Severe Weather 101
- What is lightning?
- Lightning is a channel of electrical charge called a stepped leader that zigzags downward in roughly 50-yard segments in a forked pattern. This step leader is invisible to the human eye, and shoots to the ground in less time than it takes to blink. As it nears the ground, the charged step leader is attracted to a channel of opposite charge reaching up, a streamer, normally through something tall, such as a tree, house, or telephone pole. When the oppositely-charged leader and streamer connect, a powerful electrical current begins flowing. A bright return stroke travels about 60,000 miles per second back towards the cloud. A flash consists of one or perhaps as many as 20 return strokes. We see lightning flicker when the process rapidly repeats itself several times along the same path. The actual diameter of a lightning channel is one-to-two inches.
Lightning is one of the oldest observed natural phenomena on earth. It can be seen in volcanic eruptions, extremely intense forest fires, surface nuclear detonations, heavy snowstorms, in large hurricanes, and obviously, thunderstorms.
- What causes thunder?
- Lightning causes thunder! Energy from a lightning channel heats the air to around 18,000 degrees Fahrenheit. This causes the air to rapidly expand, creating a sound wave known as thunder. The stepped leader causes the initial tearing sound, and the ground streamer causes the sharp click or crack heard at a very close range, just before the main crash of thunder.
Thunder can be heard up to 25 miles away from the lightning discharge. At these distances, thunder sounds like a low rumble because the higher frequency pitches are more easily absorbed by the surrounding environment, and the sound waves set off by the lightning discharge have different arrival times.
- Where does lightning usually strike?
- Lightning strikes the ground somewhere in the U.S. nearly every day of the year. Thunderstorms and lightning occur most commonly in moist warm climates. Data from the National Lightning Detection Network shows that over the continental U.S. an average of 20,000,000 cloud-to-ground flashes occur every year. Around the world, lightning strikes the ground about 100 times each second, or 8 million times a day.
In general, lightning decreases across the U.S. mainland 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 U.S. 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 U.S. Regions along the Pacific west coast have the least cloud-to-ground lightning.
- What causes lightning?
- The creation of lightning is a complicated process. We generally know what conditions are needed to produce lightning, but there is still debate about exactly how a cloud builds up electrical charges, and how lightning forms. Precipitation and convection theories both attempt to explain the electrical structure within clouds.
Precipitation theorists suppose that different sized raindrops, hail, and graupel get their positive or negative charge as they collide, with the heavier particles carrying negative charge to the lower part of the cloud.
Convection theorists believe that updrafts transport positive charges found near the ground upward through the cloud while downdrafts carry negative charges downward.
You can read more about lightning at the National Weather Service's JetStream Online School for Weather.
- Where does lightning strike?
- Tall objects such as trees and skyscrapers are commonly struck by lightning. Mountains also make good targets. The reason for this is their tops are closer to the base of the storm cloud. Remember, the atmosphere is a good electrical insulator. The less insulation the lightning has to burn through, the easier it is for it to strike. However, this does not always mean tall objects will be struck. It all depends on where the charges accumulate. Lightning can strike the ground in an open field even if the tree line is close by.
- How is electrical charge distributed through a thunderstorm?
Charge distribution in storm clouds [+]
What we do: NSSL researchers use a 3-D cloud model to investigate the full life-cycle of thunderstorms. The model has shown how graupel or other droplets could help form regions of lower charge within the storm.
NSSL researchers were pioneers in the science of launching instrumented weather balloons into thunderstorms. This capability allowed NSSL to collect weather data in the vicinity of tornadoes and drylines, and all the way up through a thunderstorm, gathering critically needed observations in the near-storm environment of thunderstorms. In addition, these mobile labs and ballooning systems provided the first vertical profiles of electric fields inside a thunderstorm leading to a new conceptual model of electrical structures within convective storms.
One way researchers test their theories is by making measurements of severe thunderstorms in the field and later analyzing the results. Large-scale field experiments involving many instruments with a primary focus on atmospheric electricity include the Deep Convective Clouds and Chemistry experiment (DC3), the MCS Electrification and Polarimetric Radar Study, the Severe Thunderstorm Electrification and Precipitation Study and the Thunderstorm Electrification and Lightning EXperiment.