Chapter 10 Thunderstorms and Tornadoes

Which of the following conditions are favorable for the development of tornadic thunderstorms in a given area?

All of the above conditions occurring (more or less) simultaneously.

Multicell thunderstorms owe their existence to:

All of the above can lead to multicell thunderstorms.

Which of the following statements is false?

Lightning strikes are always fatal.

How does a cumulonimbus cloud achieve charge separation?

The exact mechanism is as yet unknown.

The so-called Tornado Belt of the United States is located

in the central plains

The onset of precipitation marks the beginning of a thunderstorm's _______ stage.

mature

Which of the following is not a requirement for thunderstorm development?

Strong vertical wind shear.

Which below is the best indication that a severe thunderstorm is about to produce a tornado?

a wall cloud

Cars are often good protection from lightning. What makes them so?

The steel skin of the body conducts the electricity away from the car's inhabitants.

Most tornadoes have wind speeds:

less than 97 kts.

Thunderstorms which produce tornadoes

have rotating updrafts

What differentiates a multicell thunderstorm from a supercell thunderstorm?

Supercells are characterized by rotating updrafts.

A hook-shaped echo on a radar screen often indicates

the possible presence of a tornado-producing thunderstorm

Which of the following is NOT a characteristic of lightning?

Lightning is a form of alternating current.

A tornado-like event that forms over water is a

waterspout

The Enhanced Fujita scale pertains to

the strength of a tornado.

A Doppler radar determines precipitation ____ by measuring changes in the ____ of the reflected radiowave

velocity, frequency

The morning forecast for your area calls for hot, humid conditions during the afternoon, followed much cooler temperatures after the passage of a cold front. Which of the following would most likely be issued that morning for your area?

Severe thunderstorm watch.

Lightning may occur

• within a cloud

• from a cloud to the ground

• from one cloud to another cloud

What differentiates air mass thunderstorms from severe thunderstorms?

Strong vertical wind shear.

Which of the following factors is most important in determining the strength of a tornado?

central pressure

Most tornadoes move from:

southwest to northeast

SW toward the NE

A tornado cloud that does not touch the ground is called

a funnel cloud.

A typical diameter of a tornado would be:

250 meters

On a surface weather map, this marks the boundary where a warm, dry air mass encounters a warm, moist air mass.

dry line

Radar

Radio Detection and Ranging. Transmits and receives microwave radiation. Power that comes back depends on the size of the target, the composition of the target, and the number of targets.

Radar reflectivity

A measure of the power returned to the radar. The more that comes back, the higher the reflectivity.

Bright spots on radar

Heavier precipitation

Green spots on radar

Not precipitation; bugs, debris, etc. up-drafted by the storm.

What do doppler radars detect?

Velocity of precipitation and speed of wind. Only radial velocity.

Radial velocity

Velocity towards and away from the doppler radar.

Weather radar limitations

Beam of radiation spreads with range.
Limited range because of the earth's curvature.

Thunderstorm

The process by which unstable parcels rapidly rise, condense, and produce precipitation and lightning. The most obvious manifestation is cumulonimbus clouds.

Ingredients for thunderstorms

Warm air in low levels, moist air in low levels, cool air in upper levels.

Peak time for thunderstorms

4:00PM. When the Earth is warmest.

Severe thunderstorm

Winds gusting greater than 50 knots, hail greater than quarter sized, and tornadoes.

Vertical wind shear

Wind speed and direction that changes with height.

Air mass thunderstorms

Thunderstorms that develop away from front that rarely become severe. Triggered by localized convective liftings or orographic lifting.

Stages of development of air mass thunderstorms

Cumulus cloud
Updraft through the cumulus cloud
Becomes cumulonimbus cloud with updraft and downdraft

Downdraft

Cold air descends to ground

Updraft

Warm air ascends through clouds

Gust front

Cold air produced by thunderstorms. Leading edge of precipitation.

Reason for thunderstorms

Redistribution of heat and wind

Squall line

Narrow bands of thunderstorms. Can form fronts. Can form as disorganized thunderstorms agglomerate into an organized line of thunderstorms. Can produce hail or tornadoes. Biggest hard is straight line winds. Composed of multiple closely-spaced thunderstorms along the line. Can produce severe weather. Long lifetimes that constantly regenerate along the gust front.

Supercell

Highly organized, generally isolated thunderstorms that is characterized by a storm-scale vortex called a mesocyclone. Produce strong and violent tornadoes.

Mesocyclone

Rotating updraft

Lightning

Atmospheric spark. 5 times hotter than the surface of the sun. Deals with the imbalance of charges.

Thunder

Expansion of air caused by lightning increasing pressure.

Tornadoes

Violently rotating vortex in contact with both the parent cumulonimbus cloud and the ground. Comes out of a mesocyclone. Winds can exceed 300 mph. Strong winds create significantly reduced pressure because of the pressure gradient force.

Funnel cloud

A vortex not in contact with the ground

Condensation funnel

The region of the tornado manifested by condensation

Enhanced Fujita Scale

Tornado intensity is rated according to the damage produced, not by the wind speeds. Dependent on the severity of damage and on the structure that is damaged.

EF-0

65-85mph. Light damage. Some damage to siding and shingles.

EF-1

86-110mph. Moderate damage. Considerable roof damage. Winds can uproot trees and overturn single-wide mobile homes. Flagpoles bend.

EF-2

111-135mph. Considerable damage. Most single-wide mobile homes destroyed. Permanent homes can shift off foundations. Flagpoles collapse. Softwood trees debarked.

EF-3

136-165mph. Severe damage. Hardwood trees debarked. All but small portions of houses destroyed.

EF-4

166-200mph. Devastating damage. Complete destruction of well-built residences, large sections of school buildings.

EF-5

>200mph. Incredible damage. Significant structural deformation of mid- and high-rise buildings.

Tornado watch

Conditions are supportive of tornadoes. Covers a large area. Valid for 6+ hours.

Tornado warning

A tornado has been spotted on the ground. Covers an area about the size of a county. Valid for ~1 hour.

ordinary cell thunderstorms

A thunderstorm produced by local convection within a conditionally unstable air mass.

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 thunderstorm cycle and is characterized by heavy showers, lightning, thunder, and violent vertical motions inside cumulonimbus clouds.

dissipating

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, 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.

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. Formed by the horizontal spreading of cool air that originated inside a thunderstorm.

downburst

A severe localized downdraft that can be experienced beneath a severe thunderstorm.

microburst

A strong localized downdraft less than 4 km wide that occurs beneath thunderstorms.

heat burst

A sudden increase in surface air temperature often accompanied by extreme drying.

squall line

A line of thunderstorms that form along a cold front or out front or out ahead of it.

bow echo

A line of thunderstorms on a radar screen that appears in the shape of a bow.

derecho

Strong, damaging, straight-line winds associated with a cluster of severe thunderstorms that most often form in the evening or at night.

wall cloud

An area of rotating clouds that extends beneath a supercell thunderstorm and from which a funnel cloud may appear.

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 electric 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 traveling at or above the speed of sound.

stepped ladder

An initial discharge of electrons that proceeds intermittently toward the ground in a series of steps in a cloud-to-ground lightning stroke.

dry lightning

Lightning that occurs with thunderstorms that produce little, if any, appreciable precipitation that reaches the surface.

tornado

An intense, rotating column of air that often protrudes from a cumuliform cloud in the shape of a funnel or a rope whose circulation is present on the ground.

funnel cloud

A tornado whose circulation has not reached the ground. Often appears as a rotating conelike cloud that extends downward from the base of a thunderstorm.

tornado watch

A forecast issued to alert the public that tornadoes is imminent.

tornado warning

A warning issued when a tornado has actually been observed either visually or on a radar screen.

Fujita scale

A scale that describes tornado intensity by observing damage caused by the tornado and their rotational wind speed.

Enhanced Fujita Scale

A scale that describes tornado intensity by observing damage caused by the tornado.

landspout

Relatively weak nonsupercell tornado that originates with a cumuliform cloud in its growth stage and with a cloud that does not contain mid-level mesocyclone.

waterspout

A column of rotating wind over water that has characteristics of a dust devil and tornado.