These powerhouses have their origins in a strong, volatile atmosphere. The atmosphere has nicely figured out a way to make those tall, towering clouds form. The essential requirement is for rapid upward motions to develop.

It is all based upon Archimedes' bathtub experience. He was the ancient thinker who, while taking a bath, discovered that the deeper he went into the water, the more water was displaced, and the more that was displaced, the less he seemed to weigh. According to legend, he ran directly from the bathtub through the streets of Syracuse screaming, "Eureka!"

What he found was the basic principle of buoyancy: Something that is less dense, surrounded by something more dense, will float. People float, and ships float on water because they are able to displace enough water to have a volume that is less dense than the surrounding water. A cork rises in a basin of water because of that very principle. A cloud, or a pocket of air, can act in the same way as a cork.

Stability and Instability

Braving the Elements

"The calm before the storm," is a proverb that seems to make plenty of meteorological sense. Before Hurricane Andrew slammed across southern Florida, the skies were partly cloudy, the wind was light, and the temperatures were mild. But in a few short hours, one of the most intense hurricanes in U.S. history was bearing down in that very same area. Sinking motions on the periphery of the storm account for both the calm before, and after, the storm.

If some object is given a shove and keeps going, its condition is said to be unstable. In the opposite sense, if the object comes back, the condition is stable.

In the atmosphere, if a pocket of air rises and just keeps going, the air is unstable. Of course, as the pocket rises, the pressure around lessens, so the pocket expands. That expansion automatically causes a cooling. Now cooler air has a greater density than warmer air, so the pocket will tend to return to the ground from where it originated. But if the air outside the pocket is cooling faster because of particular weather conditions, then the pocket will have a free ride. As it continues upward, it will eventually condense (when its relative humidity reaches 100 percent). A cloud is born. If the cloud stays warmer than its surroundings, it will grow bigger and bigger. Soon, the cloud will reach as high as 30,000 feet, then 40,000 or 50,000 feet. At that point, we're into some real severe weather!

The following figure sorts out Archimedes's principle applied not to taking a bath, but to looking at the atmosphere.

Unstable air.

Unstable air.

Lapse Rates

The change in temperature with elevation is the lapse rate. The rising pocket has one lapse rate; the air surrounding it has another. They could be equal, but they're usually not. The pocket's lapse rate is simply a function of the rate of expansion, which is predictable and known. If the pocket is dry, it will cool at a rate of 5.5 degrees for every thousand feet it rises. That is called the dry adiabatic lapse rate. "Adiabatic" means no change in heat—the temperature changes not because a fire was lit or because ice was tossed in, but because the volume of the pocket changed. The molecules became farther apart, so the temperature went down. I know we're getting in pretty deep, but do you know what you just learned? The First Law of Thermodynamics!


The change in temperature with elevation is the lapse rate. If a pocket of air is dry, it will cool at a rate of 5.5 degrees for every thousand feet it rises. That is called the dry adiabatic lapse rate.

Once the pocket forms a cloud, the cooling rate decreases because the condensation process releases some heat. The cooling rate becomes about 3 degrees for every thousand feet of lift. That's called the moist adiabatic lapse rate, but, because some heat is absorbed in the process, it is not truly adiabatic. So it is sometimes called a pseudo-adiabatic lapse rate.

In any case, if the pocket's lapse rate is less than the environment's lapse rate, the pocket will not cool as much. It will be warmer, less dense, and Eureka! A storm is born.

The environment's lapse rate is measured every day by sending balloons equipped with instruments into the atmosphere. The temperature throughout the atmosphere is monitored. That observed environmental lapse rate is then compared with the theoretical rates for rising pockets of air, and a determination of stability is made. If the environment is really cold in the upper levels of the atmosphere and the air near the ground is especially warm, there's a good chance these pockets will be warmer than their surroundings. The pockets will have a free rise to the top, and look out! Nature's most violent weather will develop. The upward motions can reach expressway speeds.


If a rising pocket is saturated, it will cool at a lower rate of about 3 degrees Fahrenheit. That is called the moist adiabatic lapse rate.

Lightning Strikes

A thunderstorm is made possible by both electronics and dynamics. We just looked at some of those dynamics. Now, bring on the electronics.

The following figure shows the progression of events that allows those tall clouds with strong updrafts to become electrifying. The atmosphere normally has small, electrically charged gases called ions. Those ions have a net negative charge in the upper reaches of the atmosphere. Because opposites attract, those negative charges induce positive charges on the surface of the earth. The positive charges collect at the surface. That is called a fair weather field because it occurs when the weather is quiet.

Now let's put one of those tall, towering clouds into that atmosphere. Because of the earth's original field, cloud droplets and ice crystals will line up with negative charges at the base and positive charges on top—again opposites attract. The earth's original positive charge induces a negative charge at the lower end of these elements. The upper atmosphere's negative charge induces a positive charge on the top end of these droplets. The next step gets a bit tricky and is not so clearly understood.

Charging of the atmosphere.

Charging of the atmosphere.

Magically these cloud elements begin to separate, with the positive charge on the top of the droplets, ice crystals transferring to the bottom of the droplets, and ice rising within the cloud. This transfer of charge and separation is mysterious, but ice seems to be an effective agent for the transfer. In any case, positive charges cluster in the top of the clouds, and negative charges collect at the base. There are smaller pockets of positive charge at the cloud base, but overall the charge is negative there.

Weather Words

"Thunder is good, thunder is impressive. But lightning does the work."

—Mark Twain

Now that a net negative charge is induced at the base of a cloud, a new, net positive charge is forced at the ground. No longer is it a fair weather field. When the difference in charge becomes strong enough, a current begins to flow, step-by-step, from the cloud toward the ground. When the entire channel is carved out, the final surge of electricity is sent from the ground up to the cloud. That is the lightning stroke. The current reaches 100,000 amps. That energy is huge.

So the lightning that appears to travel from cloud to ground actually goes from ground to cloud. Other lightning discharges will remain within a cloud or go from cloud to cloud. The entire process—going from a puffy cumulus cloud to a rip-roaring lightning storm—can unfold within an hour.

The temperature of that channel can hit 54,000 degrees, causing the atmosphere to rapidly expand. After expanding, the atmosphere quickly contracts. A shock wave is set off, and the expanding and collapsing channel becomes the source of the thunder. Mark Twain was right. Lightning really does the work. Without lightning, there wouldn't be thunder. And without a cloud, there couldn't be lightning.


Lightning travels at the speed of light—186,000 miles each second. Sound travels about 0.2 mile per second. So you will see lightning sooner than you will hear its accompanying thunder. For every five seconds between lightning and thunder, a storm is one mile away. So if your room lights up, and you count to 15 before you hear thunder, the storm is three miles away. When lightning is really close, the thunder happens at nearly the same time and is it loud!

Lightning can take on many different forms and shapes:

Most recently, a new form of lightning has been noted from spacecraft. Charged spikes originate from the tops of clouds and extend into space. Also sometimes the sky is observed to have a green tinge during violent storms. The origin of that green color is mysterious; some say it's related to the presence of hail.

Weather Words

"Avoid the ash, it attracts the flash. Stay away from the oak, it draws the stroke."

—Weather lore

Avoid the Ash

Lightning is responsible for about a hundred deaths each year in the United States. Exposed objects are most susceptible to a lightning strike. No, it really isn't a good idea to sit under an apple tree—or any tree—during a thunderstorm. Some trees such as ash and oak have a high water content. These are the absolute worst. Because water is a good conductor of electricity, those trees especially should be avoided.


People already struck by lightning do not carry an electrical charge. They can be handled safely and should be taken care of immediately. Prompt mouth-to-mouth resuscitation, cardiac massage, and prolonged artificial respiration can revive a person who may appear to be dead.

On the subject of water, stay off (and out of) the water when thunderstorms are threatening. You are much more exposed to lightning over the open water than over land. Also the turbulence associated with thunderstorms is enough to make plenty of waves. Stay away from metal objects, too.

If you are in an open area and suddenly feel a tingling sensation, or your hair begins to stand on end, lightning is about to hit you. Crouch down toward the ground in a small ball. By crouching, you reduce your surface area and the area exposed to the electrical field.

The best advice during a thunderstorm is to go indoors, don't talk on the phone, don't take a bath or shower, and stay away from windows. An all-metal automobile will offer protection by creating a shell that will deflect the electricity. But if the car has a plastic exterior, that shell will be broken, and the protection is lost. Also, you might want to turn off your computer. Lightning could cause a shocking experience, not to mention the loss of your data files.

Braving the Elements

Can lightning strike twice? It sure can—even the same person. Roy "Dooms" Sullivan, a ranger at Shenandoah National Park, had the distinction of being struck seven different times by lightning. He survived, but he lost the nail of his big toe in 1942 and his eyebrows in July 1969. In July 1970, he was burned on his left shoulder. His hair caught on fire in April 1972, and again in August 1973. He was struck on his ankle in June 1976, and in June 1977, he received chest and stomach burns.

book cover

Excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, Ph.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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