It's a clear July morning with temperatures in the high 80s - a perfect day to lay 20 or 30 square of shingles. It's a little humid, but the winds out of the south give you just enough relief as you begin to sweat.

It's a clear July morning with temperatures in the high 80s - a perfect day to lay 20 or 30 square of shingles. It's a little humid, but the winds out of the south give you just enough relief as you begin to sweat. The high alto-cirrus clouds look like silver brushstrokes as they casually sweep to the north. After lunch there is a warm, freshening breeze, bringing somewhat cooler temperatures out of the northwest. Soon the cumulonimbus clouds scatter overhead, sending shadows across the roof deck and relieving you of the direct radiation of the harsh afternoon sun. You pick up the pace as the shadows dance by with more frequency. An hour or so before quitting time, you notice that the shadows are permanent now. You look up to see a giant blue-grey battleship parked over the entire county. The air grows still and all the birds silently seek shelter in the trees. A large anvil-shaped wedge begins to shear off in the highest jet stream as the bottom of the cloud mass flattens out into a low, slate-colored plate close overhead.

"Get a move on, boys," the foreman calls out. "You've got a half hour to cap that ridge so we can get out of here before all hell breaks loose."

Suddenly you're aware of a buzzing and crackling in your ears, and a strange tingling sensation crawls up your neck. You begin to take a deep breath when your chest begins burning from within and you feel like you're being hit by a semi. Suddenly your world becomes a solitary black hole into which you silently and helplessly fall.

Types of Lightning

Since the era of Benjamin Franklin, facts about lightning have been well documented. The principle is similar to static electricity generated as you scuffle in your socks across a nylon carpet. That simple act may generate an electrical potential of up to 10,000 volts, which is noticeable only when you touch a metal doorknob and discharge the potential in a spark. Lightning is a massive, powerful electrostatic discharge produced by both immature and mature thunderstorms, emitting visible blue-white light, temperatures up to 50,000 degrees F and a wide range of electromagnetic radiation. The surrounding air rapidly heats up, producing a violent shock wave, which breaks the sound barrier.

There are dozens of types of lightning including intra-cloud, sheet, anvil crawling, bead, ribbon (or streak), staccato (or triggered), heat (or summer), ball, sprite (or jet), cloud-to-cloud, ground-to-cloud, and cloud-to-ground lightning.

During a typical thunderstorm, a cold front collides with a warm front. The warm air rises, forcing the cold air down to earth and initiating the lateral rotational mechanism that forms a thunderstorm. Lightning is generated as moisture particles become electrostatically induced (charged) as rain and ice crystals oscillate in updrafts and downdrafts from a few feet per second to 100 mph. The top of the mature thunderstorm cloud becomes negatively charged during the day by gamma rays irradiating our atmosphere. The heavier, negatively charged air molecules in the cloud are repelled by these negative charges, and they attract positive charges from the earth until they become sandwiched between positively ionized molecules at the top and bottom of the cloud, essentially "damming up" the potential energy.

As the differing potential between these opposite charges increases, the attraction to the positive ground of earth also increases until an invisible, negatively charged stroke containing hundreds of millions of electrons is induced to leave the bottom of the cloud. A single stroke of lightning can precede the storm as far as six to 10 miles. Negative lightning makes up about 95 percent of all lightning. This negative electron flow, about the width of a pencil, is called the stepped leader. As this negative stroke approaches the surface of the earth in halting, downward steps, positively charge particles quickly gather underneath the cloud and seek the highest conductive structure as a path of least resistance. This can be a tall tree, a scaffold, a chimney, a church steeple or an elevated roof deck. The rising energy charge is called the streamer, which can reach over five miles in length. When a single leader and streamer meet in the air, they produce a completed DC circuit with a large potential of over 100 million volts, causing the current to surge back up the now positively charge leader into the cloud. This return stroke takes about a quarter of a second and is the most luminous part of the strike. Sometimes the return stroke becomes multiple, causing a flickering effect too fast for the eye to separate. The energy produced by the return stroke often superheats the leader's air channel, causing it to expand so rapidly it produces an audible shock wave known as thunder. The speed of light (299,792,458 meters per second) is much faster than the speed of sound (which varies according to air temperature and pressure, but would be approximately 339 meters per second on a dry, 85-degree day). This is why we see the lightning before we hear the thunder.

As with most dosage formulas found in nature, something exceedingly violent is typically very short-lived. The life cycle of a large thunderstorm from developmental stage, through mature stage to dissipation stage is usually less than one hour (typically 30 to 50 minutes). While most of the lightning strikes occur within 3 to 5 miles of the center of the cumulonimbus cloud, there is a 10 percent chance of a severe strike as far as 10 miles from cloud center. At 15 miles from the center, you can still see lightning, but you cannot hear thunder.

What Are the Effects?

The potential effects of being struck by lightning are as varied as the types of lightning. Apparently it is common for victims to avoid prompt emergency medical attention, in the hope that the symptoms might subside in a few hours or days. Hospital admissions are few and therefore their records are not reflected in the state data banks. Most of these injuries occur to postal and construction workers who find themselves out in the open in the summer months. The third most commonly affected group is made up of those persons using telephones or electrical equipment during the lightning strike. Some victims claim being struck by lightning on multiple occasions in different locations. One Virginia park ranger, Ray "Doom" Sullivan, was reportedly struck by lighting on seven different occasions in as many different state parks, but as his occupation put him in frequently exposed locations, you may assume he would be more prone to the hazard.

As the stepped leader approaches the ground, initiating the streamer, the negatively charged air molecules in the proximity become positively charged. This produces the acrid smell of ozone as a byproduct. There may be a distinct buzzing or crackling sound heard as the electrons discharge in rapid succession. The buildup of positive static charges from the ground also causes the skin on the shoulders, neck and head to prickle and the head and body hair to stand on end as it is attracted to the negatively charged air above. There may be a bluish glow, or aura, building up around other people and objects. If you detect this, quickly move away from them, as they may produce a serious step charge across the induction field between you.

When people are hit directly by a lighting strike, their entire physiology is immediately affected. The human body was once described by Geddes as "an electolytic resistor surrounded by an insulator (skin)." According to Ohm's Law (E=I x R), up to 14 million volts can be generated from a 20,000-amp lightning strike. In a direct hit, the electrical charge strikes the victim first. If his skin resistance is high, the majority of the current will flash around the skin and clothing to ground; this effect is aptly named "St. Elmo's Fire." An indirect splash hit is when the energy effectively bounces off of another object and strikes the victim secondarily on route to ground. A ground strike may hit the earth near the victim and is conducted likewise through the victim from entry to discharge points. Physical injuries may be due to electrical damage, intense heat of plasma and the mechanical energy produced. Attachment to conductive items may also prove hazardous after fatal voltage strikes them indirectly. All lightning victims should always be suspected of multiple traumas, particularly if the victim is discovered on the ground below a possible work point.

Overall, the critical areas of lightning damage are usually to the circulatory system (as primary conductor of current), the lungs (extremely profuse with blood and delicate) and the central nervous system (spinal cord and brain). The cardiopulmonary damages often require immediate medical attention in order for the victim to survive. Myocardial infarction and various cardiac arrhythmias may prove fatal as well. The air in the lungs expands rapidly with increased heat, sometimes damaging the heart and lungs within the pleural cavity and causing pulmonary and/or cardiac arrest.

Burns are often linear, punctured and full thickness except where produced by ignited clothing. Surface burns and subcutaneous fluid explosions from production of steam from sweat glands are often present. Blunt force trauma injuries are common as the body goes into involuntary, rapid and violent muscular spasms and collapse. Considerable hemorrhaging often occurs, as well as thrombosis caused by intravascular coagulation and suffusion. The violence of force and heat of the strike is often sufficient to tear or rupture the aortic arch and the victim quickly bleeds out. The pericardium surrounding the heart can rupture from internal pressures caused by the heating of the blood in the chest. Fatal arrhythmias are often delayed up to 12 hours. The path of the thorasic burns can create thermal damage to the gastrointestinal tract as severe as necrosis. The organs affected by electric shock and explosive heating may include the intestines, gallbladder, pancreas, spleen, liver, appendix, as well as abdominal trauma due to falls.

Severe neurological damage is almost always extensive in lightning victims, so they are usually monitored closely for 48 hours in ICU. Loss of consciousness often occurs due to the violent electrical surge to the central nervous system. The medulla oblongata of the brain seems particularly sensitive to surges of high voltage. More often than not, lightning survivors exhibit temporary or permanent spinal paralysis of the lower extremities. They can appear cold and clammy with mottled complexion, often escalating to an insensate coma with no pulse. The cerebellum forms lesions and voids, leading to amnesia and prolonged confusion as post-strike symptoms. The pressure changes forming the high-pressure shock waves may measure up to ten atmospheres, causing both external and internal damage to the ears in 30 percent to 50 percent of victims. Ear damage can include torn eardrums, shattered and dislocated bones, vertigo and tinnitus. Fifty-five percent of strike victims suffer ocular cataracts due to thermal and electrical surges and contusions from the shock wave. A dozen other minor and major ocular lesions, opacities and hemorrhagic defects may also occur in the eyes. Many of these symptoms may occur months or even years after exposure.

The electrical charge often leaves the body in unique "Lichtenberg Figures," sometimes called "lightning flowers" due to their ornately figured burn patterns. Discharge points are often marked by severe burns and tissue destruction. Boots and shoes may be blown off with such force that they can fracture the bones of the feet. Entry and exit points on the body are often marked by the rupture of capillaries and veins from the current or shock wave. Sometimes all the body hair of the victim is singed off on prolonged strikes.

Mild to severe behavioral and psychological effects may follow a lightning strike. An altered consciousness is often common to those who had a direct hit. Their sense of reality, short and long-term timing and sequential memories may be drastically distorted or suspended all together. Later symptoms may range from mild to severe headaches and distractibility to persistent psychiatric disorders such as schizophrenia and dementia. A progression of mental impairments is quite common over the next few months after injury

What to Do When Lightning Strikes

The electrocuted victim is usually knocked prone and discharged to ground. The victim may not actually hold a stored charge that could affect you, but the storm is obviously within striking distance, so remain cautious for lightning at the end of the storm. Loss of consciousness, head injuries, lack of breathing, absence of pulse or serious burns all require that you call 911 immediately for emergency medical assistance. Check the victim's ABCs (airway, breathing, circulation) for 30 seconds (minimum). If breathing or pulse is not obvious, initiate CPR if you have been trained and certified. Remember that the injuries from electrical currents may prohibit a successful CPR, but once you've initiated it you must continue until (1) the victim is pronounced deceased by a licensed medical professional; (2) you are replaced by someone with equal or better training; or (3) you collapse, physically unable to continue resuscitation.

Be aware that more storms may be brewing. Stay alert. If another storm is pending in your area, administer the breathless victim two quick rescue breaths and move him quickly with a shoulder drag to a safer location. Large thunderstorms often produce multiple accident scenes, so emergency medical services are often delayed when coming to remote areas. Therefore, .if the victim is wet and cold, add clothing or your own body heat to prevent hypothermia from damaging core organs until help arrives. A Mylar thermal blanket is a welcome addition to any first aid kit. If it's available, place dry material between the victim and the ground, as it will prevent a heat sink from lowering the temperature of the heart and lungs.

Keep and eye out for flash flooding and fires, as they often accompany thunderstorms. You may have sought shelter in low areas, so be aware that all this water has to go somewhere, and it might already be on its way. A sudden flow of as little as 500 cubic feet of water in a flash flood can generate an impact force of up to 30,000 foot-pounds. You may survive the initial crest impact, but floodwaters often bring a wave of dangerous debris at high speeds to strike, entangle and drown a weakened victim and the rescuer.

Storm Warning Signs

Some clues can warn of an approaching thunderstorm. Sometimes the air gets much more humid as the cold front approaches the warm front. A haze builds and visibility decreases across longer views. Small cumulous clouds multiply and expand to large "cauliflower"-headed clouds in less than an hour after noon. Hard edges start to form on the bottom of these clouds, while the top edges may appear fuzzy as ice crystals are formed at higher altitudes. At this point, you may assume a thunderstorm is well on its way. The warm air temperatures of the morning drop precipitously, as the cold front appears to line up the random clouds forming a squall line. Increased vapor condensation makes clouds on the horizon refract more light and appear darker. The formation of condensation in the shape of an anvil below the high-velocity jet stream indicates a storm cloud with a potential for dangerous winds, hail and lightning, due to its maximum vertical ceiling height. If you see this formation, it is time to seek shelter. Storms are often accompanied by a sharp rise in wind speed at ground level as the cold air is thrust down from higher, colder altitudes. This is often call "wind shear," and it can be strong enough to lift tractor-trailers off the highway and knock down acres of trees simultaneously. It certainly can sweep a roofing crew off of a high-rise building. Some thunderstorms are produced by a warm front meeting an occluded front, and these are often less distinctive in size and shape but may be equally destructive as a storm developing from a warm front meeting a cold front.

If you have any questions concerning the probability of a thunderstorm in your work zone during the high-probability summer months (May through September), you should not hesitate to check the early morning weather reports before and after you arrive at the site. Satellite imagery and Doppler radar have vastly improved the accuracy of local forecasting. Some industries which by their nature expose workers to potential thunderstorm risks and lightning hazards avail themselves of real-time weather downloads on laptop computers. Forewarned is forearmed. At the very least, an inexpensive National Oceanic and Atmospheric Administration (NOAA) weather radio can be employed with 24/7 reception nationwide. Many have hazardous weather alerts or automatic alarms, which may sound even when the radio is turned off. If nothing else is available, leaving a portable AM radio on may be helpful. If the station does not include a weather report, the radio may indicate an approaching thunderstorm as lightning interferes with AM radio signals quite easily.

NOAA recommends the 30-30 rules when lightning is a potential hazard. When you see lightning, count the time until you hear the thunder. If it take 30 seconds or less, the storm is less than 6 miles away, and you are in the potential strike zone. It is important that you stop your activities immediately and seek shelter in a substantial building away from open windows and doors. Remember that ball lightning may easily travel inside a building on warm, moist air currents, through wall openings as small as 12 inches and seek out a ground inside. After the storm has passed, wait at least 30 minutes from the last rumble of thunder before exposing yourself outside. Often underestimated, the "clear blue" 6- to 10-mile area behind a thunderstorm is just as dangerous as the leading edge. The flash-to-bang principle states that you can divide the interval (in seconds) between the flash and bang by five to obtain the approximate distance of the storm (in miles). Divide by 3 to obtain distance in kilometers. If the interval decreases, the storm is moving toward you. If it's increasing the storm is moving away.

Protecting Yourself in the Event of a Storm

If you are suddenly caught out in the open on a roof deck when lightning begins striking nearby, you may not have the time necessary to reach a safe shelter. Regardless of the risk, your immediate directive should be to get down from any elevated locations such as roofs, ladders or scaffolding. At the very least, try to reduce your elevation to the lowest possible height above ground, such as moving toward the eaves on the leeward (downwind) side of the roof. Determine if there are any other probable attraction points for lightning, such as service entrances, chimneys, dormers, penthouses, antennas, vent stacks, water towers, cooling towers, condensers or air conditioners. Remain as far from these objects as feasible. Stay away from any metallic objects on the roof and certainly avoid contact with a lightning rod or woven copper grounding strap. Do not gather together on a roof, but rather spread yourselves out as far as 15 feet, if possible. Place any type of insulation between you and the roof deck (fold your jacket into a pad). Sit on your heels, feet close together and bend forward into a ball grasping your legs with your arms, hands over ears, with you head in between your knees (or as close as possible). This keeps you as low in elevation as possible. Immediately remove any metal or graphite tools or nails from you belt. Unless it contains metal, keep you hat on with your back to the wind to protect your head in case of a hailstorm.

If caught in the open on the ground, avoid single trees for shelter but crawl into any available bushes or thick wood lot if available. Seek low ground or a depression or ditch but locate yourself as far away as possible from any standing or running water. If the gullies and ditches are filling up with water, move away from the high water mark as it rises.

While vehicles resting on rubber tires (even with steel belts) are often safer than other locations, your electrical isolation may be compromised if the vehicle has grounding straps or heavy rain is pouring over its sides and tires to the ground. Never attempt to leave a vehicle during the storm, even if it has been struck by a downed hot power line. Avoid contact with any plumbing or electrical equipment, as these are common circuits to established grounds. If you must leave a threatening situation, by all means, use extreme caution. Jump wide and roll from a compromised vehicle, being careful not to touch the metal body in the process.

A certain percentage of lightning fatalities are caused by "step voltage," which is passed from the strike zone through the damp, charged atmosphere surrounding a strike victim to induce indirect current electrocution of another victim as far away as 120 feet. Ground current typically affects only the feet and legs of this standing two-legged victim, but may prove fatal to livestock, whose vital organs are in the electrical route between front and rear legs. Roofers resting on their hands and knees on the roof deck or climbing down a ladder and are struck by step voltage may also experience a higher fatality rate. Workers who are gathering at the top of a ladder egress from a roof likewise share a common induction field. So it is important to remember to maintain a 15- to 50-foot separation distance from other workers if lightning is imminent.

According to a study by the University of Kansas Center on Aging, two thirds of those victims who die from lightning, die within one hour of the strike. About 80 percent succumb to cardiac arrest with minutes. Adequate and frequent training and drilling in first aid and CPR can prove to be the difference between losing a worker and having a near miss.

Don't Take Storms for Granted

Unfortunately, more construction workers are killed and injured on the job in thunderstorms than hurricanes and tornadoes, as the potential threat of physical harm is often considered minimal in a thunderstorm. Obviously, exposure on wide-open roof decks dozens or even hundreds of feet in the air will create a high probability of lightning strikes with a high severity of physical damage. Rooftop rescues may be made difficult by multiple victims and potential fire hazards, as well as the limited means of access and egress for victim and rescuers. Your best defense, as in most safety issues, can be found above your shoulders and between your ears. Plan proactively for any and all emergency responses, including thunderstorms.

Plan ahead and drill for an emergency roof evacuation prior to the predominant thunderstorm months. Receive training to help you be aware of the subtle clues in the morning hours that depict a likely afternoon thunderstorm. Make sure all of your crew members are certified and trained in first aid and CPR. Stay in touch with your local weather service on the job. Know what you should do and when you should do it whenever the storm cues are presenting themselves. If residential security is important, don't be reluctant to roll out the tarps and put the tools away early. Even if the storms eventually track around your jobsite, your prompt decision to protect your exposed roofers from potential lightning is always the best bet

SIDEBAR: Lightning Strikes: What Are the Odds?

The fact is that lightning strikes somewhere on the earth about 100 times per second (8.6 million times a day) from over 2,000 simultaneous thunderstorms worldwide. The National Lightning Detection Network (NLDN) determined that over 90 percent of all lightning strikes occur during the months from May to September, with the greatest number (73 percent) in the afternoon hours between 2 and 5 p.m., when thermal ground heating coincides with atmospheric cooling.

According to National Oceanic and Atmospheric Administration (NOAA), from 1992 to 1994 thunderstorms caused 44 percent of the fatalities (51), 19 percent of the injuries (345) and 3 percent of the physical damage ($32 million) for all convective-weather incidents in the United States. Unlike hurricanes and tornadoes, most lightning causalities involved only one person. Compared to all weather-related fatalities in the same period of time, lightning ranked third in related deaths, behind flash floods and river flooding. It is estimated by NOAA that the 100,000 thunderstorms in the United States per year produce over 20 million cloud-to-ground strikes annually.

From 1990 to 2000, Florida ranked the No. 1 state for lightning strikes (100 days per year) and lightning deaths (126 victims per year). Texas was second with 52 deaths, and Colorado third with 39. Alaska was the least likely state to be struck, with no lightning fatalities in 10 years. When states are ranked according the lightning deaths per million residents, however, Wyoming and New Mexico rank first and second. As far as lightning-plagued cities are concerned, Lightning Alley is a large urban/suburban area surrounding Interstate I-4 between Orlando and St. Petersburg where the nation's largest number (over 10,000) of lightning strikes are recorded annually year after year. For reasons unknown to most scientists, Tuscon, Ariz., runs a close second. NASA produced a study of lightning strike zones recorded in Florida from 1959 to determine the surface areas most commonly producing cloud-to-ground lightning streamers. While a significant number (34 percent) were of unknown description, the vast majority of identified lightning strike zones included open areas and water.