From the ABC 7 Weather team

Thunderstorms: Air-Mass Storms & Supercells

May 8, 2012 - 07:40 PM
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Well, it’s the time of the year where our weather attention turns to the possibility of severe weather… and that means possible thunderstorms and even tornadoes!

Well, it’s the time of the year where our weather attention turns to the possibility of severe weather… and that means possible thunderstorms and even tornadoes!

Did you know that most tornadoes emanate from a particular type of thunderstorm? Do you happen to know what the unique name of that thunderstorm is?

If you answered “A supercell thunderstorm”, then you know your weather!

For most of the year, the Mid-Atlantic region is affected by regular ol’ thunderstorms. Run-of-the-mill thunderstorms take several different forms, but the most common type we find in the summer time is called an “air-mass thunderstorm”. An air-mass thunderstorm is a type of thunderstorm that forms generally from the basic combinations of heat and humidity that typically affects the region in summer. Sometimes a weak circulation several miles up in the sky (what you sometimes hear referred to as an “upper-level storm” or “upper-level circulation”) aids to help in the air near the ground to begin to rise eventually forming this storm.

The characteristics of an air-mass storm are fairly simple. Air near ground level begins to rise. In just a few hundred feet or so, the air cools just enough to the point where the invisible water (vapor) begins to condense into very fine liquid water droplets. These fine drops of water form a cloud. The air continues to rise nearly straight up and as the thundercloud forms, it eventually reaches a level in the sky where the air temperature reaches freezing. At this point, the fine liquid droplets collide and form even bigger drops of water. The larger water droplets don’t fall to the ground just yet, because the speed of the rising air holds the raindrops within the cloud.

 

Image courtesy of NSSL

 

 Remember we’ve reached the freezing level too, so fine ice particles in the cloud also begin to form. As the rising air continues to form the “cumulonimbus” cloud, the raindrops and ice particles begin to bounce into one another. As liquid water and ice interact within a cloud, electrons begin to get “rubbed” away and begin to collect at the top of the cloud bringing an overall positive charge to the top of the cloud. The positive charge at the top of the cloud then in turn creates a negative charge at the bottom of the cloud. This cumulonimbus cloud drifts along in the tranquil breeze and starts to induce a positive charge on the ground. After enough time, the attraction between the positive charge building along the ground and the negative charge within the bottom of the cloud becomes so strong that an electrical connection is made: lightning!

 

Image courtesy of NSSL

 Most of the time, this process only takes about 30-45 minutes. Once the thunderstorm runs its course for about another 30-45 minutes, it reaches a point where the rising air can’t support the large raindrops so the thunderstorm begins to collapse and weaken, eventually fading away all together. Many times when the storm begins to “collapse”, a miniature area of rain cooled air spills out of the storm and spreads away as it hits the ground. This little area of cooler air can travel for tens of miles and aid in the formation of even more air-mass thunderstorms. This little pool of cooler air is called “outflow” that you often hear us refer to in the ABC7 weather center when we’re tracking thunderstorms in the summer.

 

Image courtesy of: Skywarn Storm Spotter Guide

 Now that we’ve covered the process of how an air-mass thunderstorm is formed, let’s talk about the godfather of all thunderstorms: the supercell thunderstorm!

If you’re an avid severe weather junkie, then I don’t need to tell you that a supercell thunderstorm is once heck of a beast. The basic structure of a supercell thunderstorm is the same as a regular storm, but it’s really a difference in the surrounding weather that creates this monster storm.

After years of research, meteorologists have found that it’s all about the wind in the atmosphere that is the reason we have supercell thunderstorms. Wind sheer: A difference in wind speed, in relationship to altitude in the atmosphere, AND/OR a change in wind direction in relation to altitude. Wind sheer is a very important phenomenon in weather when speaking of this type of storm. [As an aside, what’s REALLY interesting too about wind sheer is that it comes in two forms, a change in direction of wind or a change in wind speed… I find that kind of cool!] Take all that we know now about air-mass thunderstorms, now put that storm in a place where you’ve got a nice south breeze at about 10-15 mph, but about 2,000-4,000 feet up the winds change to the southwest and blow at 50-60 mph. Recent research has shown that it’s the winds in the first few thousand feet about the ground that can mean the difference between a weak tornado, and a monster/violent/mile wide vortex of destruction. In this type of environment, the nature of the wind field puts a spin in the air. This spin is translated up into the air as the monster storm begins to form. As it takes shape, the entire supercell begins to spin. At this point we also have a special name for a spinning supercell: a mesocyclone (meso = a word that distinguishes distance, in this case about 100 miles or so, and cyclone = meaning spinning).

 

Image courtesy of NSSL

 Within a supercell environment, the rate at which the air rises in the forming thunderstorm happens at a MUCH faster rate than an air-mass storm. Typical speeds can range from 50mph to 70mph straight up, and in some extreme cases up to 100mph-120mph! Can you imagine a wind blowing 100mph into the air?! Remember in our air-mass storm, we eventually reached an altitude at which the air reached the freezing point, that led to the formation of ice and eventually helped create our lightning. In the case of the supercell, the air is rising so rapidly that the ice crystals bump into each other and attach forming little balls of ice or hail. If the air rises above 50mph or so, big enough clumps of ice can form that once they can’t be held up by this “updraft” any longer they can fall causing major bouts of damage and even harm! It’s hail that ranges on the size of about one inch that begins to cause problems, this is why it’s our nationwide standard now for a storm to be classified as “severe” because of large hair – it needs to be at least 1” in size. So our supercell storm is spinning, it’s formed some impressive hail, now it begins to take this spinning air and begins to twist it up and down. I’ve described the updraft, but in the formation of tornadoes a column of falling air seems to also play a big role in the formation of tornadoes. Once supercells reach a point where big raindrops and hail begin to fall to the ground, they begin to take the cooler air around it along for the journey. It’s this “downdraft” in combination with the updraft that we think splits spinning air in two, and can create a tornado. As the smaller, more compact spinning air gets stretched, just like a spinning figure skater who brings her outstretched arms into her body, the spinning air begins to spin faster and a higher wind speeds are produced.

Here are a few images describing the event.  RFD stands for the "rear, flank, downdraft."

 

Image courtesy of: Skywarn Storm Spotter Guide

 

Image courtesy of: Skywarn Storm Spotter Guide

 

Image courtesy of: Skywarn Storm Spotter Guide

 

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