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Chapter 10 Study Guide
Key Terms
ordinary cell (air mass) thunderstorms: (also called air-mass thunderstorm) A thunderstorm produced by local convection within a conditionally unstable air mass. It often forms in a region of low wind shear and does not reach the intensity of a severe thunderstorm.
cumulus stage: The initial stage in the development of an ordinary cell thunderstorm in which rising, warm, humid air develops into a cumulus cloud
mature stage: The second stage in the three-stage cycle of an ordinary thunderstorm. This mature stage is characterized by heavy showers, lightning, thunder, and violent vertical motions inside cumulonimbus clouds.
dissipating stage: The final stage in the development of an ordinary cell thunderstorm when downdrafts exist throughout the cumulonimbus cloud.
multicell thunderstorm: Thunderstorms often in a line, each of which may be in a different stage of its life cycle.
overshooting top: A situation in a mature thunderstorm where rising air, associated with strong convection, penetrates into a stable layer (usually the stratosphere), forcing the upper part of the cloud to rise above its relatively flat anvil top.
gust front: A boundary that separates a cold downdraft of a thunderstorm from warm, humid surface air. On the surface its passage resembles that of a cold front
straight-line winds: Strong winds created by a thunderstorm’s downdraft that flows outward, away from the storm in a straight line, more or less parallel to the ground.
shelf cloud: A dense, arch-shaped, ominous-looking cloud that often forms along the leading edge of a thunderstorm’s gust front, especially when stable air rises up and over cooler air at the surface. Also called an arcus cloud.
roll cloud: A dense, roll-shaped, elongated cloud that appears to slowly spin about a horizontal axis behind the leading edge of a thunderstorm’s gust front.
outflow boundary: A surface boundary separating cooler, more-dense air from warmer less-dense air. Outflow boundaries formed by the horizontal spreading of cool air that originated inside a thunderstorm.
downburst: A powerful downdraft that creates a strong downward burst of wind, often associated with thunderstorms, which can cause significant damage upon reaching the ground.
Microburst: A strong localized downdraft (downburst) less than 4 km wide that occurs beneath thunderstorms. A strong downburst greater than 4 km across is called a macroburst.
heat burst: A sudden increase in surface air temperature often accompanied by extreme drying. A heat burst is associated with the downdraft of a thunderstorm, or a cluster of thunderstorms.
squall line: A line of thunderstorms that form along a cold front or out ahead of it.
bow echo: A line of thunderstorms on a radar screen that appears in the shape of a bow. Bow echoes are often associated with damaging straight-line winds and small tornadoes.
Derecho: Strong, damaging, straight-line winds associated with a cluster of severe thunderstorms that most often form in the evening or at night.
mesoscale convective complexes (MCCs): A large organized convective weather system comprising a number of individual thunderstorms. An MCC can span 1000 times more area than an individual ordinary cell thunderstorm. An MCC is a particular type of mesoscale convective system.
Supercell: A severe thunderstorm that consists primarily of a single rotating updraft. Its organized internal structure allows the storm to maintain itself for several hours. Supercell storms can produce large hail and dangerous tornadoes.
Mesocyclone: A vertical column of cyclonically rotating air within a supercell thunderstorm.
wall cloud: An area of rotating clouds that extends beneath a supercell thunderstorm and from which a funnel cloud may appear. Also called a collar cloud and pedestal cloud.
Pyrocumulonimbus: A thunderstorm cloud (cumulonimbus) that develops within the updraft above an intense wildfire.
flash floods: A flood that rises and falls quite rapidly with little or no advance warning, usually as the result of intense rainfall over a relatively small area.
Lightning: A visible electrical discharge produced by thunderstorms.
Thunder: The sound due to rapidly expanding gases along the channel of a lightning discharge.
sonic boom: A loud explosive-like sound caused by a shock wave emanating from an aircraft (or any object) traveling at or above the speed of sound.
stepped leader: An initial discharge of electrons that proceeds intermittently toward the ground in a series of steps in a cloud-to-ground lightning stroke.
return stroke: The luminous lightning stroke that propagates upward from Earth to the base of a cloud.
dart leader: The discharge of electrons that proceeds intermittently toward the ground along the same ionized channel taken by the initial lightning stroke.
heat lightning: Distant lightning that illuminates the sky but is too far away for its thunder to be heard.
dry lightning: Lightning that occurs with thunderstorms that produce little, if any, appreciable precipitation that reaches the surface.
St. Elmo’s fire: A bright electric discharge that is projected from objects (usually pointed) when they are in a strong electric field, such as during a thunderstorm.
Tornado: A rapidly rotating column of air that extends from a thunderstorm to the ground, characterized by a funnel shape and capable of causing significant destruction.
funnel cloud: A funnel-shaped cloud of condensed water, usually extending from the base of a cumuliform cloud. The rapidly rotating air of the funnel is not in contact with the ground; hence, it is not a tornado.
Tornado Alley: A region in the Great Plains of the United States extending from Texas and Oklahoma northward into Kansas and Nebraska where tornadoes are most frequent.
Dixie Alley: Region in the southern United States, typically over Mississippi and Alabama, where tornadoes often form.
suction vortices: Small, rapidly rotating whirls perhaps 10 m in diameter that are found within large tornadoes.
tornado watch: A forecast issued to alert the public that tornadoes may develop within a specified area, usually a portion of one or more states, over the next few hours.
tornado warning: A warning issued when a tornado has been observed or is indicated on radar. It may also be issued when the formation of tornadoes is imminent. Tornado warnings typically cover parts of one or more counties and last from 30 to 45 minutes.
tornado emergency: A type of tornado warning issued when a particularly strong tornado poses the potential for major damage or loss of life.
Fujita Tornado Damage Scale: A scale developed by T. Theodore Fujita for classifying tornadoes according to the damage they cause and their rotational wind speed. (See also Enhanced Fujita Scale.)
Enhanced Fujita Scale (EF Scale): A modification of the original Fujita Scale that describes tornado intensity by observing damage caused by the tornado.
tornado outbreak: A series of tornadoes that forms within a particular region—a region that may include several states. Often associated with widespread damage and destruction.
supercell tornadoes: Tornadoes that occur within supercell thunderstorms that contain well-developed, mid-level mesocyclones.
hook echo: The shape of a hook on a Doppler radar screen that indicates the possible presence of a tornado.
Tornadogenesis: The process by which a tornado forms.
non supercell tornadoes: A tornado that occurs with a cloud that is often in its growing stage, and one that does not contain a mid-level mesocyclone, or wall cloud. Landspouts and gustnadoes are examples of non supercell tornadoes.
Gustnadoes: A relatively weak tornado associated with a thunderstorm’s outflow. It most often forms along the gust front.
Landspout: Relatively weak non supercell tornado that originates with a cumuliform cloud in its growth stage and with a cloud that does not contain a mid-level mesocyclone. Its spin originates near the surface. Landspouts often look like waterspouts over land.
Waterspout: A column of rotating wind over water that has characteristics of a dust devil and a tornado.
NEXRAD: An acronym for Next Generation Weather Radar. The main component of NEXRAD is the WSR 88-D, Doppler radar.
Chapter 10 Study Guide Questions
What are the three ingredients needed for thunderstorm development?
Moisture in the atmosphere to provide fuel for storm growth
Instability to allow air to rise and form clouds
A lifting mechanism, such as a front, mountain, or heat from the Earth's surface to initiate the upward motion.
Understand the three-part thunderstorm development.
These components work together to create the conditions necessary for thunderstorms to form, with each stage playing a crucial role in the development of severe weather phenomena.
Cumulus Stage: This is the initial stage where warm, moist air rises and cools, leading to the formation of cumulus clouds.
Mature Stage: The thunderstorms become fully developed, characterized by heavy rainfall, lightning, and thunder as the updrafts and downdrafts work together.
Dissipating Stage: The storm begins to weaken as the downdrafts dominate, causing the precipitation to decrease and eventually cease.
How are severe thunderstorms different than non-severe thunderstorms?
Severe thunderstorms are defined by their potential to produce extreme weather conditions such as large hail, damaging winds of 58 mph or greater, and tornadoes, whereas non-severe thunderstorms typically exhibit lighter rainfall and do not pose a significant threat to life or property. Additionally, severe thunderstorms are often associated with supercell structures, which can lead to more organized and prolonged storm activity, whereas non-severe thunderstorms tend to be more transient and less intense.
What is an updraft and downdraft?
An updraft is a localized upward movement of air within a thunderstorm, caused by rising warm air that can lead to the development of precipitation, while a downdraft is a downward movement of air that results from the cooling of air due to precipitation and evaporation, causing heavier rain and potentially leading to gusty surface winds.
Where do thunderstorms form most frequently across the U.S., why?
Thunderstorms form most frequently across the U.S. in the eastern part of the country and the southeastern states, primarily due to the warm, moist air from the Gulf of Mexico that interacts with cooler air masses. This convergence creates instability in the atmosphere, setting the stage for thunderstorm development.
What is an ordinary thunderstorm? How is this different than a multicell or supercell thunderstorm?
An ordinary thunderstorm, also known as a single-cell thunderstorm, is a localized storm that generally lasts for a short duration, typically less than an hour. It is characterized by a single updraft and is less capable of producing severe weather compared to multicell or supercell thunderstorms. In contrast, multicell thunderstorms consist of a group of cells in various stages of development, which can lead to more intense precipitation and storm dynamics. Supercell thunderstorms, on the other hand, are highly organized and possess a rotating updraft, making them the most severe type of thunderstorm capable of producing large hail, strong winds, and tornadoes.
What is a multicell thunderstorm? How is this different than an ordinary thunderstorm? Supercell?
A multicell thunderstorm is comprised of multiple cells that can form and move together, allowing for the development of new updrafts as older ones dissipate. This results in a longer-lasting storm system that often produces heavy rainfall, hail, and gusty winds. Unlike ordinary thunderstorms, which are short-lived and isolated, multicell thunderstorms can develop into severe weather events due to their organized structure. In comparison to supercell thunderstorms, multicell storms lack the rotating updraft and are generally less intense, though they can still produce hazardous weather. Supercells are distinct due to their well-defined structure, including a mesocyclone, which significantly contributes to their ability to produce severe turbulence and the potential for tornado formation. Furthermore, supercells can persist for several hours and often exhibit a characteristic hook echo on radar, indicating their powerful and organized nature. In summary, while multicell thunderstorms can bring severe weather, supercells possess the unique features that make them the most dangerous type of thunderstorm.
What is a squall line, where are they most commonly found?
A squall line is a narrow band of severe thunderstorms that can produce heavy rains, strong winds, and sometimes tornadoes.
They are most commonly found in the central and eastern United States, often occurring ahead of a cold front.
What is a supercell? How is it different from an ordinary cell or multicell thunderstorm?
A supercell is a highly organized thunderstorm characterized by a rotating updraft known as a mesocyclone. Unlike ordinary cells, which are relatively short-lived and lack rotation, and multicell thunderstorms that consist of multiple cells in various stages of development, supercells can persist for several hours and are capable of producing severe weather phenomena, including large hail, damaging winds, and tornadoes.
What are the different types of multicell thunderstorms?
There are several types of multicell thunderstorms, including:
Cluster storms: These consist of a group of cells that form in close proximity and can produce heavy rainfall and localized severe weather.
Squall line: A linear arrangement of storms that can produce damaging winds and heavy rain along its leading edge.
Multicell line: An elongated group of cells that may develop along a cold front, often leading to organized severe weather events.
Prefrontal squall line: A specific type of squall line that forms ahead of a cold front, typically associated with significant weather activity.
What are the three types of supercell thunderstorms? Name them and their hazards.
Classic supercell: Known for its rotating updraft, this type can produce large hail, damaging winds, and tornadoes.
HP (High-Precipitation) supercell: Characterized by heavy rainfall and precipitation, it often leads to flash flooding and severe wind gusts.
LP (Low-Precipitation) supercell: Generally produces less rain but can still generate severe winds and tornadoes, often leading to significant damage in localized areas.
What is wind shear and how does it create rotation?
Wind shear refers to the change in wind speed and direction with altitude, which can create a horizontal spinning effect in the atmosphere. This rotation can be tilted vertically by updrafts of a thunderstorm, leading to the development of the rotating updraft characteristic of supercells.
Understand the basics of lightning formation – why charges separate, the difference between a positive and negative strike, cloud-to-cloud vs. cloud-to-ground.
Lightning formation occurs when collisions between ice particles in a cloud lead to the separation of electrical charges, with positive charges accumulating at the top of the cloud and negative charges settling at the bottom.
A positive lightning strike typically originates from the positively charged top of the cloud, while a negative strike begins from the negatively charged base.
Cloud-to-cloud lightning involves discharges between clouds, whereas cloud-to-ground lightning refers to the electrical discharge that travels from the cloud to the ground, posing significant risks to people and structures.
Understand the basics of thunder, why does it form, why does it sound different with each strike?
Thunder forms due to the rapid expansion and contraction of air surrounding a lightning bolt, creating a shockwave that produces sound. The variation in sound can be attributed to distance from the observer, the temperature and humidity of the air, and the presence of obstacles, which can all affect the propagation of sound waves. Understanding these factors can help us better grasp the nature of thunder and its relationship with lightning.
Understand wind shear and the role it plays in tornado formation.
Wind shear refers to the change in speed and direction of winds at different altitudes, which can create a rotational effect conducive to tornado development. When warm, moist air near the surface meets cooler, drier air aloft, it can lead to severe thunderstorms. These conditions, combined with significant wind shear, can result in the formation of rotating updrafts that may develop into tornadoes.
Where do most tornadoes form in the US? Why?
Most tornadoes in the US form in an area known as "Tornado Alley," which includes parts of Texas, Oklahoma, Kansas, Nebraska, and South Dakota. This region experiences a unique combination of geography and climate that allows warm, moist air from the Gulf of Mexico to collide with colder, drier air from Canada and the Rocky Mountains, creating ideal conditions for severe thunderstorms and tornado development.
Which month is the peak for tornado formation in the US?
Typically, the peak month for tornado formation in the US is May, as it coincides with the transition from spring to summer, when atmospheric conditions are most favorable for severe weather events.
What is the EF scale? Discuss how the Enhanced Fujita Scale is used to assess tornado strength.
The Enhanced Fujita Scale is a system used to classify the intensity of tornadoes based on the damage they cause to structures and vegetation. It ranges from EF0, indicating light damage, to EF5, which signifies incredible damage with winds exceeding 200 mph. This scale helps meteorologists and emergency management agencies to assess the tornado's strength and predict its potential impact.
What is the difference between a tornado watch and a tornado warning?
A tornado watch is issued when conditions are favorable for the formation of tornadoes, meaning that severe thunderstorms capable of producing tornadoes are possible in the area. In contrast, a tornado warning is issued when a tornado has been sighted or indicated by radar, signaling the immediate threat to life and property, and requiring people to take protective action.