Severe Storms/Thunderstorms

This page is to be used for the Severe Storms topic of the Meteorology event.

Air Mass Thunderstorms
In the United States, Air Mass Thunderstorms frequently occur when maritime tropical (mT) air 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 are 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 1.0 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".

Mesoscale Convective Complexes
A mesoscale convective complex (MCC) is a unique kind of mesoscale convective system which is defined by characteristics observed in infrared satellite imagery. They are long-lived, nocturnal in formation and commonly contain heavy rainfall, wind, hail, lightning and possibly tornadoes.

Single Cell
Single cell thunderstorms, also known as pulse storms, usually last 20 to 30 minutes. Although severe weather is uncommon in single cell storms, heavy rainfall, downbursts, hail, and even weak tornadoes are possible. Single cell thunderstorms form in environments with low wind shear.

Multi-Cell Cluster
A multi-cell cluster storm is a group of cells moving as a single unit. Each constituent cell of a multi-cell cluster is at a different stage of thunderstorm development. New cells develop on the upwind side of the cluster, mature cells can be found in the center, and dissipating cells are on the downwind side. Alike single cells, multi-cell clusters can still produce heavy rainfall, downbursts, and hail, but the risk of flash flooding can be significant, and tornadoes are less likely.

Multi-Cell Line
A squall line is a line of severe thunderstorms that can form along and/or ahead of a cold front. It contains heavy precipitation, hail, frequent lightning, strong straight line winds, and possibly tornadoes and waterspouts. Squall lines typically form in unstable atmospheric environments where low-level air can rise unaided after being initially lifted (e.g., by a front) to the point where condensation of water vapor occurs. Heat is released during condensation, resulting in the rising air becoming lighter than nearby air at the same height. This leads to an increase in the speed of the rising air which sometimes reaches speeds above 30 mph. In models this initial lifting is specified through an idealization of the flow associated with the front or other lifting mechanism or through the use of observational flow information. The gust front is located along the line where these winds meet -- which extends from the surface well up into the the storm.



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 shear starts a portion of air in the lower atmosphere spinning in a tube like formation around a horizontal axis. The updraft found in a supercell can cause the "tube" to angle upwards until the air is rotating around a vertical 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 precipitation is falling. Hail may be present, however.
 * The wall cloud is the portion of the supercell between the precipitation free base and precipitating 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 descending column. Very few of these turn into tornadoes.
 * The rear flank downdraft is the downdraft on the back side of a supercell. It is dry air that wraps around the mesocyclone and is important to tornado development.  After it descends to the ground, it moves east into the updraft.
 * The forward flank downdraft is the downdraft on the leading side of a supercell. It is the main downdraft and the area with the heaviest precipitation.
 * The flanking line is a line of cumulus clouds on the southwest side of the supercell. It forms along the rear flank downdraft.
 * The gust front is the boundary between the updraft and downdraft of a thunderstorm. In supercells it is located between the updraft and the rear flank downdraft.
 * The mesocyclone is the rotating updraft of a supercell.

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 usually 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.



Meteorologist also now reference a 2nd "alley" referred to as "Dixie Alley." Dixie Alley includes states of the SE United States such as Missouri, Tennessee, Arkansas, Alabama and Mississippi. 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 caused by a tornado can be related to the high winds, as this is the essence of a tornado. However, a lot of damage is also caused by the flying debris resulting from the destruction of some structures. Their 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.

Dissipation

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: Neither. This is because a lightning strike is not a single brilliant bolt, but actually several strokes. First, there is a stream of electrons that moves downwards from the cloud. This is called the initial leader(or Step 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 (or Return Streamer) and is what we think of "lightning".

Cloud lightning
Lightning does not always strike the ground. It can either occur between two separate clouds, or within the same cloud, which is the most common. When it occurs with in the same cloud, it will usually 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 happens so far away that the observer does not hear it, because the sound dissipates 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, usually the anvil, and travels horizontally for several miles before turning and moving downward to meet the initial leader.

Ball Lightning
The entire existence of ball lightning can be disputed because of it's lack of observation. Ball lightning has been spotted hundreds of times around the world, but very rarely by meteorologists. Observers say that ball lightning appears as a sphere, differing in size from between a few inches in diameter to several meters, and varies in color between red, orange, yellow, even green or white. It can appear after a large thunderstorm. It travels mostly horizontally, from about waist high to several meters off the ground. Usually ball lightning comes with a bad smell. It can come in through open doors or windows, including closed screens, and sometimes chimneys. No ball lightning stays for more than a few seconds, and it moves at a brisk pace- several meters per second. Sometimes observers report that it will "bounce" between puddles.

Because even the existence of ball lightning can't be proven, not very much is known about it other than its appearance. As of right now, no theories have been suggested that can explain the strange movement, appearance, and how it can produce a constant stream of light and energy.

It is thought that UFO sightings after a large storm can actually be ball lightning. So the next time you see a ball of light high in the sky after a large storm, you may not be seeing a UFO, but instead a rare example of ball lightning.

Effects
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.