Severe Weather 101
Frequently Asked Questions About Lightning
- What is lightning?
- Lightning is a giant spark of electricity in the atmosphere between clouds, the air, or the ground. In the early stages of development, air acts as an insulator between the positive and negative charges in the cloud and between the cloud and the ground. When the opposite charges build up enough, this insulating capacity of the air breaks down and there is a rapid discharge of electricity that we know as lightning. (The actual breakdown process is still poorly understood.) The air breakdown creates ions and free electrons that travel down the conducting channel. This current flow temporarily equalizes the charged regions in the atmosphere until the opposite charges build up again.
Lightning from thunderstorms begins in a strong electric field between opposite charges within the storm cloud, and can stay completely within the cloud (intra-cloud lightning) when the charge regions are similar strength (balanced) or can reach the ground (cloud-to-ground lightning) when one of the regions is much stronger than the other (unbalanced).
Lightning is one of the oldest observed natural phenomena on earth. It can be seen in volcanic eruptions, extremely intense forest fires (pyrocumulonimbus clouds), surface nuclear detonations, heavy snowstorms, in large hurricanes, and obviously, thunderstorms.
- What are cloud flashes?
- A cloud flash is lightning that occurs inside the cloud, travels from one part of a cloud to another, and some channels may extend into clear air.
- What is a “stepped leader?”
- A stepped leader is the development of the downward lightning channel. Negatively-charged lightning channels in particular do not propagate continuously, but in relatively short “steps” where the air ahead is becoming ionized as multiple low-conductivity “streamers.” A streamer that develops more current and better conductivity can become the next step that connects to the “leader” channel.
- Is it possible to have thunder without lightning?
- No, it is not possible to have thunder without lightning. Thunder starts as a shockwave from the explosively expanding lightning channel when a large current causes rapid heating. However, it is possible that you might see lightning and not hear the thunder because it was too far away. Sometimes this is called “heat lightning” because it occurs most often in the summer.
- Is lightning always produced by a thunderstorm?
- Thunderstorms always have lightning (thunder is caused by lightning, and you can't have a thunderstorm without thunder!), but you can have lightning without a thunderstorm. Lightning can also be seen in volcanic eruptions surface nuclear detonations, and in heavy snowstorms (“thunder snow”).
- What causes thunder?
- Thunder is caused by lightning. The bright light of the lightning flash caused by the return stroke mentioned above represents a great deal of energy. This energy heats the air in the channel to above 50,000° F in only a few millionths of a second! The air that is now heated to such a high temperature had no time to expand, so it is now at a very high pressure. The high pressure air then expands outward into the surrounding air compressing it and causing a disturbance that propagates in all directions away from the stroke. The disturbance is a shock wave for the first 10 yards, after which it becomes an ordinary sound wave, or thunder.
Fun fact: thunder can seem like it goes on and on because each point along the channel produces a shock wave and sound wave, so what you hear as thunder is actually an accumulation of multiple sound waves from the different portions of the lightning channel.
- What is dry lightning?
- Dry lightning is lightning that occurs without rain nearby. The NOAA Storm Prediction Center routinely issues forecasts for dry lightning because this kind is more likely to cause forest fires.
- What is a “bolt from the blue”?
- A “bolt from the blue” is a cloud-to-ground flash which typically comes out of the side of the thunderstorm cloud, travels a relatively large distance in clear air away from the storm cloud, and then angles down and strikes the ground. These lightning flashes have been documented to travel several miles away from the thunderstorm cloud. They can be especially dangerous because they appear to come from clear blue sky.
A helmeted bicyclist experienced a lightning strike to the head under fair weather conditions with a cloudless sky. It was determined that the bolt probably originated in a thunderstorm that was about 16km (approximately ten miles) away and obscured by mountains.
- Does lightning always strike the tallest object?
- Never say always! Lightning usually strikes the tallest object. It makes sense that the tallest object is most likely to produce upward streamers to connect with the downward lightning leader.
- What type of electricity is lightning?
- Lightning is an electrostatic discharge accompanied by the emission of visible light and other forms of electromagnetic radiation.
- How many volts and watts are in lightning?
- Lightning can have 100 million to 1 billion volts, and contains billions of watts.
- Why are positive lightning bolts deemed more dangerous than the more common negatively charged bolts?
- You don’t want to run into either, but positive lightning may be considered more dangerous because its peak electric current is often stronger, the flash duration (continuing) is typically longer, and its peak charge can be much greater than a negative strike. The longer-duration current is thought to make it more likely to ignite fires, as well.
- Does lightning strike from the sky down, or the ground up?
- The answer is both. Cloud-to-ground (CG) lightning comes from the sky down, but the part you see comes from the ground up. A typical cloud-to-ground flash lowers a path of negative electricity (that we cannot see) towards the ground in a series of spurts. Objects on the ground generally have a positive charge under a typical thunderstorm. (The charge that builds up in a small area of the Earth’s surface and the objects on it is determined by the net charge above it since the Earth’s surface is relatively conductive and can move charge in response to the thunderstorm.) Since opposites attract, an upward streamer is sent out from the object about to be struck. When these two paths meet, a return stroke zips back up to the sky. It is the return stroke that produces the visible flash, but it all happens so fast - in a few thousandths of a second - so the human eye doesn't see the actual formation of the stroke. Natural lightning can also trigger upward discharges from tall towers, like broadcast antennas. For more information on cloud-to-ground (and other types of lightning) visit the Severe Weather 101: Lightning Types page.
- How hot can lightning make the air?
- Energy from lightning heats the surrounding air anywhere from 18,000 degrees Fahrenheit to up to 60,000 degrees Fahrenheit.
- What causes lightning to be colored rather than the usual white or blue?
- Lightning can appear to be many different colors depending on what the light travels through to get to your eyes. In snowstorms, where it is somewhat rare, pink and green are often described as colors of lightning. Haze, dust, moisture, raindrops and any other particles in the atmosphere will affect the color by absorbing or diffracting a portion of the white light of lightning.
- How does the Earth benefit from lightning?
- The earth benefits from lightning in several ways. First, thunderstorms and lightning are part of the Earth’s global electric circuit. Thunderstorms and electrified clouds are like batteries that cause the Earth to have negative charge and the atmosphere to have positive charge.. This maintains the fair weather electric field, which is about 100 V/m near the surface. There is always a steady current of negatively-charged ions flowing upwards from the entire surface of the Earth (and positive ions downward from the atmosphere). Thunderstorms help transfer the negative charges back to Earth (lightning is generally negatively charged). Without thunderstorms and lightning, the earth-atmosphere electrical balance would disappear in 5 minutes. Lightning also makes ozone-producing chemicals.
- What happens to the ground when lightning strikes it?
- What tends to happen when lightning strikes ground is that it fuses dirt and clays in to silicas. The result is often a glassy rock (called a fulgurite) in the shape of a convoluted tube. Fulgurite has been found all over the world, but is relatively rare. The color depends on the minerals in the sand that was struck.The shape in the ground is the shape of the path the lightning current followed in the ground. There is often damage to grasses along this path too.
Lightning traveling down a tree trunk turns water to steam. If it gets under the bark into the surface moisture of the wood, the rapidly expanding steam can blast pieces of bark and branches from the tree, and the wood along the path is often killed. The charge carried by the lightning is then dissipated along the surface of the Earth. If you are near something that was hit by lightning such as a tree or fence, this process can be very dangerous as all of this current does not get dissipated instantaneously. The lightning may hit a tree then branch off and hit something else, or after the current travels through the tree trunk, it can also travel through the immediately surrounding area, and into anything or anyone nearby. This process, however, is fairly quick, so the ground or whatever was struck does not remain electrically dangerous afterwards.
The lightning current can travel even farther through water, metal fences, power lines or plumbing. Lightning current may enter a building and transfer through wires or plumbing and damage everything in its path. Similarly, in urban areas, it may strike a pole or tree and the current then travels to several nearby houses and other structures and enter them through wiring or plumbing.
- Can lightning strike the same place twice?
- Lightning does hit the same spot (or almost the same spot) more than once, contrary to folk wisdom. It could be simply a statistical fluke (i.e., with all the lightning that occurs, eventually lightning will strike somewhere near a previous lightning strike within a short period of time). It could also be that something about the site makes it somewhat more likely to be struck. Typically, when lightning strikes something on the ground, the object that is struck sends a faint channel upward that joins the downward developing flash and creates the connection to the ground. Taller objects are more likely than shorter objects to produce the upward channel. But it is also possible that something that locally affects the ability of the ground to conduct electricity (such as the salt or moisture content of the ground at the time, the presence or absence of rock, standing water, pipes or other metal objects in the ground), the terrain shape, the shape of leaves or twigs, or something else might make a particular location more likely than another nearby location to be struck.
- When and where does lightning most frequently strike?
- Lightning comes from a parent cumulonimbus cloud. These thunderstorm clouds are formed wherever there is enough upward air motion, convective instability, and moisture to produce a deep cloud that reaches up to levels colder than freezing.
These conditions are most often met in the warm seasons (spring, summer, early autumn). In general, the US mainland has a decreasing amount of lightning toward the northwest. Over the entire year, the highest frequency of cloud-to-ground lightning is in Florida between Tampa and Orlando. This is due to the presence, on many days during the year, of a large moisture content in the atmosphere at low levels (below 5,000 feet), as well as high surface temperatures that produce strong sea breezes along the Florida coasts. The western mountains of the US also produce strong upward motions and contribute to frequent cloud-to-ground lightning. There are also high frequencies along the Gulf of Mexico coast, the Atlantic coast in the southeast US, and inland from the Gulf. Regions along the Pacific west coast have the least cloud-to-ground lightning.
- How do storms become electrified?
- Clouds become electrified when strong updrafts (fueled by convective instability and moisture) produce a mixture of larger ice particles (graupel), small ice crystals, and supercooled liquid water drops and ice crystals at temperatures less than freezing (0 deg C). In this environment, rebounding collisions between the graupel ice crystals cause charge to be transferred between the particles. This is called the noninductive process because it does not require a pre-existing electric field to polarize the particles. The exact physical mechanisms are not completely understood, but it involves a transfer of mass from one particle to the other, and the sign of charge is dependent on temperature and the growth rates of the particles. The graupel and crystals gain opposite signs of charge, and then they form separate charge regions as the graupel fall faster in the updraft.
A secondary process may occur when electric fields increase and cause droplets to become polarized (ions within droplets driven by the electric field to oppose sides of the droplet). If part of the droplet freezes onto an ice particle and the rest breaks away, some net charge of ions from the drop can be captured by the ice. This is known as an inductive process since it requires an appreciable electric field to occur.
- Does lightning happen during the winter?
- Lightning occurs less frequently in the winter because there is not as much instability and moisture in the atmosphere as there is in the summer. These two ingredients work together to make convective storms that can produce lightning. Without instability and moisture, strong thunderstorms are unlikely.
During the winter, the land surface is cooler because there is not as much heating by the sun to warm it up. Without warm surface temperatures, the near-surface air wouldn't rise in the atmosphere very far. Thus, the kinds of deep (8-15 km deep) thunderstorms that develop in the summertime wouldn't develop.
Warm air holds more water vapor. And, when water vapor condenses into liquid water cloud drops, latent heat is released which fuels the thunderstorm. So, warm, moist air near the surface (and the proper conditions aloft to give you lots of instability) can result in deep convection, which may produce lightning discharges.
- What is thundersnow?
- Although thunderstorms are less common in the winter, sometimes lightning can occur within snowstorms. Called thundersnow, relatively strong instability and abundant moisture may be found above the surface, such as above a warm front, rather than at the surface where it may be below freezing. Thundersnow is sometimes observed downstream of the Great Salt Lake and the Great Lakes during lake-effect snowstorms, too.
- How many flashes a year are there?
- Over the contiguous 48 states, an average of 20,000,000 cloud-to-ground flashes have been detected every year since the lightning detection network (NLDN) covered all of the continental US in 1989. In addition, about half of all flashes have more than one ground strike point, so at least 30 million points on the ground are struck on the average each year in the US. Besides cloud-to-ground flashes, there are roughly 5 to 10 times as many cloud flashes as there are ground flashes.
- How can I stay safe from lightning?
- NOAA's National Weather Service is an excellent source for information on indoor and outdoor lightning safety and lightning risks.
- What are the odds of being struck by lightning?
- According to the NWS, the chance of an individual in the U.S. being struck during a given year is one in 1.2 million. The odds of being struck in your lifetime (estimated to be 80 years) are 1 in 15,300. You can read more about where these numbers come from on the National Weather Service website. A lot depends on your exposure, however. You can reduce your risk of being struck by lightning by going to a good shelter such as an enclosed building (see link above) if there is a thunderstorm near you! The most dangerous times of a storm can be the beginning and end of lightning production. If the first flash is a CG, it will come without any warning from previous thunder. The last flash of a storm may come many minutes after the next-to-last flash, so it is important to wait long enough for conditions to be safe again.
- Where can I get information on lightning strikes that occur in my area?
- There are several companies that collect and archive this data including Vaisala and Earth Networks which operate networks in the United States. We actually purchase lightning data ourselves (we do not have the funds to maintain our own network) and have strict rules about how we can use it.