Ah thunderstorms, an every summer afternoon constant growing up on the Southern Gulf Coast of Florida! Almost every morning we could look down the street, towards the direction of the Gulf, and see the start of a thunderstorm building up. Then anytime from about 2-4pm the storm would roll through, dump some rain on us, maybe some thunder and lightning, then be gone for the day.
What is a thunderstorm? A thunderstorm is a rain shower during which you hear thunder. Since thunder comes from lightning, all thunderstorms have lightning. A thunderstorm is classified as “severe” when it contains one or more of the following: hail one inch or greater, winds gusting in excess of 50 knots (57.5 mph), or a tornado.2
Thunderstorms come with a list of potential dangers:
- Straight-Line Winds
- Flash Floods
While a thunderstorm itself is not usually considered to cause fatalities or injuries, those come as a results of the dangers within a thunderstorm.
Thunderstorms are usually about 15 miles in diameter and last about 30 minutes. There are an average of 100,000 thunderstorms in the United States per year, and about 10% of these are considered “severe” thunderstorms.
Three basic ingredients are required for a thunderstorm to form: moisture, rising unstable air (air that keeps rising when given a nudge), and a lifting mechanism to provide the “nudge.”
The sun heats the surface of the earth, which warms the air above it. If this warm surface air is forced to rise—hills or mountains, or areas where warm/cold or wet/dry air bump together can cause rising motion—it will continue to rise as long as it weighs less and stays warmer than the air around it.
As the air rises, it transfers heat from the surface of the earth to the upper levels of the atmosphere (the process of convection). The water vapor it contains begins to cool, releases the heat, condenses and forms a cloud. The cloud eventually grows upward into areas where the temperature is below freezing.
As a storm rises into freezing air, different types of ice particles can be created from freezing liquid drops. The ice particles can grow by condensing vapor (like frost) and by collecting smaller liquid drops that haven’t frozen yet (a state called “supercooled”). When two ice particles collide, they usually bounce off each other, but one particle can rip off a little bit of ice from the other one and grab some electric charge. Lots of these collisions build up big regions of electric charges to cause a bolt of lightning, which creates the sound waves we hear as thunder.2
Be Weather Alert
Severe Thunderstorm Watch: Be prepared. Conditions are favorable for development of a severe thunderstorm.
Severe Thunderstorm Warning: Take shelter. A severe thunderstorm has either been spotted or is radar-indicated.
There are 4 main types of thunderstorms: Single Cell, Multi-Cell, Squall Line, and Supercell Thunderstorms. They each build differently, and pose different threats.
Single Cell Thunderstorm
A Single Cell Thunderstorm is one in which the updraft that creates the storm goes up and straight back down. These are generally formed during the spring and summer months from the updraft of the heat from the sun. Single Cell Thunderstorms typically last only about 20-30 minutes, and are generally a weak thunderstorm. Though they aren’t severe, they are still capable of producing plenty of rain, and possibly hail or weak tornadoes. Single Cell Thunderstorms are rare, because most of the time a thunderstorm is created from multiple updrafts, which produces a Multi-Cell Thunderstorm.
When multiple Single Cell Thunderstorms combine together, they are considered a Multi-Cell Thunderstorm. A Multi-Cell Thunderstorm is more dangerous than a Single Cell, and is also more common. They also develop in the late spring and summer months, again from the rise in heat from the sun. Multi-Cell storms can have gusty winds, and have the potential for hail and a lot of lightning. Usually one of the biggest dangers with a Multi-Cell is the possibility of flash flooding because they drop a tremendous amount of rain over a short period of time, and often build as they move, meaning they can be over the same area for quite a while.
Squall Line Thunderstorms
A Squall Line Thunderstorm has a leading edge where the most dangerous areas of the storm are usually concentrated. These can stretch across hundreds of miles and are capable of producing winds in excess of 60 mph. While they can include tornadoes and hail, the wind is usually the biggest threat with a Squall Line. Flash floods can occur, but usually only when the line becomes stationary over an area.
Most dangerous, but least common, is the Supercell Thunderstorm. A Supercell has a rotating updraft, which allows it to stay together much longer than most thunderstorms, and also become the most organized. Almost all violent tornadoes are formed in a Supercell Thunderstorm, especially when you see the telltale “hook” shown at the bottom left of this radar image. Supercells are capable of producing large, damaging hail, torrential downpours, and dangerous lightning.
During a Thunderstorm and After
Most of the time, you have plenty of advanced warning that a thunderstorm is on it’s way. Depending on the type and size of the storm, you may want to take some basic precautions before the storm hits. Secure anything outside so it isn’t damaged by the wind gusts. If possible, move anything that can be damaged by hail under cover, such as grills outside and putting vehicles in a garage. On the inside, it may help to shut the shades or blinds over windows; if anything hits and breaks a window, it keeps the glass from shattering far into the house. Unplug any sensitive electronics in case of power surges, and turn off your air conditioning (it’s actually sensitive to power surges and very expensive to fix!). Keep your tv/radio tuned to the news to make sure you are aware of any immediate weather threats and/or changes.
If you are getting a severe thunderstorm, you may want to make sure access to your “safe room” in your home is clear. This should be the same place you would go during a tornado…. a small interior room on the lowest floor with no windows. This should also be the same place that you keep your In-Home Emergency Kit ready.
Once a thunderstorm has passed, keep your local weather on for at least 30 minutes to make sure nothing else is developing behind the initial storm. Also, wait at least 30 minutes from the last sound of thunder before going outside as lightning may still be in the area. After it’s safe to do so, assess any damage that your home may have sustained during the storm.
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