Meteorology/Severe Storms

Topic of Meteorology: Severe Storms

Severe Storms General Advice
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Air Mass Thunderstorms
In the United States frequently occur in which maritime tropical (mT) air that moves northward from the Gulf of Mexico. These warm, humid air masses contain abundant moisture in their lower levels and can be unstable when heated from below or lifted along a front.

Life Cycles of Thunderstorms
For the average thunderstorm there are three stages:

The Cumulus
The Cumulus stage is dominated by rising currents of air (updrafts) and the formation of a towering cumulonimbus cloud. Falling precipitation within the cloud causes drag on the air and initiates a downdraft that is further aided by the influx of cool, dry air surrounding the cloud, a process termed entrainment. This stage then progresses to the mature stage.

Mature Stage
The mature stage is marked by the downdraft leaving the base of the cloud and the release of precipitation. With gusty winds, lightning, heavy precipitation, and sometimes hail, the mature stage is the most active period of a thunderstorm.

Dissipating Stage


Marking the end of the storm, the dissipating stage is dominated by downdrafts and entrainment. Without a supply of moisture from updrafts, the cloud soon evaporates. It should be noted that within a single air-mass thunderstorm there may be several individual cells—that is, zones of adjacent updrafts and downdrafts.

Summary
So in summary here is the all the stages together:



Severe Thunderstorms
Severe Thunderstorms are capable of producing heavy downpours and flash flooding as well as strong, gusty straight-line winds, large hail, frequent lightning, and perhaps tornadoes.

For a thunderstorm to be officially classified as severe by the National Weather Service, it must have winds in excess of 93 kilometers (58 miles) per hour or produce hailstones with diameters larger than 1.9 centimeters (0.75 inch) or generate a tornado. Of the estimated 100,000 thunderstorms that occur annually in the United States, about 10 percent (10,000 storms) reach severe status.

Regular air-mass thunderstorms are localized, relatively short lived phenomena that dissipate after a brief, well-defined life cycle(above). The key factor for a severe thunderstorm is a strong vertical wind shear. That way the cold downdraft does not cut off the updrafts, which are the thunderstorm's "fuel".

Supercells
A supercell is a large rotating thunderstorm with a mesocyclone. They can last longer than normal thunderstorms and can produce tornadoes and baseball size hail.

Mesocyclones
A mesocyclone is a large rotating vortex of air. They rotate in the same direction as a low air pressure system would in the same hemisphere as the mesocyclone. They are formed when wind sheer starts a portion of air in the lower atmospher spinning in a tube like formation around a horizontal axis. The updraft found in a supercell can cause the "tube" to angle upwards untill the air is rotating around a verticle axis.

Parts

 * The overshooting top is a dome shaped formation on the top of a supercell caused by a very strong updraft lifting a portion of clouds above the anvil.
 * The anvil is the overshooting portion at the top of the supercell. It is very cold and has almost no moisture in it.
 * The precipitation free base is a portion of the supercell from which no percipitation is falling. Hail may be present, however.
 * The wall cloud is the portion of the supercell between the percipitation free base and percipitating areas. It forms when cool air is pulled into the updraft. The air from this area quickly becomes completely saturated, and becomes visible as a cloud. The area of saturated air moves downward, so the wall cloud appears as a desending column. Very few of these turn into tornadoes.

Tornado Characteristics
Tornadoes are large clouds mostly characterized by extremely high winds. They are usually found in the most intense supercells and are caused by winds traveling in different directions, or wind shear. They usully look like large funnels touching down from the main cloud. Note that although most tornadoes look like funnel clouds, they do not necessarily need to have one, as long as the winds touch both the ground and the cloud. Consequently, a funnel cloud may occur but not a tornado if the funnel does not touch down.

Geographical and Seasonal distribution
The United States are home to the largest amount of tornadoes. Most of them occur in a central region known as Tornado Alley, which contains the states of Texas, Oklahoma, Kansas, Nebraska, and the edges of other states, depending on the definition. However, tornadoes have been observed on every continent excluding Antarctica, and every state in the United States.



There is also a pattern with the time of year and the frequency of tornadoes. The majority of tornadoes form between April and mid-June.

Tornado Hazards
Much of the damage, obviously, is caused by the high winds, as this is the essence of a tornado. However, a lot is caused by the flying debris resulting from the destruction of some structures. Thei impact can destroy other buildings more easily. Other hazards include downed power lines, broken gas lines and pumps, and fires.

The Fujita Scale
Two major scales measure tornadoes: the Fujita scale and the Enhanced Fujita Scale. Both measure from 0 to 5, but the characteristics of both are different.

The Fujita scale, or Fujita-Pearson Scale, is as follows.

An F6 category was also thought of, but it is purely hypothetical and no F6 tornado has actually existed.

The Enhanced Fujita Scale was intended to improve the Fujita scale.

Life Cycles of Tornadoes
Three stages usually categorize a tornado's life. Please note that although these are the most common, not all tornadoes follow this exact pattern and it is merely a model. Nevertheless, it is something you should know.

Formation

If conditions are right, the rotation of winds within a mesocyclone allows a vortex to form underneath it, and a funnel cloud usually forms with this. It gains energy as it descends and it becomes a tornado once it touches down.



Maturity

Once the funnel cloud becomes a tornado, it enters its mature stage. This is where all the destruction comes in.

Dissapation

When the mesocyclone loses its rotation and/or conditions are no longer right for a tornado, it begins to dissipate. The shape of the tornado can be altered into a rope-like form or some other shape, depending on the characteristics of the storm it is in.

Waterspouts
Waterspouts are similar vortexes that occur over water. They are usually less violent than regular tornadoes, although they can be rather powerful given a strong storm.



Causes of Lightning
A storm is only classified a a thunderstorm when there is lightning. Thus, its important to discuss the causes of lightning. Some cloud physicists believe that charge separation occurs during the formation of ice pellets. Experimentation shows that as droplets begin to freeze, positively charged ions are concentrated in the colder regions of the droplets, whereas negatively charged ions are concentrated in the warmer regions. Thus, as the droplets freeze from the outside in, they develop a positively charged ice shell and a negatively charged interior. As the interior begins to freeze, it expands and shatters the outside shell. The small positively charged ice fragments are carried upward by turbulence, and the relatively heavy droplets eventually carry their negative charge toward the cloud base. As a result, the upper part of the cloud is left with a positive charge, and the lower portion of the cloud maintains an overall negative charge with small positively charged pockets. As the cloud moves, the negatively charged cloud base alters the charge at the surface directly below by repelling negatively charged particles. Thus, the surface beneath the cloud acquires a net positive charge. These charge differences build to millions and even hundreds of millions of bolts before a lightning stroke acts to discharge the negative region of the cloud by striking the positive area of the ground below, or, more frequently, the positively charged portion of that cloud, or a nearby cloud.

How Lightning Strikes
Pop quiz: Does lightning start from the cloud and move down, or does it start from the ground and move up? The answer: Niether. This is because a lightning strike is not a single briliant bolt, but actually several strokes. Firt, there is a stream of electrons that moves downwards from the cloud. This is called the initial leader. As it nears the ground, electrons are pulled from the surrounding air, resulting in a ionized path from the cloud to ground. Then, electrons pour from this channel of charge. This is the main stroke and is what we think of "lightning".

Cloud lightning
Lightning does not always strike the ground. It can either occur between two seperate clouds, or within the same cloud, which is the most common. When it occurs with in the same cloud, it will usualy start in the lower portion of the anvil, and move downward.

Heat lightning
Heat lightning appears to produce no thunder. In fact, it does, but it happends so far away that the observer does not hear it, because the sound dissipate through the air.

Positive Lightning
Positive lightning occurs when there are little to no clouds. These lightning bolts originate from the top of a cloud, usualy the anvil, and travels horizontally for several miles before turning and moving downward to meet the initial leader.

Efects
According to the National Weather Service, only 10% of people that are struck by lightning are killed, leaving the remaining 90% with various injuries If you get hit by lightning, it usually damages the nervous system. When the brain is affected, the person may have difficulty with short-term memory, coding new information and accessing old information, multitasking, and being easily distracted. Lightning victims may also suffer personality changes because of frontal lobe damage and become irritable and easy to anger. In addition, some survivors complain of becoming more easily exhausted than before being struck.

Resources
"The Atmoshpere" by Frederick K. Lutgens and Edward J. Tarbuck

"Meteorology Today" by Ahrens