Formation of Lightning: The Simplified Story
As we saw in the video, lightning is a complex phenomenon, as are most things in nature. We do not yet fully understand all of
the processes that cause different parts of a thunder cloud to become charged, what parts of the cloud have what charge, and
why the lightning strikes where it does. However, a simple model for the formation of lightning explains most of the aspects of
lightning that we observe:
We do know that thunder clouds form when the sun heats the ground. This, in turn, heats up the air above it. This hot air
begins to rise up. As it rises, it cools. If it contains water vapour, this condenses out as small water drops (a cloud). This
releases more energy, which continues to warm the air and makes it rise farther and faster. If there is enough water vapour in
the air, this process builds until a fully-developed thunder cloud is formed.
At the top of the cloud, the air gradually cools off and the water and ice particles grow. When they are large enough and the air
is cool enough, it begins to fall. As the rain and ice falls, it evaporates, cooling the air even more and causing it to fall faster.
Thus, the thunder cloud develops different columns of air within it, some of which are rising and carrying up dust and water
and some of which are falling and carrying ice particles. 1) These updrafts and downdrafts rub against each other. This
rubbing of uprising dust and falling ice particles causes charging by friction. The dust loses electrons to the ice. 2) The
positive dust continues upwards gradually making the top of the cloud positive and the base of the cloud negative. 3) The
negative base of the cloud creates a positive charge in the ground under it by induction. 4) The positive ground and nearby
negative cloud pulls air molecules apart to create channels of ionized air between cloud and ground (and also between the
bottom of the cloud and the top). 5) Excess charges move through this channel of ionized air, releasing light, heat, and sound
energy as it goes. This is what we know as lightning.
LIGHTNING SAFETY TIPS
At any given moment there are over 2,000 thunderstorms in progress over the surface of the earth.
Lightning strikes the earth over 100 times each second. In the United States alone there are at least
100,000 thunderstorms each year.
Follow these guidelines:
- Stay inside. DO NOT go out unless absolutely necessary.
- Stay away from open doors, windows, telephones, metal objects, chimneys and electrical appliances.
- If outdoors, avoid the highest object in the area.
- Avoid hilltops, tall trees, open spaces, and wire fences.
- DO NOT use or handle metal objects or flammable materials.
- DO NOT use the telephone if lightning can be seen or thunder heard.
- If you are in water, i.e. lake, pond, tub, or shower, get out.
- Stop all outside work and get indoors, if possible.
- DO NOT use plug-in electrical equipment.
- Stay in your vehicle if traveling.
- If you see someone struck by lightning, call "911" immediately. Administer first aid such as mouth to mouth
resuscitation. Lightning victims are not "charged" and, therefore, not dangerous to touch.
From Newton's Apple
Scientists know that lightning results from a complicated interplay of positive and negative electrical charges occurring in the 2,000 or so
thunderstorms taking place on Earth at any given moment.
To learn about what causes lightning, scientists had to learn about the interaction of electrons and positive ions. Electrons, tiny particles orbiting
the outside of atoms, carry a negative charge. Positive ions are atoms or molecules that have lost an electron. Atoms and molecules normally
have equal positive and negative charges, making them neutral.
When different materials come in contact, electrons are transferred and one of the materials gains an excess of electrons and becomes
negatively charged. When an object with a lot of positive or negative charges gets close to an object carrying the opposite charge, a spark jumps
across the space between them to neutralize the charges.
In a thunderstorm, that spark is a lightning bolt. It's only a couple of inches wide, but it leaps between the clouds and the earth at a remarkable
90,000 miles per second. The power in the stroke is three million megawatts, comparable to all the power generated in the United States at any
one instant.
The separation of positive and negative charges necessary for lightning begins during a thunderstorm, when rising water droplets collide with
falling hailstones in the middle of the cloud. The hail strips electrons from the droplets and the top of the cloud becomes positively charged, while
the bottom becomes negatively charged. What we see as lightning happens in a two-step process. Static electricity builds up between the earth
and the cloud and a spark in the form of an invisible lightning bolt comes down from the cloud.
Just before this bolt reaches the ground, it is met with an upward moving, positively charged spark. When the two collide, an explosion occurs as
the return stroke travels up the bolt-the result: a visible flash called lightning.
From Nasa
Either lightning is attracted to testosterone, or men spend an too much time outdoors swinging metal objects about.
Men are struck by lightning 4 times more often than women. According to the National Severe Storms Laboratory
and the National Weather Service, males account for 84% of lightning fatalities and 82% of injuries.
Men can take comfort in the fact that the actual number of deaths and injuries from lightning strikes has decreased
in the past 35 years. This can be attributed to improved forecasts and warnings, better lightning awareness, more
substantial buildings, socioeconomic changes, and better medical care and communications.
Consequences
The U.S. National Weather Service recorded 3,239 deaths and 9,818 injuries from lightning strikes between 1959
and 1994. Only floods cause more weather-related deaths. But according to Dr. Elisabeth Gourbière of the
Electricité de France, Service des Etudes Médicales, only 20 percent of lightning victims are immediately struck
dead. Lightning also results in economic costs: $4 to 5 billion in lightning related losses per year in the U.S. alone.
Injuries
Lightning injuries are not the same as electrical shocks. For one thing, the contact voltage of a typical industrial
electrical shock is 20 to 63 kilovolts, while a lightning strike delivers about 300 kilovolts. Industrial shocks rarely
last longer than half a second (500 milliseconds) because a circuit breaker opens or the person is thrown far from
the live conductor. Lightning strikes have an even shorter duration, only lasting up to a few milliseconds.
Both industrial shocks and lightning strikes result in deep burns at point of contact; for lightning they are mostly on
the head, neck and shoulders. Lightning victims’ burns seem to center at the entry and exit points. Most of the
current from a lightning strike passes over the surface of the body in a process called "external flashover." Both
industrial shock and lightning victims may be injured from falling down or being thrown, and the leading cause of
immediate death for both is cardiac arrest.
If you survive a shock, you still have to deal with the consequences of the electrical burns. Industrial shock burns
can lead to kidney failure, infection, muscle and tissue damage, or amputation. Lightning burns are exceptionally
life threatening. Gourbière says that 70 percent of lightning survivors experience residual effects, most commonly
affecting the brain (neuropsychiatric, vision and hearing). These effects can develop slowly.
Lightning Avoidance
Most lightning casualties occur in the afternoon, 67% between noon and 4 p.m. local time with a peak in casualties
at 4PM. Sunday has 24% more deaths than other day, followed by Wednesday. Lightning reports reach their peak
in July. Many lightning victims had been walking in an open field or swimming before they were struck. Other
lightning victims had been holding metal objects such as golf clubs, fishing rods, hay forks, or umbrellas. But even
those not holding metal objects are as likely to be struck by lightning as a bronze statue of the same size.
When you hear thunder, you are already within the range where the next ground flash may occur.
To avoid being struck by lightning, you should seek shelter when you hear even the faintest thunder. Some of the
best places to take refuge are enclosed buildings, or cars and buses (but don't touch the metal!). In case there are no
safe spaces nearby, bend into a crouching position until there is a break in the storm. Isolated trees, telephone
booths, and open structures like gazebos or porches make poor lightning shelters. If there is a tall object nearby,
move as far away as possible - at least 2 meters. Standing next to tall isolated objects like poles or towers makes
you vulnerable to secondary discharges and explosions coming off those objects.
Lightning Questions
Read the Lightning Handout and lightning section in the text book (pg 313-315).
1) Up drafts and down drafts in a thunderstorm cause what to collide together?
2) The collisions in 1) causes what to happen?
3) What part of the thundercloud develops a positive charge and what part develops a negative charge?
4) Why does the ground under the thundercloud become positively charged?
5) What happens to allow the air to conduct the electric charge of the lightning bolt?
6) Lightning is a spark can occur between the base of the cloud and either the ____________ or the
________ of the cloud.
7) How many thunderstorms are active at any time?
8) How often is the earth struck by lightning each second?
9) How wide and how hot is a lightning bolt, and how fast does it move?
10) How much energy does the average thunderstorm produce?
11) Who gets struck more often, men or women?
12) What percentage of lightning strike victims die right away?
13) How does the charge in a lightning strike usually pass through the body?
14) If you can hear thunder, are you in danger of being struck?
15) List three things that you should do in a thunder storm and three things that you should not do?
16) a) What type of charging causes the top and bottom of a thunder cloud to become charged?
b) What type of charging causes the ground under the cloud to become charged?
c) What type of charging causes a person to be charged when hit by lightning?
17) Why do you think that is lightning necessary for the earth and the atmosphere?