Transcript Chapter 10

Chapter 10:
Thunderstorms and Tornados
Notes
Lightning in Arizona
Thunderstorms
Thunderstorm – a storm that generates lightning and thunder
Thunderstorms form when warm, humid air rises in an unstable environment.
Various mechanisms can trigger the upward air movement needed to create
thunderstorm producing cumulonimbus clouds, including: 1) the unequal heating of
Earth’s surface, and 2) the lifting of warm air due to the physical features of Earth’s
surface (mountains).
Stages of Thunderstorm Development
Air-Mass Thunderstorms
Air-mass thunderstorms frequently occur in 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 made unstable when heated
from below or lifted along a front.
Air-mass thunderstorms are more frequent in the spring and summer, and typically
form in the midafternoon when surface temperatures are highest.
Stages of development:
1) Cumulus stage – updrafts dominate throughout the cloud, and growth from a
cumulus to a cumulonimbus cloud occurs
2) Mature stage – most intense phase, with heavy rain and possibly small hail, in
which downdrafts are found side by side with updrafts
3) Dissipating stage – dominated by downdrafts and entrainment, causing
evaporation of the structure
Cumulonimbus Cloud
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.
The National Weather Service classifies severe thunderstorms as having
winds in excess of 93 kilometers per hour (58 mph) or producing hailstones
with diameters larger than 1.9 centimeters ( 0.75 inch) or generate a tornado.
Severe thunderstorms can remain active for several hours due to the
existence of strong vertical wind shear within the cloud. Wind shear includes
changes in wind speed and/or direction between different heights in the
cloud.
Supercell Thunderstorm
Supercell Thunderstorms
A supercell consists of a single, very powerful cell that at times can extend to
heights of 20 kilometers and persist for many hours. These clouds have
diameters ranging between about 20 and 50 kilometers.
Supercell storms are very complex. The vertical wind profile may cause the
updraft to rotate. Within this column of cyclonically rotating air called the
mesocyclone tornadoes can form.
Squall Lines and Mesoscale Convective Complexes
Atmospheric conditions that favor the formation of severe thunderstorms can exist
over a broad area. As a result, groups of individual storms clustered together can
form.
1) Squall line – an elongate band in which thunderstorms can develop
2) Mesoscale convective complex – thunderstorms organized into roughly circular
patterns.
Storms in either environment are not unrelated individual storms, they are related by
a common origin or they occur in a situation in which some cells lead to the
formation of others
Cloud-to-Ground Lightning
Thunder and Lightning
A storm is classified as a thunderstorm only after thunder is heard. Because
thunder is produced by lightning, lightning must also be present.
Lightning is similar to the electrical shock you may have experienced when
touching metal, only much more intense. During the formation of a large
cumulonimbus cloud, a separation of charge occurs; one part of the cloud
develops an excess negative charge while another part acquires an excess
positive charge.
The most common type of lightning occurs between oppositely charged zones
within a cloud or between clouds. About 80% of all lightning is this type and is
called sheet lightning. The second type of lightning , electric discharge between
a cloud and Earth’s surface, is called cloud-to-ground lightning
The electrical discharge of lightning superheats the air immediately around the
lightning channel. In less than a second the temperature rises by as much as
33,000OC, expands explosively, and produces sound waves we call thunder.
Tornado; a Violently Rotating Column of Air in Contact with the Ground
Tornadoes
Tornadoes are local storms of short duration that must by ranked high among
nature’s most destructive forces.
Tornadoes are violent windstorms that take the form of a rotating column of air that
extends downward from a cumulonimbus cloud. Pressure within some tornadoes
have been estimated to be as much 10 percent lower than immediately outside the
storm. Due to the much lower pressure in the interior of the tornado, air near the
ground rushes into the tornado from all directions.
Formation of Mesocyclone often Precedes Tornado Development
Development and Occurrence of Tornadoes
Tornadoes form in association with severe thunderstorms that produce high winds,
heavy rainfall, and often damaging hail. Less than 1 percent of all thunderstorms
produce tornadoes. Although meteorologists are still not certain what triggers
tornado formation, it is apparent that they are the product of the interaction between
strong updrafts in a thunderstorm and the winds in the troposphere.
Tornadoes can form in any situation that produces severe weather, including cold
fronts, squall lines, and hurricanes. The most intense tornadoes usually form in
association with supercells. An important precondition linked to tornado formation in
severe thunderstorms is the development of a mesocyclone, a vertical cylinder of
rotating air about 3 to 10 kilometers across that develop in the updraft of a severe
thunderstorm.
Annual Average Incidence of Tornadoes in the United States;
Average Number of Tornadoes and Tornado Days each Month
Tornadoes are most often created along the cold front or squall line on a middlelatitude cyclone or in association with supercell thunderstorms.
Throughout the spring months , air masses associated with mid-latitude cyclones
are most likely to have greatly contrasting conditions. Continental polar air from
Canada may still be very cold and dry, whereas maritime tropical air from the Gulf
of Mexico is warm, humid, and unstable. The greater the contrast, the more intense
the storm tends to be.
The two air masses are most likely to meet in the central United States, the location
of more tornadoes than anywhere else on the planet.
The region of frequent tornado occurrence changes throughout the spring and
summer. In late January and February, the center of maximum frequency lies over
the central Gulf states (Louisiana, Mississippi, Alabama). During March and April
the center moves eastward to the southeastern Atlantic states (Georgia, Florida,
the Carolinas). During May and June the center moves through the southern Great
Plains ( Oklahoma, Kansas, Missouri, Arkansas). Late June and July the center of
maximum frequency is the northern Great Plains (Nebraska, Iowa, South Dakota).
Doppler Radar
Tornado Forecasting
Because of their small (relatively) size and short duration, severe thunderstorms and
tornadoes are difficult to predict. However, prediction of these weather phenomena is
critical to the safety of people living in regions where severe thunderstorms and
tornadoes are frequent.
Tornado watches – alert the public to the possibility of tornadoes over a specified area
for a particular time interval. Watches are generally reserved for organized severe
weather events where the tornado threat will affect at least 26,00 square kilometers or
persist for at least three hours.
Tornado warning – a warning issued by the local National Weather Service office when
a tornado has actually been sighted in an area or is indicated by weather radar.
Warnings are issued for much smaller areas than watches, usually covering portions of
a county or counties. They are also in effect fro much shorter periods, typically 30 to 60
minutes.