Steve Horstmeyer's Severe Weather Primer Part 10 - Lightning
Lightning is a big, powerful, deadly electrical spark. Typical household electric current in the U.S. carries 15 to 20 amps. A giant positive lightning strike sometimes called "a bolt out of the blue" can carry 300,000 amps and the power of 1 billion volts.
Images Courtesy: NOAA. Left: Ribbon lightning, Empire State building, Center: Eiffel tower, Ribbon lightning, Right: Norman, OK
DEADLIER THAN TORNADOES
From 1959 through the end of 2015, 5124 people were killed by lightning in the United States. During that same period 4434 died in tornadoes.
You read it right! Lightning is deadlier than than tornadoes! One reason is that ALL thunderstorms, no matter how small and weak contain lightning and every lightning bolt is strong enough to be deadly. Tornadoes, on the other hand, are relatively rare in the world of thunderstorms.
Another reason lightning is deadlier than tornadoes is that most tornadoes are very weak. From 1950 through 2015 there were 60,114 tornadoes in the U.S. The official U.S. tornado database, available online at NOAA's Storm Prediction Center lists 27933 F0/EF0 tornadoes from 1950 through the end of 2015. The "Zeros" caused only 23 deaths. There were 20,221 F1/EF1 tornadoes and they caused only 227 deaths. That means the "zeros" and "ones", a total of 48,154 tornadoes - 80% of the total number - caused only 250 deaths.That's only 4.3% of the total number of tornado deaths which stands at 5790. F/EF0 and F/EF1 tornadoes are rarely deadly.
From 1959 through 2015 (57 years) an average of 77.8 people were killed each year by tornadoes. The number killed by lightning in an average year is 89.9. Note: The tornado database starts in 1950 and the lightning database starts in 1959. When comparing tornado deaths and lightning deaths I use only the years we have data for both. When writing about tornado deaths alone I use the entire 1950-2015 time period.
Obviously it is crucial to your safety that you pay attention when thunderstorms rumble through the area. There is a 100% chance of lightning any time you hear thunder. One way to do that is use the FOX19 Weather NOW app. It can follow you where ever you go and within the FOX19 NOW viewing area will give you audible alerts when lightning nears your precise location.
STATIC ELECTRICITY IS DYNAMIC AND VERY COOL
Static electricity is fairly simple. When an object or a region of an object has a negative charge it has an accumulation of electrons that exceeds the number of protons or positively charged ions. You see the effects of static electricity everywhere. Rubbing a balloon on a wall and making it cling, clothes fresh from tumbling in the dryer crackle and stick together, the ever present dust on a computer screen and in the video below, right out of high school science lab, bending a stream of water from a faucet with a comb are all examples of static electricity.
Some materials easily give up electrons and others are eager to take them. When givers and takers come in contact the flow of charged particles creates an electrical current. Objects that give up electrons become positively charged and those that receive electrons acquire a negative charge.
There are a number of ways electrons can escape from one object and accumulate on another. Sometimes just touching is enough to free electrons from the donor surface. At other times a forceful collision or vigorous rubbing is necessary. Applying pressure or heating can also free electrons from some materials. When opposite charges are separated from one another it is called charge separation.
LET'S HEAR IT FOR INDUCTION
Another way to create a charge separation is through induction. In most thunderstorms here is a large negative region near the base of the cloud #1 in the diagram below). It attracts positive charges on the ground creating what meteorologists call a "positive shadow" beneath cloud base. On the diagram below you can see the positive shadow near #2. Between the positive ground and the negative cloud base the charges are separated by insulating air.
When the difference in charge between the negative cloud base and the positive ground shadow is greater than the insulating capacity of the air between the two - ZAP! - electrostatic discharge (ESD) - the rapid flow of charged particles. ESD between cloud base and the ground creates the inseparable duo - the brilliant flash of lightning and the deafening clap of thunder . It may happen once or multiple times until there is not enough charge difference to bridge the gap between cloud base and the ground. Cloud to ground lightning is the most common type of lightning but not the only kind.
The + + + + and - - - - of Lightning
A negative lightning strike carries a negative charge from cloud to the ground (#5 on the diagram). A positive lightning strike (#6) carries positive charge from cloud to negatively charged ground (#4).
Notice that positive lightning travels farther through the air and therefore has to over come a greater insulating capacity than a strike that travels less distance. This tells us that positive lightning is much more powerful than negative lightning.
A typical negative lightning strike from cloud base to the ground travels a few thousand feet, has multiple strokes and carries a current of 10,000 to 45,000 amps with a power up to 100 million volts.
A positive lightning strike may be 10 times more powerful than a negative strike. Positive lightning is usually a single stroke, can travel 20 miles, with a current of 300,000 amps and a power of 1 billion volts. During winter and when thunderstorms are decaying positive lightning strikes are the dominant form of cloud to ground lightning.
MORE KINDS OF LIGHTNING
The negative and positive lightning described above is cloud-to-ground lightning (CG).CG lightning is numbers 5 and 6 in the diagram above. If lightning starts on a tall tower or skyscraper and goes up it is called ground-to-cloud (GC) lightning. GC lightning can be negative and positive.
For more on GC lightning watch this 2 minute and 11 second video.
The electric field inside a thunderstorm is much greater than the cloud-to-ground electric field so most lightning occurs within thunderstorms. This type of lightning is called intra-cloud (IC) lightning (just above #1 in the diagram).
If a bold of lightning shoots out of a thunderstorm into the surrounding air it is called cloud-to-air (CA) lightning. Lightning can also travel between clouds and is called cloud-to-cloud (CC) lightning. CC lightning is rarely observed.
This 48-second video below shows CA and IC lightning.
Anvil Crawlers are horizontal forking bolts that cross the sky. They progress slowly enough that you can see them "crawl". They are well above the ground often seen near the underside of the thunderstorm anvil. Look at the video below.
Bead lightning is really just the decaying stage of a lightning strike. When parts of the lightning strike are becoming less luminous it often looks like a string of beads. This animation from a high speed move made by E. Philip Krider, Institute of Atmospheric Physics, U. Arizona.
Ribbon lightning occurs when the lightning channel is being blown downwind fast enough that there is a visible gap between individual bolts. The first image is of a bolt striking the top of the Empire State Building. The right half is an enlargement. The second image is from Chardon, OH taken in 1898 by W.H. Osborne.
Sheet lightning is just regular lightning in the distance. Its light is reflected off of a high overcast and much of the sky seems to light up in a sheet.
I know you are wondering about heat lightning. It got its name because on sultry summer nights the flash of lightning is often seen across the distant sky. Heat lightning is just regular, mostly IC lightning, in a distant thunderstorm that lights up the cloud so you see direct and reflected light. Just like all lightning it forms in a thunderstorm.
ELECTRICITY IN THUNDERSTORMS
The diagram above shows a simple model of the electric charge distribution in a typical thunderstorm. Ongoing research has revealed there are thunderstorms with the opposite charge distribution from the diagram, a three-layer charge distribution and much more complex distributions.
HOW THE SEPARATION OF CHARGES COMES ABOUT IN A THUNDERSTORM
Thunderstorm electrification is very complex and not yet completely understood. The charge distribution that develops in a thunderstorm depends on air temperature inside the thunderstorm and how moist the air is. The three main players in the electrification game are graupel, ice crystals and supercooled liquid cloud drops.
Supercooled water drops are liquid drops that are colder than freezing. They can exist as liquid to -40°F because of the surface tension of the small drop.
Graupel looks like little snowballs. It is also called snow pellets and sometimes in the U.S. soft hail. The series of images below show that graupel forms when a snowflake falls through supercooled water drops. The drops collect on the snowflake and freeze eventually forming a small ball.
HOW GRAUPEL FORMS
Left above: Supercooled (now frozen) drops accreted on ice needles. Right above: Graupel, supercooled drops accreted and original particle obscured. Courtesy: USDA.
The next image shows what happens when graupel falling through a thunderstorm collides with an ice crystal.
The collision transfers charges and creates graupel and ice crystals that become positively or negatively charged depending on the air temperature. If the
temperature is -20°C (-4°F) or colder ice crystals becomes positively charged. Ice crystals are small and light and are carried high in the thunderstorm forming a positively charged region(#3). Graupel is heavier and falls to lower altitudes carrying a negative charge to near cloud base (#1).
At temperatures warmer than -20°C (-4°F) the charge changes are reversed partly explaining some of the complex charge distributions researchers have observed in thunderstorms.
THE LIGHTNING STRIKE - FASTER THAN THE BLINK OF AN EYE
1. ELAPSED TIME = ZERO. As the negative charge in lower part of a thunderstorm increases positive charges from the "ground shadow" are pulled up through trees, telephone poles, buildings and towers.
2. Invisible negative charges descend in jagged steps towards the positive charges below as the "stepped leader".
3. As the negative charges get closer to the ground invisible channels of positive charges called the "streamers" surge upwards. This is another example of induction.
4. ELAPSED TIME = 0.005 SECONDS. When the stepped leader meets the streamer, a couple hundred feet or so above the ground,the insulating capability of the air has broken down and a dimly visible surge of negative charge races down the channel.
5. At the same time a brilliant return stroke of positive charge bursts up the channel.
6. ELAPSED TIME = 0.005001 SECONDS. The colossal current heats the air to as much as 30,000°C (53,540°F). The air around the channel rapidly expands and - KABOOM! - a shock wave we hear as thunder.
The image below was taken in Tucson, AZ and reported in 2012 at the 22nd Annual International Lightning Detection Conference. The paper is located here:
From when the stepped leader meets the streamer and the lightning channel is completed until then end of the first flash 1 microsecond passes. That's one-millionth of a second. Way too fast for you to see if lightning is traveling up or down. In fact different parts of the strike travel in both directions.
If there is enough charge remaining additional lightning strokes will occur.
A BOLT OUT OF THE BLUE
A positive CG lightning strike is often called "a bolt out of the blue". Because a positive strike can travel so far from a thunderstorm the sky may be clear and deep blue. With absolutely no warning, lightning strikes.
Positive lightning bolt. Courtesy: Martin Kucera http://floridalightning.com/
LIGHTNING FACTS AND SAFETY
ONE-MISSISSIPPI, TWO-MISSISSIPPI, THREE-MISSISSIPPI ...
FIVE WAYS LIGHTNING CAN KILL YOU
1. Direct Strike
2. Side Flash (Side Splash)
3. Ground Current