Download Chapter 13 – Severe Weather

Chapter 13 – Severe Weather
At any given moment, nearly 2000
thunderstorms are in progress around the world.
Both geography and air mass movements
make thunderstorms most common in the
southeastern United States.
Overview of Thunderstorms
How thunderstorms form
For a thunderstorm to form, three conditions
must exist:
•An abundant source of moisture in the lower
levels of the atmosphere
•lifting of the air mass in order for
condensation to occur (necessary for cloud
formation & growth)
•unstable atmosphere.
Lightning is the transfer of electrical charge caused by
the rapid rushes of air in a cumulonimbus cloud.
Friction between the updrafts and downdrafts within a
cumulonimbus cloud removes electrons from some of the
atoms in the cloud.
Atoms that lose electrons become positively charged
ions, and atoms that receive the extra electrons become
negatively charged ions.
Eventually, the differences in charges break down, and a
branched channel of partially charged air, called a
stepped leader, is formed between the positive and
negative regions.
When the stepped leader nears the ground, a
branched channel of positively charged particles,
called the return stroke, rushes upward to meet
it and illuminates the connecting channel with
about 100 million volts of electricity.
Stepped leader moving in ~50m steps.
© 1969 Martin Uman -- From: Uman, Lightning
Thunder
A lightning bolt heats the surrounding air to
about 30,000 C, about five times hotter than
the surface of the Sun.
The thunder you hear is the sound produced
as this superheated air rapidly expands and
contracts.
Thunderstorm and lightning safety
Each year in the United
States, lightning
causes about 7500
forest fires and an
average of 300 injuries
and 93 deaths to
humans.

Assume LSP.
Crouch with feet as
close together as
possible. Have heels
touch. Place hands
over ears.
-REMEMBERDO NOT LIE FLAT
ON THE GROUND
30-30 RULE

30 Seconds
◦ MOVE INDOORS if thunder is heard within 30
seconds of seeing Lightning

30 Minutes
◦ SUSPEND ACTIVITIES for 30 minutes after the
last observed lightning or thunder.



By referencing the time in seconds from
seeing the lightning (the FLASH) to hearing
the accompanying thunder (the BANG), we
can range lightning's distance.
A "Flash" to "Bang" of five seconds equals
lightning distance being one mile away.
A "Flash" to "Bang" of ten = two miles;
etc…..
Supercells
Severe thunderstorms can develop into
self-sustaining, extremely powerful storms
called supercells.
These furious storms can last for several
hours and can have updrafts as strong as
240 km/h.
An anvil-shaped cumulonimbus cloud is
characteristic of many severe
thunderstorms.
Precipitation in the form of balls or lumps of ice.
It forms because of two characteristics common to
thunderstorms.
1. water droplets rise to the heights of a
cumulonimbus cloud where the temperature
is below freezing, encounter ice pellets, and
freeze on contact with the pellets, which
causes the ice pellets to grow larger.
2. There needs to be an abundance of strong
updrafts and downdrafts moving side by side
within a cloud.
During summer and fall, the tropics experience
conditions ideal for the formation of large, rotating,
low-pressure tropical storms called tropical
cyclones.
Location
Favorable conditions for cyclone formation
exist in all tropical oceans except the South
Atlantic Ocean and the Pacific Ocean off the
west coast of South America.
Cyclone formation
Tropical cyclones require two basic
conditions to form
1) an abundant supply of warm ocean
water
2) some sort of mechanism to lift warm
air and keep it rising.
Cyclone formation
The first indication of a building tropical cyclone
is a moving tropical disturbance.
When a disturbance over a tropical ocean
acquires a cyclonic circulation around a center
of low pressure, it has reached the
developmental stage and is known as a
tropical depression.
Cyclone formation
When wind speeds around the low-pressure
center of a tropical depression exceed 65 km/h,
the system is called a tropical storm.
If air pressure continues to fall and winds
around the center reach at least 120 km/h, the
storm is officially classified as a cyclone.
Section 13.3
Tropical Storms
Cyclone formation
Once winds reach at least 120 km/h,
another phenomenon occurs—the
development of a calm center of the storm
called the eye.
The eye of the cyclone is a span of 30 to
60 km of calm weather and blue sky.
Section 13.3
Tropical Storms
Cyclone formation
The strongest winds in a hurricane are
usually concentrated in the eyewall—a tall
band of strong winds and dense clouds that
surrounds the eye.
Section 13.3
Tropical Storms
Cyclone formation
A hurricane will last until it can no longer
produce enough energy to sustain itself.
This usually happens when the storm has
moved either over land or over colder water.
Section 13.3
Tropical Storms
Like most storms, cyclones begin with
warm moist air rising.
Section 13.3
Tropical Storms
The Saffir-Simpson
hurricane scale
classifies hurricanes
according to wind speed,
potential for flooding due
to the effect on the height
of sea level, and potential
for property damage.
Damage
Hurricanes can cause extensive damage,
particularly along coastal areas, which tend
to be where human populations are the
most dense.
Winds
Much of the damage caused by hurricanes
is associated with violent winds.
Storm surge
A storm surge occurs when hurricane-force
winds drive a mound of ocean water toward
coastal areas where it washes over the land.
Hurricane advisories and safety
The National Hurricane Center issues a
hurricane warning at least 24 hours before a
hurricane is predicted to strike.
Awareness, combined with proper safety
precautions, has greatly reduced death tolls
associated with hurricanes in recent years.
A tornado is a violent, whirling column of air in
contact with the ground.
Before a tornado reaches the ground, it is called a
funnel cloud.
A tornado forms when wind speed and direction
change suddenly with height, a phenomenon
associated with wind shear.
Although tornadoes rarely exceed 200 m in
diameter and usually last only a few minutes, they
can be extremely destructive
Section 13.2
Severe Weather
Tornado classification
The Fujita tornado intensity scale, which
ranks tornadoes according to their path of
destruction, wind speed, and duration, is
used to classify tornadoes.
Tornado distribution
Most tornadoes—especially violent ones—form in
the spring during the late afternoon and evening,
when the temperature contrasts between polar air
and tropical air are the greatest.
This type of large temperature contrast occurs
most frequently in the central United States.
Many of the more than 700 tornadoes that touch
down in the United States each year occur in a
region called “Tornado Alley,” which extends from
northern Texas through Oklahoma, Kansas, and
Missouri.
If you are caught in a tornado
Indoors: take shelter in the
southwest corner of a
basement, a small downstairs
room or closet, or a tornado
shelter.
Outdoors: find lowest spot & lie flat and face-down,
protecting the back of your head with your arms.
Avoid seeking shelter under bridges. Stay away from
objects that might become debris, i.e. cars.
Car: Drive away from storm if possible. If not, park
the car quickly & off road. Find shelter if possible. If
not, follow outdoor guidelines.
An individual
thunderstorm can
unleash enough rain to
produce floods, and
hurricanes also cause
torrential downpours,
which result in
extensive flooding.
 Floods can also occur
when weather patterns
cause even mild storms
to persist over the
same area.

Low-lying areas are most
susceptible to flooding, making
coastlines particularly vulnerable
to storm surges during
hurricanes.
Rivers in narrow-walled valleys
and streambeds can rise rapidly,
creating high-powered and
destructive walls of water.
Droughts are
extended periods of
well-below-average
rainfall.
Droughts are
usually the result of
shifts in global wind
patterns that allow
large, high-pressure
systems to persist
for weeks or months
over continental
Because the sinking air prevents humid air
from rising, condensation cannot occur, and
drought sets in until global patterns shift
enough to move the high-pressure system.
Section 13.4
Recurrent Weather
Heat waves
An unpleasant side effect of droughts often
comes in the form of heat waves, which
are extended periods of above-average
temperatures.
Heat waves can be formed by the same
high-pressure systems that cause droughts.
Section 13.4
Recurrent Weather
Heat waves
Because of the dangers posed by a
combination of heat and humidity, the
National Weather Service (NWS) routinely
reports the heat index.
The heat index assesses the effect of the
body’s difficulty in regulating its internal
temperature as relative humidity rises.
Section 13.4
Recurrent Weather
The opposite of a heat wave is a cold wave,
which is an extended period of belowaverage temperatures.
Cold waves are also brought on by large,
high-pressure systems. However, cold waves
are caused by systems of continental polar or
arctic origin.
Because of the location and the time of
year in which they occur, winter highpressure systems are much more
influenced by the jet stream than are
summer high-pressure systems.
Section 13.4
Recurrent Weather
The winter location of the jet stream can
remain essentially unchanged for days or
even weeks. This means that several polar
high-pressure systems can follow the same
path and subject the same areas to
continuous numbing cold.
Section 13.4
Recurrent Weather
Wind-chill index
The effects of cold air on the human body are
magnified by wind. Known as the wind-chill
factor, this phenomenon is measured by the
wind-chill index.