• Michael Thornton

How do Thunderstorms, Hail, and Tornadoes work?

Updated: Oct 11

How does a Thunderstorm form?

For a Thunderstorm to form it needs 3 key ingredients. These ingredients include instability, moisture, and a boundary for the storms to initiate. You may be asking yourself "what is instability?" Instability means that the atmosphere is unstable. When the atmosphere is unstable the temperature will begin to drop significantly as you get higher into the atmosphere. So how do we get Thunderstorms if the air cools down as you increase in height?

Well, during the day the earth's atmosphere heats up because of radiation from the sun. If you have a blue sky on a severe weather day, then the atmosphere will be able to heat up much more quickly because nothing is blocking radiation from getting to the Earth's surface. This is why on cloudy days where severe weather is expected storms might tend to be weaker. After the radiation has reached the ground, conduction begins to occur.

Conduction is the process of the ground transferring the heat from the sun back into the air. Once conduction has completed, the warm air will rise and convection will occur. You may be asking yourself "how can moisture play a role in Thunderstorm development?" Well, simply put, if you have no moisture in the atmosphere, you will get no storms.

The reason for this is because the air will be too dry for storms to form. When the dewpoint is around 55 degrees Fahrenheit storms are able to form more easily and can become severe. Within every Thunderstorm exists an updraft, downdraft, frozen liquid, water, and water vapor. Did you know? Some Severe Thunderstorms can reach as high as 70,000 feet into the atmosphere.

To put that into context planes fly at around 31,000 - 38,000 feet.

What is a Severe Thunderstorm?

A Severe Thunderstorm is a Thunderstorm that is either producing wind speeds over 58mph or hail up to at least 0.75 inches. Severe Thunderstorms can be discrete, meaning that they are by themselves, or they can be multi-cellular, meaning that there is more than 1 storm in the area. Severe Thunderstorms can also come in the form of a squall line. Not all Severe Thunderstorms produce hail, severe winds, and tornadoes. Some just produce hail that can reach up to the size of a softball and others can produce wind speeds up to 100mph.

What is Hail?

Hail occurs when water droplets are carried high into an updraft where the temperatures drop significantly. Hailstones can grow in two separate ways. The first being the "Dry growth". Dry growth occurs when supercooled water and ice crystals collide with each other and then accumulate. The other way is known as "Wet growth".

Wet growth occurs when the ice is in an area where the temperature is below freezing, but not very cold. In this case, when the ice collides with the supercooled water, the water will actually take longer to freeze. Did you know? On July 23rd, 2010, the largest hailstone ever recorded fell in a small town in South Dakota called Vivian. This hailstone measured in at 8 inches in diameter and 18.25 inches in circumference.

What is a derecho, bow echo, and squall line?

Photo courtesy of Josh Swangstu

I know all too well about Derechos, bow echos, and squall lines as I spent the first 18 years of my life in Wisconsin. So what is a squall line? A squall line is a multicellular storm that moves in a linear pattern. Squall lines are usually ahead of the cold front and often intersect the warm front. Now that you know what a squall line is, we can move onto a bow echo. A bow echo is another form of a squall line, but rather than just being linear, they look similar to a bow that you would shoot an arrow with.

1998 Great Lakes Derecho in Wisconsin

Finally, a derecho is the most dangerous type of squall line. It can last for hours spanning across multiple states. A derecho can even cause more damage than a tornado due to its size. In 1998, a derecho formed in Minnesota and move Southeast into Wisconsin where a 120mph wind gust was recorded. This derecho killed 4 people and caused $172 million in damages. To put that into perspective, with inflation that is $275 million today. All squall lines have the ability to be extremely dangerous; please take them seriously!

What is a tornado?

Photo courtesy of StoughtonTornado.org

Before we can talk about tornadoes, we must first talk about what a Supercell is. A Supercell is a Severe Thunderstorm that has a rotating updraft. This rotating updraft acts as a barrier against the environmental wind, meaning that the environment can not destroy the updraft. A Supercell can last for hours and travel across an entire state if it is in the right environment. The life cycle of a Tornado consists of 5 different stages.

Photo courtesy of Dale Bernstein

Those 5 stages are: the dust whirl stage, the organization stage, the mature stage, the shrinking stage, and finally the decaying stage. In the dust whirl stage, the tornado has continuous circulation from the cloud to the ground. The vortex will be invisible as the pressure is not low enough for condensation. So how can you tell if it's actually a tornado or just a funnel? Well, if you see dust being picked up under the funnel then the funnel is in fact actually a tornado.

Photo courtesy of Colin McDermott

During the organization stage, the tornado fully condenses to the ground because the pressure has finally become low enough. During this stage, tornadoes can also rope out and dissipate. When the tornado reaches its mature stage, the tornado is at its most intense. Once the tornado reaches the shrinking stage, the vortex will begin to shrink, while the rotation could actually increase. The tornado will usually take a left path once this process has begun.

Finally, once the tornado has reached the decaying stage the tornado begins to rope out as the RFD (Rear Flank Downdraft) occlusion is completed.

About the writer: Michael Thornton graduated from Rose State College majoring in Emergency Management. Currently, he is the Assistant Director at Latimer County Emergency Management and is an Oklahoma Emergency Management Association member.


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