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Severe Thunderstorms and Their Hazards…

Updated on July 3, 2017

Rain is arguably the most commonly occurring weather event. But at times, simple rain does not occur alone. During the spring and summer months, weather systems that bring rain often spawns stronger storms. Most recognize these storms as thunderstorms.

A thunderstorm is a rainstorm that is accompanied by lightning, thunder, and sometimes gusty winds. They are caused by an updraft, which occur when warm, moist air rises upwards into the atmosphere. Once aloft, this warm updraft begins to cool. This results in the formation of towering cumulonimbus clouds. Inside these clouds, the cool air triggers condensation of the moisture in the air. When this moisture becomes too heavy to remain airborne, it falls to the ground as rain.

During a thunderstorm, rain tends to be heavier than that that occurs in a simple rain shower. This is because the rain within a developing thunderstorm doesn’t just bring water to the ground. These heavy rains also drag cooler air down with them as well. The combined warm updraft and cool downdraft create a storm cell that reaches high into the atmosphere. As the storm intensifies, lightning and thunder are also produced; the result; a thunderstorm. And since warm updrafts can occur anywhere in the world where warm, moist air rises quickly, most people—no matter where they live—have experienced a thunderstorm at some point in their life.

How a thunderstorm forms...
How a thunderstorm forms...

Only about 10% of all thunderstorms are classified as severe. But what exactly distinguishes a “severe” thunderstorm from any other thunderstorm? According to the National Weather Service (NWS), a thunderstorm is recognized to be severe when it exhibits (or has the potential to exhibit) the following criteria:

  • Strong winds of 58 mph (93 kph) or higher, AND…
  • Hailstones that measures 1-inch (2.54 centimeters) or more in diameter, OR…
  • A confirmed tornado or funnel cloud (this also includes those storms with a higher probability of producing a tornado).

Unofficially, the presence of (potentially) dangerous lightning can also be considered a criterion for a severe thunderstorm. And it is these elements of a severe thunderstorm that makes them hazardous.

What Makes Severe Thunderstorms Dangerous?

When it comes to severe thunderstorms, thunder isn’t dangerous. However, heavy rain, lightning, hail, and heavy winds potentially are. So to understand why severe thunderstorms can be dangerous, it is necessary to understand the individual hazards they create.

Heavy Rain

Depending on the individual conditions (geography, previous weather conditions, amount of rain unleashed, etc.), heavy enough rains produced during a severe thunderstorm can result in flooding. Flooding typically occurs when intense rain falls over a short period of time (or prolonged rain falls over longer period), causing a river or stream to overflow onto the surrounding area. Flooding can also result from the failure of a water control structure, such as the failure of a levee, dam, or an overwhelmed water and/or sewage drainage system.

Even more dangerous than flooding is the prospective hazard of flash flooding. Flash floods are exactly what the name suggests: floods that happen in a flash! Flash floods generally develop within 6 hours of a heavy rain event (or other cause, such as an aforementioned dam or levee failure). What makes flash flood particularly dangerous is the rapid rise of water over low-lying areas. In some cases, flooding may even occur well away from where heavy rain initially fell. These sudden downpours can rapidly change the water levels in a stream or creek and turn small waterways into violent, raging rivers (this is especially common in the Western and Southwestern U.S., where low lying areas may be very dry in one moment, and filled with rushing water from upstream the next). Urban areas are especially prone to flash floods due to the large amounts of concrete and asphalt surfaces that do not allow water to penetrate into the soil easily…and where the majority of water drainage systems fail. Flash flooding kills more people than any other single severe thunderstorm hazard.

Lightning

Aside from the millions of volts of heat and electricity that the average lightning bolt contains, the unpredictability of lightning makes it a particularly dangerous severe thunderstorm hazard. On example of this is where (and when) lightning can potentially strike. Although most lightning will strike within 10 miles (16 kilometers) of its parent thunderstorm (cloud), an errant lightning bolt can strike much farther away. This phenomenon is known as “a bolt from the blue,” due to the fact that a lightning strike that occurs some distance from the parent thunderstorm often appears to strike from a clear blue sky. In reality, it is simply a bolt that manages to stray some distance from a thunderstorm cloud. On rare occasions, lightning-detection equipment has recorded lightning striking up to 50 miles away from the parent thunderstorm.

An example of "a bolt from the blue."
An example of "a bolt from the blue."

Another reason lightning is so dangerous is because it is inherently destructive due to the power it carries. On average, a lightning bolt carries an estimated 100-million volts of electric potential, and is capable of producing a heat potential upwards of 50,000 degrees Fahrenheit (27,760 degrees Celsius). This means that any potentially combustible object that is struck directly by lightning can—and most likely—will catch fire. This includes dwelling that house residential quarters and businesses, infrastructure-related items such as utility poles, as well as trees. In fact, about 20% of the wildfires that occur in the U.S. each year are caused by lightning. Lightning strikes on power lines and elements of the electrical infrastructure can cause physical damage, as well as power outages. Finally, an average of 50 people a year dies from lightning-related injuries. Despite this number, that amounts to a fatality rate of 10% overall from the number that are actually struck by lightning annually.

Fires to an apartment building, caused by lightning...
Fires to an apartment building, caused by lightning...

Hail

Hail is a form of precipitation that occurs when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere, where they freeze into balls of ice. Normally, when these balls of ice become too heavy to stay aloft, much like rain drops forming from moisture in the clouds, they fall to the ground as hail. However, oftentimes hailstones may cycle through

The hazards stemming from hail in a severe thunderstorms lies in proportion to the size of the hailstones themselves. This is to say that the larger the hailstones produced by a storm, the more extensive the property damage…and potential for injury to those exposed to them. For example, the typical level of damage from hail (based on size) is as follows:

  • Quarter-size (1 in./2.54 cm) - Damage to shingles
  • Golf ball (1.75 in./4.44 cm) - Dents on cars
  • Baseball (2.75 in./6.98 cm) - Windshields smashed
  • Softball (4.5 in./11.43 cm) - Holes in roofs


Large hailstones, collected in the aftermath of a severe thunderstorm...
Large hailstones, collected in the aftermath of a severe thunderstorm...

What’s more, of the estimated $10 billion in damage annually that severe thunderstorms have caused in the past decade, hail accounts for at least half the cost to both property and agricultural insurance. Vehicle damage is the most widely known hazard of hailstorms. Large hail can easily shatter the windows and cause dozens of large dents (extreme hail events can completely total a vehicle). And every year, farmers are forced to worry about potential crop damage from severe thunderstorms that produce damage (ad destructive) hail. For them, even a moderately intense hail event can result in a total loss to an agricultural crop. Buildings can likewise suffer damage from an intense enough hail event produced during a storm. Hail can easily shatter windows, even breaking through walls and roofs.

Hail damage to property...
Hail damage to property...

While hail in and of itself very rarely kills human beings, injuries—both direct and indirect—can, and do occur (such as from accidents that can result from a traveling automobile suddenly having its windshield shattered from a large hailstone impact, or from flying glass).

Damaging Winds

Damaging winds from severe thunderstorms are much more common than damage from any tornadoes they may produce. In fact, the damage from the winds produced by a severe thunderstorm can be so intense that many often attribute the damage caused to tornadoes. The actual culprit in most cases is what’s known as “straight-line winds.”

Almost all of the damaging winds that are produced during severe thunderstorms are the result of a downburst of air (i.e., a downdraft). In a thunderstorm, millions of falling rain drops, the buildup of friction causes rising air to begin to fall towards the ground. Also, some of the falling rain will evaporate. Through evaporation heat energy is removed from the atmosphere cooling the air associated with the precipitation. As a result of the cooling, the density of the air increases causing it to sink toward the earth. When this dense rained-cooled air reaches the surface it spreads out horizontally, with the leading edged of the cool air forming a gust front. When these gust fronts hit the ground, they spread out, causing the straight-line winds that sometimes, create the damage they are known for.

Furthermore, the damaging winds produced by severe thunderstorms can distinguish themselves in their intensity, their extent, and duration. In fact, meteorologists have categorized damaging thunderstorm winds based on their individual attributes. They include:

  • Downbursts - A downburst is an area of strong, often damaging wind produced by thunderstorm-related downdrafts, with the overall size of the downburst affecting an area from 4 to 6 miles (8 to 10 kilometers) in diameter at the point of contact with the ground.
  • Microbursts – A microburst is a small downburst that affects a area of less than 2½ miles (4 kilometers) in diameter at the point of contact with the ground. Microbursts tend to last for only 2-5 minutes in duration and—despite their small size—can produce destructive winds up to 168 mph (270 km/h). They also create hazardous flying conditions for aircraft pilots, and have been responsible for several notable airplane crashes.
  • Macrobursts - A macroburst is larger than a microburst, with a horizontal extent more than 2½ miles (4 km) in diameter from the point of contact with the ground. Also, a macroburst is not quite a strong as a microburst but still can produce winds as high as 130 mph (210 km/h). Damaging winds generally last longer, from 5 to 20 minutes, and can produce tornado-like damage up to an EF-3 on the Fujita Scale.
  • Derechos - A derecho is a widespread, long-lived windstorm associated with a band of rapidly-moving severe thunderstorms. Derechos are composed of a "family of downburst clusters.” By definition, they are defined as cluster of severe thunderstorms that cut a swath of wind damage that extends for more than 250 miles (about 400 kilometers), includes wind gusts of at least 58 mph (93 km/h) along most of its length, and includes several, well-separated 75 mph (121 km/h) or greater wind gusts.


An example of a severe thunderstorm downburst...
An example of a severe thunderstorm downburst...

The potential for death, injury, and/or property damage from these powerful winds are many. Those caught outside during these storms are vulnerable to being injured or killed by falling trees, and people at sea risk injury or drowning from storm winds and waves that can overturn boats. Occupants of cars and trucks also are vulnerable to falling trees and utility poles. High profile vehicles such as semi-trailer trucks, buses, and sport utility vehicles may be blown over. At outside events such as fairs and festivals, people may be killed or injured by collapsing temporary structures and flying debris. Even those indoors may be at risk for death or injury. Mobile homes, in particular, may be overturned or destroyed, while barns and similar buildings can collapse. People inside homes, businesses, and schools are sometimes victims of falling trees and branches that crash through walls and roofs; they also may be injured by flying glass from broken windows. On occasion, severe structural damage to buildings can pose danger to those within.

Overall, severe thunderstorms occur in every state in the continental U.S, in varying levels of probability. The states with the lowest occurrences of severe thunderstorms are in the West, while the state of Florida not only experiences the greatest number of severe thunderstorms, but also of lightning strikes. As with any severe weather event, the potential for destruction and the number of potential casualties are dependent on the intensity, extent of the affected area, and the duration of each storm (few severe thunderstorms last longer than 30 minutes).

Overall, severe thunderstorms occur in every state in the continental U.S, in varying levels of probability. The states with the lowest occurrences of severe thunderstorms are in the West, while the state of Florida not only experiences the greatest number of severe thunderstorms, but also of lightning strikes. As with any severe weather event, the potential for destruction and the number of potential casualties are dependent on the intensity, extent of the affected area, and the duration of each storm (few severe thunderstorms last longer than 30 minutes).

The variation in potential danger from storm to storm is partly the reason why the NWS issues weather advisories based on this potential. These advisories include a “Severe Thunderstorm Watch,” and a “Severe Thunderstorm Warning.” A severe thunderstorm watch means that conditions are just conducive for the development of severe weather, and DOES NOT indicate that severe weather has been reported. While no immediate action on the part of the general public is required for the issuance of a watch, citizens should keep up to date on the current weather situation and be prepared to seek shelter if necessary. However, a Severe Thunderstorm Warning indicates that severe weather is imminent in your area or is already occurring (based on either human observation or Doppler radar). A warning requires that those in the affected area take shelter.

A severe thunderstorm watch means that a severe thunderstorm is expected in within 6 hours of the issuance of the watch. A watch can be issued NWS's Storm Prediction Center, or by one of the local branches of the NWS forecast office. When the Storm Prediction Center issues a watch, it is normally for a large area of expected storm activity, usually covering an area approximately 120 to 150 miles wide and 300 to 400 miles long (36,000 to 60,000 square miles). However, local NWS forecast offices issue warnings, which usually are done so on a county-by-county basis.

Lightning

There is no safe place outside when severe thunderstorms are in the area. If the sound of thunder can be heard, lightning is within striking distance (to estimate the number of miles distant a thunderstorm is, count the number of seconds between a flash of lightning and the next clap of thunder. Then, divide this number by five. This will yield the distance in miles). As the old adage goes, When Thunder Roars, Go Indoors! There is little that can be done to reduce the risk of a potential lightning strike if outdoors during a severe thunderstorm. Hence, the only completely safe action is to get inside a safe building or vehicle.

To avoid being caught outdoors during a severe thunderstorm, focus on being proactive. Know the weather patterns of any area there are plans to visit, and listen to the weather forecast for any planned destination. This is because the forecast may be very different from area to the next. If there is a high chance of thunderstorms, stay inside. But if outdoors, avoid open fields, the top of a hill or a ridge top. Stay away from tall, isolated trees or other tall objects. If camping in an open area, set up camp in a valley, ravine or other low area (tents offers NO protection from lighting). Avoid water, wet items (such as ropes), and metal objects, such as fences and poles. Water and metal do not attract lightning but they are excellent conductors of electricity (as the current from a lightning flash will easily travel along conducting materials for long distances)

Lightning deaths by location/activity
Lightning deaths by location/activity

Hail

Many strong and severe thunderstorms produce hail. Large hail, or flying glass large hailstones may have broken can injure both people and pets. Hail can be smaller than a pea, or as large as a softball, and can be very destructive to automobiles, glass surfaces (skylights and windows), roofs, plants, and crops. In a hailstorm, take cover immediately. Pets and livestock are particularly vulnerable to hail, so bring animals into shelter before storms begin.

Scares left by hail impacts against the skin...
Scares left by hail impacts against the skin...

Damaging Winds

Downbursts and straight-line winds associated with severe thunderstorms can produce winds 100 to 150 miles per hour (or more), enough to flip cars, vans, and semi-trucks. The resulting damage can equal the damage of most tornadoes. If a severe thunderstorm warning is issued, take shelter the same way you would if a tornado were approaching your area. Leave structures that are susceptible to being blown over in high winds, such as a mobile home, and head for a secured structure.

Being aware of the hazards of severe thunderstorms will enable one to make decisions that may help you mitigate the risk for serious injury...or worse. Of course, every possible risk can't be addressed, nor is there an absolute guarantee these information will ensure an absolute level of safety during severe weather. However, these are most the most well-known hazards, and the most practical course of actions one can take that will help ensure a reasonable level of safety during a severe thunderstorm.

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