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( 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 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.

Ball Lightning
The entire exsitance of ball lightning can be disputed because of it's lack of observation. Ball lightning has been spotted hundrends 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 veries 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 severl 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. Somtimes observers report that it will "bounce" between puddles.

Because even the exsitance 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 sugestied 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.

Saffir-Simpson Intensity Scale
tegory	Winds (1 min sustained winds in mph and km/hr)	Summary	People, Livestock, and Pets	Mobile Homes	Frame Homes	Apartments, Shopping Centers, and Industrial Buildings	High-Rise Windows and Glass	Signage, Fences, and Canopies	Trees	Power and Water	Example 1	74-95 mph

64-82 kt

119-153 km/hr	Very dangerous winds will produce some damage	People, livestock, and pets struck by flying or falling debris could be injured or killed. Older (mainly pre-1994 construction) mobile homes could be destroyed, especially if they are not anchored properly as they tend to shift or roll off their foundations. Newer mobile homes that are anchored properly can sustain damage involving the removal of shingle or metal roof coverings, and loss of vinyl siding, as well as damage to carports, sunrooms, or lanais. Some poorly constructed frame homes can experience major damage, involving loss of the roof covering and damage to gable ends as well as the removal of porch coverings and awnings. Unprotected windows may break if struck by flying debris. Masonry chimneys can be toppled. Well- constructed frame homes could have damage to roof shingles, vinyl siding, soffit panels, and gutters. Failure of aluminum, screened-in, swimming pool enclosures can occur. Some apartment building and shopping center roof coverings could be partially removed. Industrial buildings can lose roofing and siding especially from windward corners, rakes, and eaves. Failures to overhead doors and unprotected windows will be common. Windows in high- rise buildings can be broken by flying debris. Falling and broken glass will pose a significant danger even after the storm. There will be occasional damage to commercial signage, fences, and canopies. Large branches of trees will snap and shallow rooted trees can be toppled. Extensive damage to power lines and poles will likely result in power outages that could last a few to several days. Hurricane Dolly (2008) is an example of a hurricane that brought Category 1 winds and impacts to South Padre Island, Texas.

Category	Winds (1 min sustained winds in mph and km/hr)	Summary	People, Livestock, and Pets	Mobile Homes	Frame Homes	Apartments, Shopping Centers, and Industrial Buildings	High-Rise Windows and Glass	Signage, Fences, and Canopies	Trees	Power and Water	Example 2	96-110 mph

83-95 kt

154-177 km/hr	Extremely dangerous winds will cause extensive damage	There is a substantial risk of injury or death to people, livestock, and pets due to flying and falling debris. Older (mainly pre-1994 construction) mobile homes have a very high chance of being destroyed and the flying debris generated can shred nearby mobile homes. Newer mobile homes can also be destroyed. Poorly constructed frame homes have a high chance of having their roof structures removed especially if they are not anchored properly. Unprotected windows will have a high probability of being broken by flying debris. Well-constructed frame homes could sustain major roof and siding damage. Failure of aluminum, screened-in, swimming pool enclosures will be common. There will be a substantial percentage of roof and siding damage to apartment buildings and industrial buildings. Unreinforced masonry walls can collapse. Windows in high-rise buildings can be broken by flying debris. Falling and broken glass will pose a significant danger even after the storm. Commercial signage, fences, and canopies will be damaged and often destroyed. Many shallowly rooted trees will be snapped or uprooted and block numerous roads. Near-total power loss is expected with outages that could last from several days to weeks. Potable water could become scarce as filtration systems begin to fail. Hurricane Frances (2004) is an example of a hurricane that brought Category 2 winds and impacts to coastal portions of Port St. Lucie, Florida with Category 1 conditions experienced elsewhere in the city.

Category	Winds (1 min sustained winds in mph and km/hr)	Summary	People, Livestock, and Pets	Mobile Homes	Frame Homes	Apartments, Shopping Centers, and Industrial Buildings	High-Rise Windows and Glass	Signage, Fences, and Canopies	Trees	Power and Water	Example 3	111-130 mph

96-113 kt

178-209 km/hr	Devastating damage will occur	There is a high risk of injury or death to people, livestock, and pets due to flying and falling debris. Nearly all older (pre-1994) mobile homes will be destroyed. Most newer mobile homes will sustain severe damage with potential for complete roof failure and wall collapse. Poorly constructed frame homes can be destroyed by the removal of the roof and exterior walls. Unprotected windows will be broken by flying debris. Well-built frame homes can experience major damage involving the removal of roof decking and gable ends. There will be a high percentage of roof covering and siding damage to apartment buildings and industrial buildings. Isolated structural damage to wood or steel framing can occur. Complete failure of older metal buildings is possible, and older unreinforced masonry buildings can collapse. Numerous windows will be blown out of high-rise buildings resulting in falling glass, which will pose a threat for days to weeks after the storm. Most commercial signage, fences, and canopies will be destroyed. Many trees will be snapped or uprooted, blocking numerous roads. Electricity and water will be unavailable for several days to a few weeks after the storm passes. Hurricane Ivan (2004) is an example of a hurricane that brought Category 3 winds and impacts to coastal portions of Gulf Shores, Alabama with Category 2 conditions experienced elsewhere in this city.

Category	Winds (1 min sustained winds in mph and km/hr)	Summary	People, Livestock, and Pets	Mobile Homes	Frame Homes	Apartments, Shopping Centers, and Industrial Buildings	High-Rise Windows and Glass	Signage, Fences, and Canopies	Trees	Power and Water	Example 4	131-155 mph

114-135 kt

210-249 km/hr	Catastrophic damage will occur	There is a very high risk of injury or death to people, livestock, and pets due to flying and falling debris. Nearly all older (pre-1994) mobile homes will be destroyed. A high percentage of newer mobile homes also will be destroyed. Poorly constructed homes can sustain complete collapse of all walls as well as the loss of the roof structure. Well-built homes also can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Extensive damage to roof coverings, windows, and doors will occur. Large amounts of windborne debris will be lofted into the air. Windborne debris damage will break most unprotected windows and penetrate some protected windows. There will be a high percentage of structural damage to the top floors of apartment buildings. Steel frames in older industrial buildings can collapse. There will be a high percentage of collapse to older unreinforced masonry buildings. Most windows will be blown out of high-rise buildings resulting in falling glass, which will pose a threat for days to weeks after the storm. Nearly all commercial signage, fences, and canopies will be destroyed. Most trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months. Long-term water shortages will increase human suffering. Most of the area will be uninhabitable for weeks or months. Hurricane Charley (2004) is an example of a hurricane that brought Category 4 winds and impacts to coastal portions of Punta Gorda, Florida with Category 3 conditions experienced elsewhere in the city.

Category	Winds (1 min sustained winds in mph and km/hr)	Summary	People, Livestock, and Pets	Mobile Homes	Frame Homes	Apartments, Shopping Centers, and Industrial Buildings	High-Rise Windows and Glass	Signage, Fences, and Canopies	Trees	Power and Water	Example 5	> 155 mph

> 135 kt

> 249 km/hr	Catastrophic damage will occur	People, livestock, and pets are at very high risk of injury or death from flying or falling debris, even if indoors in mobile homes or framed homes. Almost complete destruction of all mobile homes will occur, regardless of age or construction. A high percentage of frame homes will be destroyed, with total roof failure and wall collapse. Extensive damage to roof covers, windows, and doors will occur. Large amounts of windborne debris will be lofted into the air. Windborne debris damage will occur to nearly all unprotected windows and many protected windows. Significant damage to wood roof commercial buildings will occur due to loss of roof sheathing. Complete collapse of many older metal buildings can occur. Most unreinforced masonry walls will fail which can lead to the collapse of the buildings. A high percentage of industrial buildings and low-rise apartment buildings will be destroyed. Nearly all windows will be blown out of high-rise buildings resulting in falling glass, which will pose a threat for days to weeks after the storm. Nearly all commercial signage, fences, and canopies will be destroyed. Nearly all trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months. Long-term water shortages will increase human suffering. Most of the area will be uninhabitable for weeks or months. Hurricane Andrew (1992) is an example of a hurricane that brought Category 5 winds and impacts to coastal portions of Cutler Ridge, Florida with Category 4 conditions experienced elsewhere in south Miami-Dade County.

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

"Meteorology Today" by Ahrens