The resistivity of air is 3 gigaohms/m for a 1-cm cross section near the surface, thus the internal resistance of this hypothetical battery would be too great to be feasible.

Thunderstorm structure

## Thunderstorm structure

### What is a thundercloud?

Lightning is associated with convective activity. Thunder (and thus lightning) is used by the professional weather observer to classify the severity of convective activity. Cumulonimbus clouds are the largest form of convective cloud and typically produce lightning. Cumulonimbus clouds with lightning activity are generally referred to as thunderclouds.

### How are thunderclouds charged?

The classical thundercloud model can be described as a positive electric dipole with a positively charged region above a negatively charged region. A weaker, positively charged region at the base of the cloud gives it more of a double-dipole structure, but because of the weak strength of the lower charged region, both the positive dipole and the double-dipole can be used to describe the general structure of a thundercloud.

The three centres of accumulated charge are commonly labeled p, N, and P. The upper positive centre, P, occupies the top half of the cloud. The negative charge region, N, is located in the middle of the cloud. The lowest centre, p, is a weak, positively charged center at the cloud base. The N and the P regions have approximately the same charge, creating the positive dipole. Malan (1963) documented charges and altitudes above ground level for the p, N, and P regions of a typical South African thundercloud (1.8 km above sea level) as +10 coulombs (C) at 2 km, -40 C at 5 km, and +40 C at 10 km. These are representative of values that can vary considerably with geography and from cloud to cloud.

### Is there an association between lightning activity and radar echoes?

There is a general association between radar reflectivity and negatively charged lightning flashes. Lightning discharge sources are located near, but not necessarily within, the area of highest reflectivity (MacGorman et al. 1983). This is supported by Mazur et al. (1983) and Mazur et al. (1985). In two studies of thunderstorms developing off Wallops Island, Virginia, Mazur found that the region of maximum flash density was close to the leading edge of the precipitation core, defined by 50 dBZ weather radar reflectivity. Though Mazur did not state the polarity of these flashes, it is inferred that they come from the negative charge centre. Lopez et al. (1990) also observed that, in a Colorado thunderstorm, the peak lightning activity occurred in the gradient areas of high reflectivity.

Charge generation in thunderclouds

## Charge generation in thunderclouds

### How do clouds get charged?

It is not fully known how thunderclouds get charged. There are two general theories to explain the charge buildup required to electrify a thundercloud. They are the convective theory and the gravitational theory.

The convective theory proposes that free ions in the atmosphere are captured by cloud droplets and then are moved by the convective currents in the cloud to produce the charged regions.

The gravitational theory assumes that negatively charged particles are heavier and are separated from lighter positively charged particles by gravitational settling.

For the gravitational theory to work, there must be some charge exchange process between particles of different sizes. Charge can be exchanged between particles in various states by inductive and non-inductive processes. The most promising is the non-inductive exchange between ice crystals and hailstones, referred to as the ice-ice process.

The effectiveness of the ice-ice process lies in the thermo-electric properties of ice. The mobility of the (OH3)+ defect in ice is greater than the (OH)- defect and the number of defects increase with temperature. When warm and cold ice particles come in contact, the positive defect flows faster from the warmer to the colder particles than the converse, giving the colder particles a net positive charge. Therefore in the typical scenario, a warm hailstone or snow pellet will acquire a net negative charge as it falls through a region of cold ice crystals.

Theories of thundercloud charge generation are still very speculative. The favorability of one process over another has fluctuated over time due to the inadequate number of laboratory experiments and scarcity of useful field observations. One clear conclusion is that there is no unique mechanism to generate the required charge under all conditions. For example, the ice-ice process, presently the most favored, does not explain warm cloud lightning, albeit a not too frequent event. As research develops, the most likely explanation will lie in a combination of factors.

The lightning flash

## The lightning flash

### Why does lightning occur?

The charge buildups in thunderclouds are unstable. When electric fields generated by the charge buildups become too strong (typically 3-4 kilovolt/cm at the altitude of the negative charge region of the cloud) electrical breakdown of the air occurs and charge is exchanged within the cloud or to the ground. Charge is exchanged by a lightning flash.

### Is all lightning the same?

Lightning can occur in four ways. Lightning can travel between points within a cloud, from a cloud to clear air, from a cloud to an adjacent cloud, and from a cloud to the ground. These flashes are referred to as intracloud, cloud-to-air, cloud-to-cloud, and cloud-to-ground, respectively.

Intracloud (IC) flashes, redistributing the charge within the cloud, account for over half the lightning flashes in northern latitudes (Uman and Krider 1989). Cloud-to-cloud and cloud-to-air flashes are less common. Aside from aviation, these three types of flashes have little effect on people.

Cloud-to-ground (CG) flashes are very common and have been well documented. They exchange charge between the cloud and ground. These flashes affect people greatly, causing injury and death, disrupting power and communications, and igniting forest fires. Because of these effects, the cloud-to-ground flash has been the topic of much research.

The cloud-to-ground lightning flash can lower positive (+CG) or negative (-CG) charge, depending on the source of the flash. This can be determined by the polarity of the stroke's current. Characteristics of negative and positive cloud-to-ground flashes are summarized below.

Some characteristics of positive and negative cloud-to-ground flashes
Characteristic Negative Positive
Percent occurrence 90 10
Average peak current (kiloamperes) 30 35
Average current half life (microsecond) 30 230
Average number of strokes 3-4 1
Percent containing long continuing current 20 80

Ground-to-cloud flashes (those that originate from the ground) occur as well, as observed from large buildings such as the Empire State Building, but they are not normally distinguished from CG flashes in studies.

### Does lightning go up or down?

The answer is ... both! A cloud-to-ground lightning flash, by definition, originates from the cloud but flashes often originate from the ground, as suggested by the lightning branch structures observed in photographs.

To properly answer whether lightning goes up or down one must look at the processes involved in a lightning flash.

The negative cloud-to-ground lightning flash can be broken down into three stages. The stepped leader, the return stroke, and the dart leader.

The stepped leader is a small packet of negative charge that descends from the cloud to the ground along the path of least resistance. In its path, the leader leaves a trail of ionized gas. It moves in steps, each typically tens of metres in length and microseconds in duration. After a step, the leader pauses for about 50 microseconds, then takes its next step. The leader charge packet sometimes breaks up to follow different paths, giving lightning its forked appearance.

As the stepped leader approaches the ground, electrons on the surface retreat from the leader creating a region of positive charge. Corona discharges (dielectric breakdowns in the air, also known as St. Elmo's Fire) are released from tall objects on the surface and reach out to the approaching leader. When the downward moving leader connects with a surface corona discharge, a continuous path between the cloud and the ground is established and a powerful return stroke is triggered. The return stroke rapidly moves as a wave upwards into the cloud following the ionized trail of the stepped leader, stripping the electrons from its path.

After the return stroke, the lightning flash may end or, if enough charge in the cloud is collected, a dart leader may come down from the cloud following a direct path to the surface. In turn, the dart leader triggers a second return stroke.

A single lightning flash can comprise several return strokes. The average number of return strokes in a lightning flash is 3 or 4, each stroke typically separated by 40 to 80 milliseconds.

### Are lightning flashes positive or negative?

The positive cloud-to-ground flash is less common than the negative. Coming from higher altitudes in the cloud, positive flashes make up about 10% of all lightning flashes. They are usually composed of a single stroke, and have longer, continuing currents (see Table 1). From the forestry perspective, positive flashes are of greater concern because the longer currents are more likely to start fires.

Several studies have concentrated on the characteristics of the positive flash, but results are inconclusive due to the number of observations. The percentage of positive flashes appear to increase with latitude and with the height of local terrain. Also, positive flashes are more common in winter storms. The apparent cause of this is the lower freezing level, which places the positive charge center closer to the ground, thus increasing the likelihood of a flash.

Positive flashes are more common in stratiform clouds, while negative flashes tend to occur in areas of strong convection. Also, thunderstorms that predominantly consist of negative flashes in their early stages often end with positive discharges as the storm matures and the anvil spreads out.

A popular theory is that horizontal wind shears force a tilting of the dipole axis providing a route for the positive flash, but this has yet to be shown conclusively.

Lightning detection

## Lightning detection

### How is lightning detected? How are lightning maps made?

Most forest and weather services now use lightning detection systems manufactured by Vaisala - Global Atmospherics Inc. (GAI) of Tucson, Az and Time of Arrival (TOA) Systems Inc. of Melbourne, FL. These lightning detection systems determines the time and location of a lightning flash using two methods: magnetic direction finding (MDF) and time of arrival (TOA). In turn, these data are archived and maps can be processed to show the location and polarity of lightning flashes that occur over a period of time.

Both methods work on the principle of detecting the electromagnetic signature of a lightning flash. The antenna's bandwidths are from 1 kilohertz (kHz) to 1 megahertz (MHz). The direction finder can discriminate cloud-to-ground flash from other forms of lightning or noise by the electromagnetic signature. When the stepped leader reaches the ground, the return stroke is triggered, producing a sharp voltage rise. This telling factor distinguishes a cloud-to-ground flash from other electromagnetic noise.

The magnetic direction finder (MDF) senses the electromagnetic field radiated by a lightning flash using two erect, orthogonal wire loop antennas and a horizontal flat plate antenna. The radiated field of a lightning flash induces a current in the loops. The voltage signal measured in the loops is related to the flash's generated magnetic field strength by the cosine of the angle between the loop antenna and the direction to the flash. By comparing the voltage signals from the two loops, a direction to the flash can be determined. The flat plate antenna is used to resolve the 180 degree ambiguity associated with the calculations. In turn, the direction finder sends the data of each registered lightning flash to a position analyzer. The position analyzer triangulates data from direction finders to locate the position of a lightning flash. If the flash is in line with or directly between two direction finders (called the baseline), the position analyzer uses the ratio of the signal strengths as well.

The time of arrival (TOA) detector consists of an array of four antennae. The direction of a lightning flash is determined by comparing the time each antennae senses the flash.

### Are there national networks?

In 1989, the US National Lightning Detection Network® was established, combining several smaller lightning detection networks into one integrated, national system. This system is managed by Vaisala-GAI and consists of over 100 sensors with a detection efficiency of 80% to 90% for flashes with peak currents of 5 kA or more. Flash data is processed every 30 seconds (s) and maps are produced showing accumulated lightning activity. Vaisala-GAI also manages the Canadian Lightning Detection Network for Environment Canada. Both systems function on a subscription basis.

The US Precision Lightning Network is a network that became operational ion 2004. It consists of a network of sensors with a detection efficiency of over 90% for most of the US and southern Canada. Stroke reports provided by the network include location, time, polarity and amplitude.