Severe weather hazards are dangerous weather phenomena that threaten life and property. Within this topic, we discuss tornadoes, lightning, hail, flash flooding, and downbursts.
Why do I care? Many types of severe weather affect life and property in the Southeast U.S. The information under this topic will give a quick insight into the deadliest and most dangerous hazards and how often they occur across the Southeast.
Natural hazards to humans include earthquakes, tsunamis, wildfires, and many different types of severe weather. Figure A shows the percentages of all U.S. deaths caused by different types of natural hazards. More than 3/4 of these deaths are due to severe weather (including lightning, tornadoes, and floods) and other climate hazards such as droughts and heat waves. In this webpage we discuss some of the most frequent types of severe weather hazards, including tornadoes, lightning, flash flooding and hail. Hurricanes also affect life and property in both the direct effects of the strong winds and the coastal storm surge which can inundate property near the shoreline.
Also see Drought and Heat
A tornado is a violent column of rotating air that is in contact with the ground. Tornadoes occur most often in the Great Plains and the Midwest (also known as "tornado alley"), but they do occur in other parts of the United States. In fact, every single state in the United States has had at least one tornado.
The Southeast experiences a good number of tornadoes per year, with Florida having the most. The 1953-2004 period averaged about 145 tornadoes per year in the Southeast. However, the average number of tornadoes reported each year has been increasing steadily. This could be due to the fact that as the population increases and spreads out, the chance of someone observing and reporting a tornado will increase as well. Additionally, each year can vary a lot from the average. For example, in 2008, the Southeast had 509 tornadoes, 364 more than the 1953-2004 average!
Tornadoes range in strength from the very small tornadoes with wind speeds up to 75 miles per hour and brief appearances to the strongest tornadoes, with winds in excess of 200 miles per hour and funnel widths of up to a mile in diameter. The small tornadoes are the most frequently seen but cause little damage unless they hit a vulnerable structure. The largest tornadoes cause immense devastation over wide swaths and are the cause of most of the tornado injuries and deaths in the Southeast.
Each year, approximately 2,000 people die from lightning strikes all around the world. In the US, there are about 44 people who die each year from lightning (30 year average), but many of those fatalities occur in the Southeast. During a span of 10 years (1999-2008), 149 people have died in the Southeast from lightning strikes. Additionally, all but one state (Virginia) rank in the top 10 for lightning fatalities by state. Florida by far has the most lightning fatalities out of any state, and as one would expect. They also experience the most thunderstorms out of any state. Tampa, Florida is considered the lightning capital of the world due to the amount of thunderstorms they experience each year.
Figure F: A Hailstone
Hailstones are chunks of ice that fall from strong thunderstorms. They are highly damaging to crops, earning the nickname “the white plague”. In the U.S., hail causes over 1 billion dollars in damage to both crops and property on average each year.
Hail is formed when ice crystals in a thunderstorm grow in the presence of water vapor in the clouds. Strong updrafts in the thunderstorm keep the ice crystals from falling out of the cloud until they grow to a fairly large size. Commonly, the hailstones may cycle through a storm several times before falling to the ground, resulting in an onion-like layering in the ice as the stones move through areas of very cold temperatures near the top of the cloud, where water droplets freeze quickly in an opaque manner, and warmer areas near the bottom of the cloud where water freezes more slowly and clearly. The National Weather Service (2010) considers hail to be severe if it has a diameter of one inch or more; however, even smaller hailstones can cause significant damage to crops when they bruise fruit that was destined for the table, since the cosmetic blemishes can reduce its value significantly.
Flash flooding in the Southeast usually occurs when a large amount of rain falls in an area over a short period of time. The ground can only soak up so much water in a given time, and when the rain rate exceeds what can infiltrate into the ground, flooding is likely to occur. Hurricanes, tropical storms, and ordinary thunderstorms can produce flash flooding.
More people die from floods each year than from tornadoes, lightning, or hurricanes. Forecasters can usually predict where flooding will occur when a hurricane or tropical storm affects an area. However, when dealing with thunderstorms, predicting flash floods can be nearly impossible due to their isolated nature. Flash floods usually occur in low-lying areas where water can collect or in cities where water runoff from impermeable surfaces can fill roads or storm drains quickly.
Figure H. Flash flooding covers the street.
Downdrafts are an integral part of a thunderstorm's life cycle. They are usually fairly short-lived as the thunderstorm blows itself out. Sometimes, however, downdrafts in strong storms fall to the earth so fast that we call them downbursts. A downburst is basically a very strong downdraft that can cause damage to life or property as wind speeds can be in excess of 100 mph in some cases.
Not all downbursts are equally strong. Depending on the size of the downburst, we either call it a microburst or a macroburst. If the strong winds of the downburst extend for less than 2.5 miles, then it is called a microburst. If the strong winds stretch out to be more than 2.5 miles long, then it is called a macroburst.
Microbursts and macrobursts can create extensive damage in a relatively small area. Their damage is often mistaken for tornado damage. While the winds of microbursts/macrobursts do approach tornadic levels, the damage that they tend to cause is straight line wind damage. Basically, the wind that causes the damage usually comes from just one direction and is not twisting or turning (like a tornado). It can be hard to depict tornado or downburst damage from the ground, but if observing it from the air (like a helicopter or plane), it becomes quite easy to tell what caused the damage.
Want to learn more?
Links to National Science Education Standards:
Earth Science: EEn.2.3.2 : Explain how ground water and surface water interact.