Severe Weather

Lightning and Thunder

Lightning is simply a discharge of electricity, a giant spark, that occurs in mature thunderstorms. It is actually a flow of electrical current through the air. The lightning stroke can heat the air through which it travels to an incredible 30,000°C (54,000°F), which is 5 times hotter than the surface of the sun. This extreme heating causes the air to expand explosively, thus initiating a shock wave -- called thunder -- that travels outwards in all directions from the flash.

In an average year, lightning causes more deaths than any other thunderstorm-related phenomenon, excluding flash floods. In the United States 75-100 people are killed each year by lightning. Probably about 10 times that many people are struck and injured each year. One contributing reason for this is that lightning victims frequently are struck just before or after the occurrence of precipitation at their location. Many people apparently feel safe from lightning when not experiencing rain. Remember that thunderstorms do not have to produce rain at all.

Lightning is an electrical discharge produced to balance the differences between positive and negative charges within a cloud, between two clouds, or between the cloud and the ground. We will use an in-class handout to explain and describe the mechanisms believed to be responsible for lightning formation. Again we will only cover the basics. Lightning is complicated and by no means fully understood.

A close lightning flash striking a tree at a distance of 60 meters

"Triggered lightning", the discharge is triggered by the presence of the tall tower atop Mount San Salvatore, near Lugano, Switz.

During a thunderstorm, not only is the ground positively charged, so is everything on it. The tallest object in the area is most likely to be struck, whether it is a building, a tree, or a person standing alone in a field, especially objects that have sharp, pointed tips sticking upward.

Lightning rods are placed on buildings to protect them from lightning damage. The rod is made of metal and has a pointed tip, which extends well above the structure. The positive charge concentration will be maximum on the tip of the rod, thus increasing the probability that the lightning will strike the tip and follow the metal rod harmlessly down into the ground.

Although lightning rods and the tallest objects around are most likely to be struck, the path taken by an individual lightning bolt is random and unpredictable. Cloud-to-ground lightning begins by traveling downward from 15,000 to 20,000 feet high in a typical summer thunderstorm cloud. When a cloud-to-ground flash comes to ground, it does not "pick" its target until it is within about 30 yards of the ground. Even though your chances of being directly struck are much lower when you are in a building or car, there have been documented cases where the lighting channel actually goes through small openings in structures and "picks" out an unlucky victim. Lighting has even been known to travel horizontally away from a thunderstorm cloud for miles before turning downward and striking the ground in an area blanketed by clear blue skies.

Cars usually offer safe shelter. When lightning strikes a car, it normally leaves the passengers unharmed because it usually takes the quickest path to the ground along the outside metal casing of the vehicle.

Different Types

Although cloud-to-ground lightning strikes pose the most danger to people on the ground, they make up only about 20% of all lightning strikes. The most common type of lightning in a thunderstorm is in-cloud lightning, which occurs within the cloud itself.

Cloud-to-cloud lightning is a common occurrence in which opposite electrical charges in one cloud attract those in another.

Lightning prediction, detection, and safety

Lightning is powerful. A lot of energy is released during a lightning strike. Here we will consider the three forms of energy listed below:

• Some of the energy is in the form of visible radiation, which is what we see.
• Some of the energy goes into heating and expanding the air, which generates sound waves that we hear as thunder.
• Other energy is released as longer wavelength radiation (that we cannot see) called radio waves.

One way to estimate how close lightning is to your location is to measure the time between when you see the lightning strike and when you hear the thunder. Sound travels much slower than light, so you see lightning first, then hear the thunder. The speed of sound near the Earth's surface is about 1 mile per 5 seconds. So for example, if you see lightning, then hear the thunder 10 seconds later:

Distance to Lightning = (speed of sound) x (time to hear thunder)

Distance to Lightning = (1 mile / 5 sec) x (10 sec) = 2 miles

To best ensure your safety, the experts suggest following the 30/30 Rule.

• When you see lightning, count the seconds until you hear thunder.
• If this time is 30 seconds or less, quickly go inside a substantial building. If such a building is not available, a metal-topped vehicle is the next best choice.
• Wait at least 30 minutes after seeing the last lightning or hearing the last thunder before going back outside.

The liklihood that cloud to ground lighting is about to occur is estimated by measuring the electric field build up in the air. The Electric Field is a measure of the strength of the charge build up. When a certain threshold is reached, lightning is about to hit somewhere. Electric field measuring equipment is used at many outdoor events (e.g., golf tournaments, state fairs, etc.) to allow authorities to give warning that people should take cover. Note that while the electric field instrumentation can provide information that tells us lightning is about to strike somewhere in the area, there is no way to predict exactly where the lightning will hit.

Cloud-to-ground that has already occurred can be located using an instrument called a lightning direction-finder, which works by detecting the radio waves produced by lightning. A network of these magnetic devices has been set up all around the United States as part of the National Lightning Detection Network. The location of each cloud to ground strike is pinpointed using triangulation. This information is desplayed on maps showing the time and location of all detected cloud to ground lightning strikes (see link on the right). This is valuable in showing the general motion of lightning producing storms and the density of lightning strikes, but it can not be used to predict when and where a newly developing storm will first produce lightning. Recent cloud to ground lightning displays

Learn to reduce your lightning risk through outdoor and home lightning safety education. A very good source of information about lightning safety and medical problems associated with being struck is provided on the National Weather Service Lightning Safety Page