Have you ever seen tall, dark puffy clouds forming on a hot humid afternoon?  These are called cumulonimbus clouds, sometimes nicknamed "thunderheads." 

They can actually form any time of day when the temperature falls rapidly higher up in the sky.  These tall dark clouds are full of moisture and contain strong up and down air currents.  Cumulonimbus clouds may tower more than 50,000 feet, and cover from just a few square miles up to two hundred square miles. 
What is Lightning?

To put it simply, lightning is electricity.  It forms in the strong up-and-down air currents inside tall dark cumulonimbus clouds as water droplets, hail, and ice crystals collide with one another.  Scientists believe that these collisions build up charges of electricity in a cloud.  The positive and negative electrical charges in the cloud separate from one another, the negative charges dropping to the lower part of the cloud and the positive charges staying ins the middle and upper parts. Positive electrical charges also build upon the ground below.  When the difference in the charges becomes large enough, a flow of electricity moves from the cloud down to the ground or from one part of the cloud to another, or from one cloud to another cloud.  In typical lightning these are down-flowing negative charges, and when the positive charges on the ground leap upward to meet them, the jagged downward path of the negative charges suddenly lights up with a brilliant flash of light. Because of this, our eyes fool us into thinking that the lightning bolt shoots down from the cloud, when in fact the lightning travels up from the ground. In some cases, positive charges come to the ground from severe thunderstorms or from the anvil at the very top of a thunderstorm cloud.  The whole process takes less than a millionth of a second.   

Kinds of Lightning

There are words to describe different kinds of lightning.  Here are some of them:

In-Cloud Lightning: The most common type, it travels between positive and negative charge centers within the thunderstorm.

Cloud-to-Ground Lightning: This is lightning that reaches from a thunderstorm cloud to the ground.

Cloud-to-Cloud Lightning: A rare event, it is lightning that travels from one cloud to another.

Sheet Lightning: This is lightning within a cloud that lights up the cloud like a sheet of light.

Ribbon Lightning: This is when a cloud-to-ground flash is blown sideways by the wind, making it appear as two identical bolts side by side.

Bead Lightning: Also called "chain lightning," this is when the lightning bolt appears to be broken into fragments because of varying brightness or because parts of the bolt are covered by clouds.

Ball Lightning: Rarely seen, this is lightning in the form of a grapefruit-sized ball, which lasts only a few seconds.

Bolt from the blue: A lightning bolt from a distant thunderstorm, seeming to come out of the clear blue sky, but really from the top or edge of a thunderstorm a few miles away.

What Puts the Thunder in the Thunderstorm?   

Lightning bolts are extremely hot, with temperatures of 30,000 to 50,000 degrees F.  That's hotter than the surface of the sun! When the bolt suddenly heats the air around it to such an extreme, the air instantly expands, sending out a vibration or shock wave we hear as an explosion of sound. This is thunder.  If you are near the stroke of lightning you’ll hear thunder as one sharp crack. When lightning is far away, thunder sounds more like a low rumble as the sound waves reflect and echo off hillsides, buildings and trees.  Depending on wind direction and temperature, you may hear thunder for up to fifteen or twenty miles.

A Deadly Danger

When it comes to deadly weather, tornadoes and hurricanes get all the publicity, but lightning is actually the worst threat, killing more people on average every year than tornadoes and hurricanes combined.  About one hundred people die from lightning every year in the United States, and hundreds more suffer lifelong injury or disability.  In fact, the National Weather Service calculates a one-in-three hundred chance that you or a family member will be struck by lightning sometime during your lifetime.

You can beat the odds fairly easily, though. The simplest way is to get inside a home or other sturdy building during a thunderstorm.  Do it immediately; don't wait for the rain to fall.   Most lightning injuries occur before the rain starts and after it stops.   Remember, if you hear thunder, lightning is close enough to strike you. 

The "30-30 Rule"

A good plan to follow is the "30-30 Rule."  It works this way: if the time between seeing lightning and hearing thunder is less than thirty seconds, you are in danger of being struck. Go inside.   After the storm is over, wait thirty minutes after the last flash of lightning or boom of thunder before going back outside.  But be careful!  Even the "30-30 Rule" cannot protect against the first lightning strike, so always know the weather forecast, and watch the sky for possible developing thunderstorms.   Sports coaches, golfers, scout leaders and campers should have a good lightning safety plan and use it when thunderstorms threaten.

Where to Go?

If you are caught outside during a thunderstorm with no buildings nearby, you should avoid open fields, beaches, lakes and swimming pools as if you life depends on it, because it does.  Lightning often strikes the tallest object around, and you don't want that object to be you.  That's also why isolated trees, picnic shelters and covered bus stops offer no protection, and may actually increase your chances of being struck.  If no other shelter is nearby, get into a car with metal sides and roof, and roll the windows up.

Nowhere to Hide? 

If no building or car is available and you must stay outside during the thunderstorm, find shelter in a dense woods or thick grove of small trees.  If you are trapped in an open space, get as low as you can in a valley or ravine and crouch down.  Stay away from metal fences, flag poles and lamp posts.  If lightning is about to strike near you, it might give a brief warning.  Your hair may stand on end, your skin may tingle, you might hear a crackling sound, and keys or other metal objects may vibrate.  If this happens to your group or sports team, spread out twenty feet or more apart and squat down with your head and feet together, your head tucked and your ears covered. (It's going to be loud!)  After the lightning flashes, keep moving to a safer place. With some pre-planning and by following some simple rules, you can avoid the danger of nature's light show and enjoy its beauty instead.

Terrifying Twisters

Some severe thunderstorms may produce tornadoes.  These are violently rotating columns of air in contact with the Earth’s surface. The United States has more tornadoes than anywhere else on earth, with about one thousand occurring every year.  The wind inside a tornado can reach speeds of more than 200 mph. Government meteorologists may issue a tornado watch if they think thunderstorms could be severe enough to produce tornadoes.   If someone reports a tornado, or if weather radar indicates a thunderstorm is strong enough to produce a tornado, local National Weather Service meteorologists issue a tornado warning.  If you hear a tornado warning, act quickly and get to a closet or hallway on the lowest floor of your home, away from outside walls and windows until the danger passes.  It is best for your family to have an emergency plan before storms hit. 

Learning Activities
Try these learning activities about thunderstorms and lightning

1. How Far Away?
The next time you see lightning, count the seconds until you hear thunder.  Since light travels faster than sound, the sound of thunder takes longer to get to you; about five seconds to travel one mile.  If you count to five just before you hear the thunder, the lightning is about one mile away.  If it is very close, the thunder will sound like a loud crack.  If the lightning is far away, it will sound more like a low rumble. If the lightning is more than fifteen miles away, you may not hear it at all.