Back To CourseEarth Science 102: Weather and Climate
13 chapters | 127 lessons
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At 6:23 pm on May 3, 1999, a mile-wide tornado touched down near Oklahoma City right in the middle of rush hour. It traveled through Oklahoma City for 38 miles and stayed on the ground for 85 minutes. Wind speed of the storm was measured at 301 miles per hour, and this was the highest wind speed ever recorded on Earth.
After it was over, 36 people had lost their lives and one billion dollars in damage had been done. The tornado was ranked an EF5, the most severe type of tornado, based on wind strength and amount of damage.
EF5 tornados are strong enough to rip entire buildings off of their foundations. In this lesson, you will learn about the different types of severe storms that can cause this kind of destruction.
The most basic type of thunderstorm is a single cell, or isolated, thunderstorm. Generally, they form in unstable conditions where there is warm, humid air. Single-cells develop when the Sun warms the Earth, causing air to rise. This is called an updraft, and it forces the air to rise above the freezing level of the atmosphere.
The moisture in the air begins to freeze, and a storm cloud starts to form. The frozen particles of moisture start to join together, forming bigger and heavier particles. Once the particles become too heavy to stay in the air, they fall back to Earth as precipitation and bring air down with them. This falling air is called the downdraft.
Rain, hail, and lightning can occur during a single cell thunderstorm. Eventually, the downdraft overpowers the updraft, and the storm ends. These types of storms only last for a short time and aren't usually severe. Severe thunderstorms form in a similar way but under even more unstable conditions and with a more powerful mechanism to start the updraft.
Severe thunderstorms usually develop along cold fronts and are often multi-cell storms. This just means that more than one storm forms along the front, so they are not isolated like single-cell storms. The key to their formation is strongly rising warm, humid air in an unstable environment.
One way they develop is through convergence, which is when the air streams flow into one another. For example, as a cold front moves east across the United States, it brings cold wind with it that is blowing down from the north. That front meets warm winds that are blowing up from the south. When the winds from the north and south meet each other, the air piles up. Then, there is nowhere for the air to go but up. Air rising in this way can lead to the development of potentially severe thunderstorms.
Another way that they can form is through orographic lifting, which happens when a cold front is forced to move from a low elevation to a higher elevation as it moves over terrain, usually mountains. This is why severe weather is common east of the Rocky Mountains. As cold fronts approach the Rockies, the air is forced to rise over them, causing storms to develop. However, there are a couple of types of multi-cell storms.
Mesoscale convective complexes are groups of thunderstorms that form in a large cluster that can span an entire state. Usually, they develop from a thunderstorm outbreak in the area of unstable air that is warm and humid. In the areas where they form, there is enough heat and moisture for air to rise and storms to form.
Individual storms form in close vicinity of each other, and the storm cells feed off the outflow from other nearby storms. The result is a nearly circular, large cluster of storms that are often severe. They form in the late afternoon due to daytime heating and last into the night, but the most severe weather happens fairly early in storm development. The storms can travel great distances before they dissipate, and last at least six hours.
Another group of severe storm systems is called squall line thunderstorms. Squall line thunderstorms are a large number of single-cell storms arranged in a line. They are similar to mesoscale convective complexes in that they are a cluster of storms, but, instead of a circular-shaped cluster, they form in a multi-cell linear cluster.
Squall lines form along cold fronts or as far as 80 miles ahead of a cold front. They form in a similar way to mesoscale convective complexes. If the air at or in front of the cold front is unstable, humid, and warm, squall line thunderstorms can develop. They usually have heavy precipitation, strong winds, small hail, and lightning. In some rare cases, tornadoes can develop.
There are other severe storms that can be isolated. Supercell thunderstorms are large, single-cell storms that can be very severe. Compared to the other storms mentioned so far, these storms are the least common and can produce the most severe weather. They usually create a lot of heavy rain, strong wind, hail, and even tornadoes about 15% of the time. Supercells typically have a diameter of 12 to 30 miles and a life span of 2 to 4 hours.
Supercells are different from other types of storms primarily because they have a rotation that leads to severe weather. There is a strong updraft, typically in the southwest part of the storm, where the rotation occurs. This rotation is caused by differences in wind direction or speed around the storm, and the southwest area where it occurs is called a mesocyclone. This is the part of the storm where a tornado may develop.
In this southwest region of the supercell, a hook shape called a hook echo often forms. The presence of the hook echo on a weather radar suggests that a mesocyclone is present and that a tornado is possible.
If you see a hook echo on a storm that is headed in your direction, you might want to find shelter right away! The other thing that makes supercells so severe is how long they last. If you remember, thunderstorms end when the downdraft overcomes the updraft that is feeding the storm. Supercells are usually so large that the updrafts and downdrafts are far apart from each other, so it can take them a long time to fizzle out and die.
Although there are several different types of severe thunderstorms, they all develop in a similar way. They develop where there is warm, humid, unstable air. The warm air rises above the freezing level where ice crystals form, creating an updraft. The ice crystals get heavier as they clump together until they eventually fall, bringing cold air down with them. This is a downdraft, and once it develops, the storm is fully developed. This is when rain, hail, and lightning occur.
If the updraft that starts storm formation is strong enough, storms can be severe. Convergence, or when air flows collide, and orographic lifting as air moves over a mountain are two common ways strong updrafts develop.
A lot of the time when a cold front is involved, more than one thunderstorm develops along the edge of it. These are called multi-cell thunderstorms. One type of multi-cell cluster is called a mesoscale convective complex. It develops when a cold front stalls and meets warm, moist air. It causes multiple storms to form in a state-sized cluster that is circular in shape. They develop in late afternoon, last for many hours, and travel long distances.
Other multi-cell storms are called squall line thunderstorms. These are similar to mesoscale convective complexes, except they form in a line, not a cluster that is out ahead of a cold front. They usually have heavy rain and strong wind.
Another type of severe storm that is not a multi-cell storm is called a supercell thunderstorm. Supercells are the most severe kind and cause strong wind, heavy rain, and tornadoes. They can be very large and last several hours. The difference between supercells and other severe storms is a rotation of winds within the storm called a mesocyclone. It is within a mesocyclone that tornadoes can develop. The mesocyclone shows up on a weather radar as a hook shape that is called a hook echo. The presence of a hook echo suggests a tornado might be developing.
|Single cell||the most basic (or isolated) type of thunderstorm|
|Multi-cell storms||means that more than one storm forms along the front, so they are not isolated like single-cell storms|
|Convergence||when the air streams flow into one another|
|Orographic lifting||happens when a cold front is forced to go from a low elevation to a higher elevation as it moves over terrain, usually mountains|
|Mesoscale convective complexes||groups of thunderstorms that form in a large cluster that can span an entire state|
|Squall line thunderstorms||a large number of single-cell storms arranged in a line|
|Supercell thunderstorms||large, single-cell storms that can be very severe|
|Mesocyclone||rotation caused by differences in wind direction or speed around the storm in the southwest area where tornadoes can be created|
|Hook echo||a hook shape on a weather radar that suggests that a mesocyclone is present and that a tornado is possible|
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Back To CourseEarth Science 102: Weather and Climate
13 chapters | 127 lessons